Ultrasound Hydrogel: A Review on Materials and Method (2024)

Sadia Afzal1, Muhammad Zahid1, S Sadia Nimra2, Zahra Fatima2, HM Fayzan Shakir3,4,5*, ZA Rehan2*

1Department of Chemistry, University of Agriculture, Faisalabad, Pakistan

2Department of Materials, School of Engineering and Technology, National Textile University, Faisalabad, Pakistan

3NPU-NCP Joint International Research Center on Advanced Nanomaterials and Defects Engineering, Xi’an, Shaanxi Province, China

4Shaanxi Engineering Laboratory for Graphene New Carbon Materials and Application, Northwestern Polytechnical University, Xi’an, Shaanxi Province, China

5School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, Shaanxi Province, China

*Correspondence to: HM Fayzan Shakir, School of Materials Science and Engineering, Northwestern Polytechnical University, 127 Youyi Xi Lu, Xi’an 710000, Shaanxi Province, China; Email: Fayzan.shakir93@gmail.com;

ZA Rehan, Department of Materials, School of Engineering and Technology, National Textile University, Sheikhupura Road, Faisalabad 37610, Pakistan; Email: Zarehan@ntu.edu.pk

DOI: 10.53964/jmpcm.2022002

Abstract

Ultrasound procedures are widely used in assessing and diagnosing a wide variety of medical conditions. For example, in ultrasound imaging, which is utilized for mapping or identifying internal aspects of the patient's body such as fetus, tendons, muscles, and other organs, etc. It is also used for treating skin conditions such as reducing wrinkles, supporting tissue healing, analysis, and improving the extensibility of connective tissues. Medical ultrasound imaging has advantages over magnetic resonance imaging, such as portability, real-time imaging, reasonable cost, and its harmless effect. Ultrasound gel is used as a coupling medium in all ultrasound procedures to replace air between the patient’s skin and the transducer, as ultrasound waves have trouble in traveling through air. But the availability and cost of commercial ultrasound gel are its major limitations. This review article describes the properties and applications of ultrasound gel and some materials that could be used in the formulation of an ultrasound gel. Generally, an ultrasound gel could be prepared by mixing these seven ingredients: vehicle, thickening agent, anti-inflammatory agent, skin conditioning agent, chelating agent, preservative, and neutralizer.

Keywords: hydrogel, ultrasound, compatibilizer, gelling agent, antimicrobial

1 INTRODUCTION

With the advancements of health technologies in low-resource sites, the equipment, supplies, and support modalities that permit these developments to be employed are not completely measured. It is common to set up a new computed tomography (CT) scan machine in a hospital, but unreliable electricity supply and unavailability of contrast agents restrict its extensive application. In low-resource settings, numerous costly high-tech gadgets are left unused due to the lack of experienced healthcare specialists and proper disposables, such as the ultrasound, particularly when it comes to the constant accessibility of ultrasound gel. In resource-limited settings, portable ultrasound is proved to be a progressively beneficial diagnostic device. Currently, costs for ultrasound are few, yet the approach to the essential consumable goods (specifically ultrasound gel) for ultrasound use remains one of the major hurdles for the implementation, which limits the extensive and routine use of ultrasonography[1,2].

Ultrasound has been used in the medical field to observe the interior of the human body in a non-invasive manner[3]. Medical ultrasound is divided into two classes: diagnostic and therapeutic. Ultrasound diagnostic procedures are used for assessing and diagnosing a wide variety of medical conditions related to internal organs to determine the presence of abnormal masses or to detect changes in the appearance of tissues, vessels, and organs. The ultrasound transducers, or probes, work at megahertz (MHz) frequencies and are always placed on the surface of the skin (but probes are sometimes placed inside the body).

Diagnostic ultrasound is further divided into two categories: anatomical and functional ultrasound. Anatomical ultrasound produces images of internal organs or other structures, whereas functional ultrasound produces information maps via data about the velocity and movement of blood or tissues and other physical features[4-6]. Ultrasonography, ultrasound imaging, is an ultrasound-based medical imaging technique that is utilized for mapping or identifying interior of the patient's body, such as fetus, muscles, tendons, and other organs, etc., and their size, structure, and pathological lesions are captured with real-time tomographic pictures[7,8]. Medical ultrasound imaging has advantages over magnetic resonance imaging (MRI) and X-ray, such as portability, real-time imaging, reasonable cost, non-ionizing radiation, and safety profile. However, the resolution of MRI systems and CT systems is usually higher than that of ultrasound imaging systems[9,10].

Therapeutic ultrasound procedures interact with the body tissues in such a way that they are either transformed or damaged without producing images. The utilization of very high-intensity waves that can destroy abnormal or diseased tissues like tumors allows certain modifications, such as pushing, moving, or heating tissue, delivering drugs to particular locations in the body, and dissolving blood clots. Therapeutic ultrasound generates high levels of focused acoustic output on particular areas to heat, ablate, or break down the tissues. High-intensity focused ultrasound (HIFU) is a type of therapeutic ultrasound that utilizes highly targeted and high-intensity sound beams. HIFU is being studied as a way to modify or destroy the abnormal or diseased tissues within the body without cutting the skin or causing injury to the neighboring tissue. Ultrasound therapeutic procedures are also used to support tissue healing, analysis, and improve the extensibility of connective tissues. These procedures are also utilized for treating skin conditions such as removing wrinkles[4,5,7].

The degree of medical advantage of therapeutic ultrasound is determined by its use, and proper use of these procedures can minimize the damage[11]. Although the long-term impact due to ultrasound contact at diagnostic intensity is still obscure, most specialists argue that the benefits can remedy its weakness[12]. The World Health Organization (WHO) has recognized ultrasound as effective, safe, low-cost, and versatile[13].

These ultrasound procedures need a special medium that can provide lubrication to the skin to aid the movement of the transducer on the skin and can replace air between the transducer and patient's skin because ultrasound waves have difficulty in traveling through the air due to very small acoustic impedance[11,14]. Thus, a coupling agent is required to guarantee a good acoustic connection between the skin and the transducer[15]. In recent years, ultrasound gel has been generally utilized as a conductive medium in several diagnostic procedures such as endoscopy, ultrasonography, electrocardiography, and transesophageal echocardiogram examinations[16].

Ultrasound gel that is used to minimize the air between the transducer and the patient’s skin is the best medium to achieve these functions. It transfers ultrasound waves between the patient’s skin and the transducer and reduces the mismatch of acoustic impedance and reflection to ensure a clear image[17,18]. The larger the difference between the acoustic impedance of the two media, the larger will be the reflection and the lesser will be the transmission of sound waves at the boundary of the two media, hence affecting the image quality. Despite the main objective of getting a clear image, ultrasound gel is not supposed to compromise the safety or comfort of the patient[19]. Commercial ultrasound gel posses all these properties and is ideal for use in ultrasonography and other ultrasound procedures, but the cost and availability of commercial ultrasound gel are its limitations[20].

The ultrasound is applied using a transducer that typically comes into contact with the skin of the patient, and the gel is utilized between the skin and the transducer to enhance the transmission of ultrasound waves as they are readily absorbed in the air, shown in Figure 1. The high-frequency vibrations are transferred into any tissue that contacts the transducer, reflected, and then captured by the transducer to produce an image on the ultrasound machine. Some ultrasound energy is additionally somewhat consumed by tissue on its route, both on its travel away from the transducer and on its return journey to the transducer. In this manner, the transducer performs two functions: transmitting and receiving sound waves.

Figure 1. Transmission of ultrasound waves with and without ultrasound gel.

A gel is a dense jelly-like material that has characteristics extending from weak and soft to hard tissues. Gels are characterized as a considerably weak cross-linked arrangement, which shows no stream within the steady-state. Gels are mostly liquid by weight, but they behave like solids due to a three-dimensional cross-linked arrangement within the fluid. It is the crosslinking inside the liquid that contributes to the structure or hardness of a gel. Simply, a gel is a distribution of particles of a liquid inside a solid, in which the liquid is the discontinuous phase and the solid is the continuous phase. In some cases, the gel is semi-solid and formed into disks, but still retains the ability to readily transmit ultrasound waves[3].

Water has often been utilized as a coupling medium in numerous treatments of ultrasound because of its desired acoustic properties. For example, a cone-shaped plastic case filled with water with a thin membrane of polyurethane wrapping the open tip has been used for the application of HIFU to stop bleeding from organs and damaged blood vessels[21,22]. However, there were several disadvantages of this coupler that would render it impractical in the clinical context. These drawbacks include circulation, confinement, sterilization, and degassing problems. HIFU hemostasis procedures utilize coupling cones made of solid titanium or aluminum, more recently[23]. Because of their robustness, they are more suitable choices for potential clinical applications. Nevertheless, the large mismatch of impedances between tissue and metal and the low efficiency of energy transfer because of the high attenuation coefficient are the main disadvantages[24].

Thus, all these drawbacks gave rise to the idea of hydrogels, for use in ultrasound procedures, which are mostly in liquid composition and also exhibit similar densities to those of liquids. Previous researches have demonstrated that hydrogels are effective coupling media for both therapeutic and diagnostic ultrasounds[25-27]. Hydrogels are networks of hydrophilic and crosslinked polymers that are swollen in water. The good mechanical properties and high water content of hydrogels have rendered them desirable for a large variety of biomedical applications. Since hydrogels are mainly composed of water, they intrinsically have low impedance and attenuation, identical to that of biological soft tissues. The benefits of the hydrogel are that they have a comparatively low cost of materials, can be shaped into rigid forms, and can be readily mounted to a ultrasound transducer. All of these advantages render them an attractive choice as single-use acoustic couplers for high-intensity focused ultrasound devices[24].

An imperative constituent in any sort of ultrasound treatment system is the procedure for transferring the acoustic energy into the patient’s body. The acoustic coupling medium gives an effective way to US transmission to the tissues from the transducer. The perfect coupler is a uniform medium that has a sound speed comparable to that of the tissue being treated and low acoustic impedance and attenuation, which would bring about refraction and reflection of the ultrasound beam at the coupler-tissue boundary[24].

1.1 Acoustic Impedance

It is a physical characteristic of tissue. It defines how much opposition an ultrasound beam experiences as it goes through a tissue. Acoustic impedance (AI) is mathematically described as

Z = ρv

Where Z is the AI,

ρ= density of the medium (in kg/m3)

v= speed of sound in the medium (in m/s).

Hence, the units forAI are kg/(m2·s) or Rayl.

Speed of sound in different materials, density, and associated acoustic impedances are given in Table 1.

Table 1. The Speed of Sound, Density, and Associated Acoustic Impedances of Several Media (Including Some Soft Tissues)

Reflections at boundaries between two different media occur because of differences in a characteristic known as the AI of each substance. Some of the wave energy is transmitted, while some are reflected at the interface between media with different AIs. The larger thegapin AI between the two media, the more prominent the reflection and the lesser the transmission[28,29].

For diagnostic ultrasound, materials having acoustic quantities comparable to soft tissue are necessary as tissue-mimicking materials. In an ultrasound image, soft tissues include the tissues of ligaments, tendons, fascia, muscles, fibrous tissue, nerves, fat, and blood vessels. Acoustic quantities comprise the sound velocity or speed of sound (SoS), attenuation coefficient (ultrasound energy lost by material), and AI[30,31]. Ideally, a tissue-mimicking material should have a density=1.043(g/cm3), SoS = 1540±10m/s, AI=1.6MRayls, and an attenuation coefficient for frequencies extending from 2-15MHz is 0.5-0.7dB/cm/MHz, as the attenuation coefficient has a linear response to frequency[32,33]. During ultrasound imaging, a material with comparable properties will guarantee image contrast, resolution, and depth of penetration identical to soft tissues[33].

The difference between the AI of soft tissue and bone, and between soft tissue and air is rather large. This large difference between the AI of skin and air is the reason to use coupling gel for imaging purposes[34,35].

Density, speed, and AI for two commercially available ultrasound gels are given in Table 2.

Table 2. Shows Density, Speed, and Acoustic Impedance for Two Commercially Available Ultrasound Gels

1.2 The Intensity Reflection Coefficient

It is described as the ratio of the intensity of the reflected wave in comparison to the transmitted or incident wave. Mathematically, this statement can be expressed as:

Where Z1 and Z2 are the AI of the two media creating the boundary. The same AI of both media gives a reflection coefficient of zero. It means that there is no reflection and total transmission of sound waves. Matching impedance (no reflection) gives an effective coupling of sound energy from one medium to another. Tracking of reflections and mapping of the intensity of the reflected sound waves made the image formed in ultrasound[36,37]. If Z1 is the impedance of gel and Z2 is the impedance of muscle, then the intensity reflection coefficient tells us that 0.05% of sound waves reflect at the boundary between gel and human skin, and 99.94% transmit through the skin, and it is the opposite without ultrasound gel.

1.3 Transducer

A transducer is a device that transforms energy from one form to another and is utilized to produce ultrasound waves. The ultrasonic transducer is in direct contact with the skin of the patient. Ultrasound waves are emitted through the transducer by the piezoelectric effect, travel to the patient's skin, penetrate, and then are reflected from hard tissues and bones and received by the transducer. Hence, the transducer acts as both a transmitter and a receiver[10,38-41]. Different forms of transducers are utilized in various areas, such as urology, cardiology, ophthalmology, gynecology, obstetrics, and orthopedics. A typical transducer is composed of different layers.

1.4 Active Layer

An active layer is generally composed of piezoelectric material, and mostly this material is piezoceramic. This active layer performs many functions: it produces an ultrasound wave in reaction to an electric signal; it also receives the wave reflected at the borderline of tissue; and by using the piezoelectric effect, it transforms the received ultrasound wave into an electric signal. The piezoelectric crystal, thus, functions as both a transmitter and a receiver of sound waves. On the other hand, the effective transmission of ultrasonic energy between the two media is prevented by the large mismatch between the acoustic impedance of the human body and piezoceramic elements[10,42].

1.5 Matching Layers

To assist the transmission of ultrasound energy, acoustic matching layers are utilized in addition to active layers[43]. These are used to match the impedance of the transducer to soft tissues and enhance the coupling efficiency of the acoustic energy to the soft tissues (transmission media) from the probe. Every matching layer has a width of ¼ of wavelength at the middle frequency of the transducer, with a value of AI close to 7Mrayl. In general, the options are limited for selecting the material with the necessary precise AI of 7Mrayl. The single-layer approach needs the development of novel materials. It is possible to utilize double or multiple layer designs by using easily accessible substances whose AI values are intermediate between 33.7Mrayl (the AI of the active layer, e.g., ceramic) and 1.5Mrayl (the AI of soft tissues). This design provides a broad spectrum of materials available, but it also increases complications in the manufacturing of the transducer[44,45].

1.6 Backing Block

Some ultrasound waves propagate towards the back from the piezoelectric material, and the backing block layer is utilized to absorb these ultrasonic waves. A backing block should have a large attenuation because a low attenuation coefficient could cause the reflection of the backward wave at the bottom of this layer to return to the piezoelectric element, thus creating noise in the ultrasound image. In addition to this material damping, some structural modifications have been employed to enhance the dispersing effects of the backing block, for example, introducing rods or grooves in the backing block[46-48]. It should have low AI as high AI causes the acoustic energy, produced by piezoelectric material, to be wasted by this layer, and as a result, only some of the ultrasound waves would be transferred to the human body. Generally, the backing block has an AI of 3 to 5Mrayl[49].

1.7 Kerfs and Acoustic Lenses

The acoustic lens performs two functions. First, it protects the ultrasonic transducer from external harm. Secondly, it focuses the ultrasound beam onto a specific point[50]. Normally, they are made of rubber materials to provide comfortable contact between the patient and the transducer. The term “Kerf” is a space between exhibited piezoelectric components that confines each component from its adjacent components to decrease the crosstalk between them because the crosstalk extremely affects the performance of the transducer. As a result, various kerf materials and shapes have been developed to reduce crosstalk[10]. A schematic representation of a transducer is given in Figure 2.

Figure 2. Schematic representation of a transducer.

Ultrasound gel is used in a variety of ultrasound operations, such as physiotherapy, cardiovascular research, cancer treatment, facial skin tightening, and ultrasonography, all of which are discussed below in detail. Ultrasound procedures use a range of frequencies (1-20MHz) depending on their applications[51,52]. For example, in diagnostic radiology, ultrasound frequencies vary from 2-15MHz roughly. Higher frequencies are not as penetrating because they are absorbed easily due to shorter wavelengths. That’s why low frequencies are utilized for deeper body parts and high frequencies are utilized for those that are superficial body structures[53,54]. The higher frequencies are advantageous in a way because they give a good distinction between a target and other things[10]. The following frequencies are usually utilized for ultrasound analysis: The following frequencies are usually utilized for ultrasound analysis (Figure 3) .

Figure 3. Different frequencies for ultrasound analysis.

2.1 Physiotherapy

Ultrasound is an excellent technology for a physiotherapist in the developing world. It is inexpensive, quick, and does not expose the patient to radiation. Physiotherapists use ultrasound more than any other electrophysiological modality. The level of medical advantage from diagnostic ultrasound remains undefined and can be governed by the application. When operated correctly, the possibility of harm or injury is considered low, enabling ultrasound a modality of moderate efficiency and low risk for physical therapy. Superficial thermal response receptors are activated by topical analgesics that produce hot or cold sensations. Aqueous gels containing a suspension of topical agents are more efficient in the transmission of ultrasound energy as compared to less effective cream-based agents[55]. Topically applied external analgesics provide some benefit for pain relief. The active ingredient in most of these is menthol, methyl salicylate, or a combination of the two. Menthol is the irritant that produces a cooling sensation, while methyl salicylate provides a sensation of heat. The amount of menthol in many of the over-the-counter drug (OTC) topical analgesics is 1-16% and the amount of methyl salicylate is 0-30%. Previous studies have reported that when it is mixed with ultrasound gel, it enhances therapeutic ultrasound by providing fast pain relief[1,56-58].

Based on the literature, it was found that aloe vera is one of the plant materials known to decrease inflammation effectively and promote wound healing. This brings us to the conclusion that aloe vera has a possible analgesic activity that could be used in physiotherapy treatment. Around 75 active components have been known to be present in the aloe vera gel. The synergistic effects of these several compounds provide biological activities that can cure severe diseases such as cancer, liver problems, skin diseases, and acquired immune deficiency Syndrome (AIDS)[59,60]. The presence of vitamin C, B1, B2, B6, and folic acid in the flesh of aloe vera assists in the reduction of pain by stimulating the immune system and the level of Prostaglandin E2 (PGE2), therefore decreasing the inflammation reaction[61]. This creature is also known to contain steroidal and alkaloid substances that carry antioxidative properties, exerting potency to cure cancer. Further, aloe vera gel could enhance the healing of cutaneous injuries and other types of burns. Hence, aloe vera gel was included as one of the ingredients in ultrasound gel to provide fast pain relief with its cool and burning sensation[55].

2.2 Cancer Treatment

Warts are defined as benign tumors that normally include the skin and other epithelial tissues, which are triggered by human papillomavirus infection. Various morphologic forms of warts appear depending on the virus type and body sites, including common warts, plantar warts, periungual warts, anogenital and filiform warts (facialwarts)[62]. The treatment of warts is rather challengeable, although many treatment modalities are accessible. Given there is no radical cure, a combination of different types of treatments is used. Lasers, intralesional bleomycin injections, cryotherapy, 5% imiquimod, topical salicylic acid, topical 5-fluorouracil[63], retinoic acid[64], vitamin D analogs[65], zinc, cidofovir, oral retinoids, topical and intralesional immunotherapy are all treatment modalities for recalcitrant warts. Human papillomavirus causes viral warts, which can grow as papillomas anywhere on the body, most commonly on the feet and hands. Some warts degenerate spontaneously, but most of them need treatment. Treatment is also tough due to the resistance of warts to standard therapy and the high probability of reappearance.

Cryotherapy, or cryosurgery, is the targeted and controlled destruction of infected tissues by the use of cold-temperature materials. It is a negligibly invasive procedure in which freezing or cold temperatures are utilized to destroy the tissues within the body for the treatment of different conditions, including various forms of cancer (kidney, liver, prostate, and lung)[66-69], cardiovascular diseases (pulmonary and arterial disorders)[70,71], and neurological conditions[72,73]. The major advantage of cryosurgical techniques is the capability to cure a volume of diseased tissues without completely disturbing the neighboring structures[74]. Therefore, cryosurgical approaches are superior to some conventional surgical methodologies with respect to shorter recovery time and lower risk[75].

After the cryogen is sprayed, transferring of heat from the tissue to the cryogen and the formation of ice in the extracellular chamber occurs. Extracellular solutes are concentrated, and these concentrated solutes create an osmotic gradient in which fluid moves to the extracellular compartment and the concentration of solutes moves inside the cell, directing to cell destruction. The ice crystals also mechanically destroy the cell membrane. Additionally, there is also intracellular ice formation that damages the organelles such as the endoplasmic reticulum and mitochondria. Due to the cold temperature, there is serious vasoconstriction and endothelial destruction that results in platelet accumulation, micro-thrombi development, and finally, ischemic necrosis of the tissue. Inflammation occurs in reaction to cell death, leading to further damage. Most of the above treatments are only partially effective, with a higher rate of reappearance.

The solution to this problem is the application of ultrasound gel on top of the wound, followed by cryospray. The ultrasound gel contributes to extend the freezing time, which maintains the temperature for a longer period and increases the success rate with minimum adverse reactions of cryotherapy. The minimum usage of cryogen seems to be more effective and safer when used with the gel[62].

2.3 Facial Skin Tightening

Ultrasound seems to be an effective and safe technique for tightening facial skin. Nonablative technologies for skin tightening are applied for skin sagging and removal of wrinkles with the least discomfort, risk of adverse effects, and minimum downtime. The excellent safety profile of these procedures is mitigated due to their limited efficiency. Recently, light, laser, and other energy processes have been altered for the treatment of sagging and wrinkled skin in a so-called non-ablative way. The attractive characteristics of nonablative skin tightening are the capability to go back to work or social activities, limited post-procedure healing time, less need for physician oversight, and low chances of adverse events in comparison to rhytidectomy or ablative resurfacing. As a result, non-ablative skin tightening is a better option for more patients than surgical or ablative skin tightening[76,77]. The replacement of invasive procedures by non-ablative skin tightening has been prevented in terms of lack of persistence, efficiency, and reliability of such procedures[78,79].

HIFU is used in the facial skin tightening process. In this highly intense focused ultrasound procedure, ultrasound gel is applied first to the targeted skin surface of the patient. Then the probe is firmly positioned and pressed evenly so that it is coupled to the surface of the skin. The ultrasound imaging function is utilized to verify that the probe or transducer is acoustically coupled with the tissues of the skin and that the geometrical focal depth for treatment lies within the middle to the deep reticular dermis layer. Treatment exposure starts with the delivery of a series of individual ultrasound impulses for nearly 2 seconds. Then, the transducer is moved to the next position and laterally repositioned from 3 to 5mm so that it is parallel and adjacent to the prior treatment line. This sequence of energy delivery is repeated multiple times. During the use of the probe, the focused ultrasonic energy is selectively absorbed and produces thermally induced zones in the geometrical focus region of the beam. An average of 110 exposure lines is positioned on the neck and face of each individual by employing the focused ultrasound procedure. Due to the variation in face size, the total number of lines is also adjusted to provide uniform spacing and density. Normally, it would take 15 to 25min for a complete face and neck treatment. The ultrasound gel is wiped out and patients are prompted that slight swelling and redness might persist for some days after this procedure.

In these procedures, epidermal damage is reduced, and thermal energy is focused on the subcutis and reticular dermis, where it is believed that immediate contraction of tissues and delayed remodeling together produces tightening of the skin. Face-lifting continues to give a higher degree of development in comparison to non-ablative skin tightening, and it can be argued that the advantage is long-lasting. Some non-ablative tightening receivers attain slight or no visible skin tightening at all, and there is no set of criteria targeting to what can be utilized to predict and prevent such poor outcomes. In addition, there is still a demand for accurate and precise three-dimensional imaging devices that can reliably evaluate slight variations in the volume and skin tightening on the average features of the face. When these are accessible and authorized, it may be easier to record significant and visible tightness after non-ablative therapy. Given the limitations and promises of non-ablative skin tightening, initial improvements are required to maintain the procedure's tolerability and safety while improving its persistency and efficiency[77,80].

Ultrasound energy exhibits features to tighten the skin. First, it is generally believed that ultrasound energy is considerably effective in inducing skin tightening due to the energy supply to the superficial musculoaponeurotic system and the deeper subcutaneous layers of the face[81-85]. In addition, as this administration can be dissociated from secondary scattering and absorption into the dermis and epidermis, the possibility of accidental skin damage can be minimized. Besides ionizing radiation, ultrasound is the only form of inducible energy that can be transmitted arbitrarily and selectively into the tissues. High-intensity ultrasound functions as follows: an ultrasonic field enables the tissues to vibrate and creates friction between the molecules, leading to the absorption of mechanical energy and secondary heat production. Overall, selective coagulative variation in the focal area of the ultrasound field is affected, but other distal and proximal tissues to the focal area remain intact[84-90].

In a prospective group study at a dermatological clinic to evaluate the efficacy and safety of this therapy, patients were treated with ultrasound gel as a coupling medium followed by topical anesthetic and then treated with an intense and focused ultrasound tightening device to the cheeks, side of the neck forehead, and submental region. Standardized pictures of front and side views were attained on different days. An average elevation of brow height of 1.7 to 1.9mm was produced by a single treatment of ultrasound of the forehead. Side effects were restricted to temporary swelling and redness, which are common in all light, laser, and other energy therapies. Most patients responded, usually with temporary mild edema and erythema. The visible signs of aging of facial skin comprise not only superficial abnormalities like fine lines and spotted red-brown dyschromia, but also coarser textural changes, such as wrinkles of the face and sagging. This study had some limitations because it was the first study of skin tightening by ultrasound on live patients, so modest treatment parameters were used. More enhanced exposure parameters (for example, more passes, various focal depths, and higher energy densities) may have led to a better tightening of tissues. In future work, high-intensity and more focused ultrasound probes could be utilized to the deeper tissue to get a high-resolution image and better tightening efficiency, which would provide improved intraoperative visualization of the layers of facial tissues and will also facilitate the precise treatment[77,80,91]. A schematic of how ultrasound travel from transducer to skin tissue is shown in Figure 4.

Figure 4. Diagram of the ultrasound device applied to the skin. A focused, I-shaped ultrasonic beam is emitted by the probe and enters the dermis. Thermal coagulative zones are produced in the dermis and subcutis.

2.4 Cardiovascular Research

Ultrasound has been widely utilized in the cardiovascular study of both mouse embryos and adults[92-94]. MRI gives three-dimensional and high-resolution images of the mouse heart the moment it is born[95], and this technique has been utilized in the fundamental extrapolation of function and mass of the left ventricle in newborn mice[96]. However, the maintenance of physiological feasibility for comparatively longer scan times from 35min to 3h, the high costs associated with this technique, and the availability of an appropriate scanner, are the major challenges that restrict its regular use for newborn characterization. Cardiac CT has been used in newborn mice for the detection of congenital cardiac abnormalities[97], but they need to inject contrast agents and its comparatively low resolution has restricted its use mainly to adult animals[98]. To overcome all these limitations, ultrasound is used in cardiovascular research[93,94]. The strengths of this technique are its rapidity (scan time is 10-15min per animal), high resolution (50mm), and relatively low cost[99]. Despite this, its low spatiotemporal resolution in newborn mice, due to their small size and elevated heart rate, has previously limited its use to Doppler for normal cardiac growth and 1-D measurements during regeneration after injury[100-106].

High-frame-rate electrocardiogram-gated kilohertz visualization (EKV) imaging generates a typical single heart cycle after scanning[94]. In summary, EKV ultrasound imaging can collect in vivo information about contemporary functional and structural maturation in neonatal mice. This allows the detection of heart growth in a non-invasive way in post-natal and young mice that is associated with in vitro observations, along with giving a way to observe the functional variations related to cardiac damage and regeneration. Thus, high-resolution EKV ultrasound imaging provides a convenient 2-D in vivo tool to investigate the effect of genetic or other changes on newborn heart development and regeneration processes after an injury[107].

2.5 Ultrasound Imaging

Even though ultrasound was discovered 12 years earlier than the X-ray (1883), but its application in the medical field was found late. The first practical use of ultrasonic technology was recorded in the First World War in submarine detection. The use of ultrasound in medicine was initiated in the 1950s[108]. In the 1930s, ultrasound therapies were utilized for therapeutic purposes such as physiotherapy, cancer medications, pregnancy, and other diseases. Diagnostic applications of ultrasound started at the end of the 1940s as a result of the collaboration between doctors and engineers from the SONAR experiment[109]. Physiotherapists started using ultrasound technology in the 1940s. Ultrasound waves have been utilized for medical diagnosis in human body imaging for more than half a century. Recently, ultrasound transmission gel has become widely utilized as a conductive medium in numerous diagnostic processes, such as endoscopy, ultrasonography, electrocardiography, and transesophageal echocardiogram examinations[29,110].

Diagnostic sonography or ultrasonography (ultrasound imaging) is a diagnostic ultrasound-based imaging technique that is used to visualize the structures of the subcutaneous body, including vessels, joints, muscles, tendons, and other inner organs for possible injury or pathology. Sonography images the soft tissues of the body in a very effective way. It is a widely used technique in hospitals and health care centers provided its ease of use, absence of ionizing radiation, portability, real-time diagnostic ability, and wide range of applications[1,18,111-113] by utilizing ultrasound waves (sound waves of a frequency that goes beyond the boundary of audible human hearing, i.e., 20kHz).

When utilized for imaging purposes, ultrasound waves are produced from a probe or transducer by the piezoelectric phenomenon (the contraction and expansion of a crystal, producing vibrations in the crystal when a voltage is applied across it). These high-frequency beams or vibrations are transferred into the tissue that comes in touch with the probe. Similarly, applying pressure to the crystal (in the form of waves reflected from tissue layers) produces a voltage that can be recorded. Therefore, the crystal serves as both a sound receiver and a transmitter. Ultrasound is somewhat absorbed by tissues on its passage, both on its route away from the probe and on its way back. The position and nature of each border between organs and tissues may be derived from the duration between when the initial signal is transmitted and when the reflections from different media boundaries are received (along with a measure of the signal’s intensity loss)[111]. As technology continues to develop, the machines become lighter with improved characteristics, longer battery life, and better image quality. After the primary investment in the ultrasound machine, it is inexpensive to operate, requiring only a power supply, an expert operator, and ultrasound gel. A gel (or other coupling media) is required to increase the conduction of sound waves from the ultrasonic transducer to the tissues of the patient[18]. An ultrasound image of a fetus is shown in Figure 5.

Figure 5. An ultrasound image of a fetus[114].

Water, alcohol, and oil could be used as ultrasound mediums to take images, but these have some drawbacks that water and alcohol are volatile and difficult to handle due to low viscosity[1,115]. Oil has many disadvantages, as it could dissolve the rubber or plastic parts of the probe and damage the transducer. Moreover, it is very messy and stains clothes[20,116]. Honey, petroleum jelly, and marmalade as commercial gel alternatives were also not perfect coupling media[117]. Cream-based agents are also less effective in transmitting ultrasound energy than aqueous gels[56,57]. The best coupling medium is a water-soluble gel, and that can be commercially available or homemade[118]. Commercial standard ultrasound gel is sterile, non-toxic, hypoallergenic, water-soluble, density=0.983g/cm3, pH=6.5±0.50, and high viscosity, with a very clear screen, etc.[119]. But the cost and availability of commercial ultrasound gel are its limitations[20].

The WHO Diagnostic Ultrasound Manual contains an ultrasound gel recipe comprising propylene glycol, carbomer, trolamine, and EDTA, but these materials are not easy to obtain[29]. Ultrasound gel with desired properties could be manufactured by following ingredients, as shown in Figure 6, and drugs put in the ultrasound gel should not increase the density or block the intensity of ultrasound waves[120].

Figure 6. Composition of an ultrasound gel.

3.1 Vehicle

It serves as a carrier of the components of the gel and partially or completely solubilizes the gel components[7]. Due to the need to match AI to the soft tissues for ultrasound gel, it was found to be the major portion of the gel, and mostly it was water, as its acoustic characteristics are very similar to biological soft tissues, and it is also referred to as a universal solvent. Pure and distilled water were utilized to formulate the ultrasound gel in different studies. The water for ultrasound gel purposes should be very pure and free from any contaminants[11,52]. Water supports the diffusion, integrity, and solubility of compounds in hydrogels, which is significant for environmental, biotechnological, pharmaceutical, and biomedical applications[121].

3.2 Thickening Agents

Gels are semisolid and transparent formulations having a high ratio of solvent/thickening agent or gelling agent[122]. These are used to give sufficient viscosity to the gel. They are also called thixotropic agents. A variety of polymers were used to produce the network of gel. Examples of suitable thickening agents include polymers or any water-absorbing materials such as polysaccharides or polyacrylic acid polymers[7].

3.3 Polysaccharides

Polysaccharides are naturally occurring polymers that are biodegradable, hydrophilic, biocompatible, and also behave as gelling agents[123]. Because of their low disposal costs and great biocompatibility, these materials are currently attracting extensive attention as biomedical materials[124]. Moreover, the diversity in their chemical arrangements enables the production of innovative functionalized compounds that can suit a wide range of applications. Natural polymers are good prospects for a variety of biomedical applications, including regenerative medicine and drug delivery, because they degrade into physiological intermediates. Polysaccharides, in particular, demonstrate outstanding properties such as non-toxicity, high water solubility, and swelling capacity, which are generated by simple chemical variations, as well as a wide range of chemical configurations[125,126].

Suitable polysaccharides for ultrasound formulation include different types of starches such as cornstarch, rice starch, and other polysaccharides like xanthan gum, guar gum, and cassava flour. Starch is the most abundant and stored form of carbohydrate in plants. It is a very precious ingredient because it is extensively used as a thickening, gelling, and water-holding agent. Chemical modifications such as substitution and cross-linking are often used to modify the gelling properties of starches. These modifications enhance the resistance to elevated temperatures, acid, and shear, increase freeze-thaw stability and reduce the retrogradation[127,128].

3.4 Cornstarch

In a blinded, randomized, cross-over study water and corn starch were used to make an alternative ultrasound gel. Both ingredients were easily available and inexpensive. This study was conducted to compare the homemade cornstarch-based gel with commercially available ultrasound gel. The cornstarch gel was made by mixing a portion of cornstarch with 10 parts of water at moderate heat with continuous stirring for 3-5min, which took approximately 10min. The images obtained by applying the cornstarch-based gel were of the same quality as those produced with the commercial gel. The commercial gel used was Medline Gel from Medline Industries, USA. In low-resource backgrounds, the cornstarch-based gel is a desirable coupling medium for utilization in ultrasound applications due to its ease of manufacturing and low cost.

This study also had some limitations, for instance, it did not answer the questions regarding ease of use and patient or sonographer’s satisfaction with use. The shelf life and sterility of the cornstarch gel were also unknown because the gel was prepared before 24h of its application. Therefore, the use of this gel on skin surfaces with open wounds is not recommended. Moreover, the gel began to separate after 24-48h of its preparation, which makes its use more difficult. Finally, the image quality of the ultrasound video clips produced by utilizing commercial ultrasound gel and cornstarch gel was examined by this study, but it failed to assess the diagnostic accuracy of the scans[18].

In low-resource settings, the inexpensive gel prepared from abundant materials could be a suitable replacement for commercial gel. Water and cornstarch (recognized as cornflour in the UK) are abundant and very inexpensive materials in various regions of the world. In one study, the corn starch gel was prepared by heating water (5cups) and cornstarch (½cup) together. The heating of water and corn starch could eliminate some pathogens; otherwise, none of these has intrinsic antiseptic properties. Therefore, after preparation, gel should be stored properly in reused and sterilized ultrasound gel bottles to avoid any contamination. The gel was used by the researchers to take standard ultrasound images of the heart, bladder, kidneys, gallbladder, and neck vessels. The cornstarch gel was better than commercial gel in terms of three image parameters, such as quality, detail, and resolution. The commercial gel consumed was Medline Gel.

There were certain limitations of this study as well. Firstly, images were obtained for this study from two healthy individuals with no identified or expected significant diseases. It was noticed that the gel was the best when it had cooled, and it was recommended to use it in a few days. Afterward, it started to separate slightly, though it was still usable. In the meantime, factors containing shelf life and storage, along with contamination and sterility considerations, should be studied further[1].

In another study, the gel was made by heating cornstarch (50g) in distilled water (500mL). The prepared cornstarch gel with the addition of aloe vera gel was utilized for physiotherapy application to achieve quick pain relief in the affected region[55]. Anti-inflammatory, antioxidant, and wound healing effects are the several central biological activities of this plant. Previous research has shown that aloe vera is an effective natural pain reliever[129] in milky white color and creamy form. The results indicated that the prepared ultrasound gel and its constituents were of constant quality and could be readily employed for ultrasound physiotherapy treatment. The prepared ultrasound gel was proved to be superior to the gel available on the market considering the pain management score. On the other hand, more studies are required to approve its efficiency and to establish commercial standards.

Some improvements are needed to enhance the hom*ogeneity of the gel. There was some clumping of starch that was observed during application on skin and confirmed by touching. Yet, the gel was not greasy upon application; the after-feel effect was good and could be easily removed by washing with water. In conclusion, several participants reported a comfortable, warm, or cool-burning sensation on their upper or lower limbs for several hours after they had left the physiotherapy lab. This indicates a potential benefit of sustained pain relief after therapeutic ultrasound treatments with the formulated gel. It was also recommended that the prepared ultrasound gel could be utilized during an ultrasound scan to obtain medical images. In the future, the formulation will be further developed into commercial standards and tested with a large number of volunteers to confirm its potential analgesic effect[55].

3.5 Rice Starch

The rice-based gel was meant to be a coupling agent in ultrasonic applications. It was prepared from a mixture of medical-grade rice starch powder, distilled water, sucrose, glycerin, carboxymethyl cellulose, methylparaben, and sodium hydroxide. In this research, 100 patients were tested for the efficiency and safety of the products by four physicians. Compared to standard ultrasonic gel, the imaging results revealed that rice gel has improved echogenicity and produced equal clarity. Thus, rice starch-based ultrasonic gel could be an option for general hospitals[8].

In another study, ultrasound gels were made with various concentrations of rice starch (RS) powder by an aqueous solution method. The formulations of gels were composed of rice starch powder, carboxyl methylcellulose (as an emulsifier for enhancing viscosity and stability), distilled water (plasticizer), sucrose (additive for binder), liquid glycerol (it mixes and binds with the RS molecules to enhance water-soluble property), methylparaben (a preservative) and NaOH (neutralizer)[119]. The phenomenon could be described as the effect of RS powders in the compositions that appear from the gelatinization of RS molecules through thermal and basic treatment. When RS powder is heated in the presence of water, the molecules of RS are modified by the destruction of its granules, and it goes through an irreversible structure. Internal binding of each granule happened due to the swelling pattern of RS powder. Gelatinization with a disorder of molecular hydrogen bonds optimizes the results in the generation of a viscous paste. This bonding was obtained from amorphous and crystalline rice starch, which had poor swelling and solubility properties. Therefore, the viscosity and transparency of rice gel are obtained by the gelatinization process[119,130].

The results revealed that the use of 2g of RS powder in the composition gives an optimized formulation with easy cleaning, pleasant odor, high viscosity, and non-irritating properties. The findings of this study led to the conclusion that the use of RS gel in ultrasound examinations is effective and safe for commercial ultrasound gel replacement. However, this gel needs further investigation on image quality during the ultrasound process[119,131,132].

3.6 Cassava Flour

Cassava root flour is inexpensive and easily available in many low-resource areas such as Asia, Latin America, the Caribbean, and Africa. When cassava root flour was transformed into a slurry with overheated water, it was just like a commercial ultrasound gel in consistency. This natural formulation has comparatively high amylopectin content, providing good stability of gel, low tendency to retrogradation, cohesive texture, and high clarity of pastes. It also has a low-temperature range of gelatinization (65-70°C) and a rapid increase in the viscosity during the gelatinization process, indicating that the CFS could be produced rapidly without requiring a high temperature[133]. The use of cassava flour slurry in gynecological and obstetric applications has been reported in low-resource countries[134].

The use of cassava root flour (commonly available in Western Africa and the Great Lakes Region) and bulla (the most common food staple used in Ethiopia) to formulate a prototype gel has been reported. The gel was made by heating cassava and bula (8 parts), water (32 parts), and salt (4 parts), yet the skin irritancy, microbial growth, and image quality were not evaluated with complete scientific accuracy. However, some primary tests indicate that the image quality of this gel is the same as the current commercially available gel. This alternative gel was proposed as a reasonable solution to chronic issues of resources observed in-field because both bulla in Ethiopia and cassava root flour in the Sun-Saharan Africa’s Great Lakes region were abundantly available at an extremely low cost. So, they are considered attractive and cheap replacements for commercial gel[115].

Aziz et al.[2] have shown a low-cost alternative to commercial ultrasound gel that could be prepared by using generally available ingredients according to a simple method without complicated supply chain management, with relative stability and sterility and the absence of unpleasant consequences. This is a comfortable relief for hospitals and suppliers, as they fight with the availability and cost of ultrasound gel for routine radiological processes. Cassava flour slurry was produced by quick boiling of water, salt, and cassava flour, which are readily available and inexpensive. It was inexpensive and simple to prepare and produce ultrasound images of similar quality in comparison to less available and more expensive commercial gel. In low-resource settings, CFS particularly benefit obstetrics and gynecological treatments. The slurry is also a model of an economical invention, where a product is manufactured at a low cost and also maintains the main features with reliable quality.

Results showed that the image quality produced by CFS was similar to ultrasound gel currently available on the market. Furthermore, all sonographers observed that the use of CFS was easy and all participants stated that CFS was comfortable and acceptable, except that itching was indicated by only one patient. The CFS also remained sterile for a minimum of five days after preparation. This study also had some limitations in that all images were obtained with high-resolution and non-portable machines. Furthermore, all of the women were undergoing a regular obstetric scan that did not require complex anatomic images, and application of this product in more invasive procedures, for instances endovagin*l ultrasounds was not tested. Finally, the viscosity of each batch was not determined, so the effect of viscosity on image quality was not assessed[2].

3.7 Xanthan Gum and Guar Gum

Xanthan gum, a biotechnologically manufactured polysaccharide, was also used as a thickening agent and increased the gliding characteristics of the ultrasound gel. Ultrasound gel made from xanthan gum scored the best as it had pleasant soothing and moisturizing effects on the skin. The 8 non-commercial gels were made one day before the scanning process and stored at ambient temperature and tested against commercial gel (Aquasonic 100, Parker Industries). Before the implementation of the study, only one research published by Binkowski et al. 2014 regarding a non-commercial gel was reported. Cold glucomannan and hot concentrated glucomannan received the lowest scores and were observed to be worse than commercial ultrasound gel for specific characteristics such as model comfort, consistency, sliding, and real-time image quality. The qualitative observations were most satisfactory for half‐strength commercial gel, hot low‐concentration glucomannan gel, and xanthine gum gel. Hot low‐concentration glucomannan obtained the best results for consistency and sliding properties. All of the formulations were readily removed from the probe except for the cold glucomannan, which was much harder to remove[117].

In another study, a variety of homemade gels were produced and analyzed to ensure the quality of the images. Guar gum (a galactomannan polysaccharide) and Glucomannan-based gel had better physical properties during preliminary testing and did not show any significant difference in phantom imaging and analysis in comparison to commercial ultrasound gel available in the market. Neither the gel needed heating, damaged ultrasonic probe, attracted insects, stained clothing samples, nor caused adverse effects on the subjects. For the preparation of the gel, 1.5 teaspoons of glucomannan and one teaspoon of guar gum were mixed with different combinations of alcohol (91% isopropyl) and water. The gels were directly prepared in the bottle to be utilized during the process of scanning by stirring the mixture at ambient temperature. A small quantity of alcohol as a bacteriostatic agent was also added to the mixture. Glucomannan powder took approximately five minutes to completely thicken, while guar gum powder thickened into a gel instantly. The alcohol was reported to thin the gel, but the high concentrations of alcohol (5% to 10%) inhibited the gel formation. In both gels containing 0% alcohol, the growth of mold was observed after eight days of its preparation. While using these homemade gels, it is recommended to use the gel within 24h to prevent the growth of bacteria, transducer damage, and ingredient separation. The results showed that alcohol-free glucomannan gel was too thick. Therefore, sonographers could tune the thickness according to their requirements. These findings encourage the utilization of glucomannan or guar gem gel as a satisfactory agent if the ultrasound gel on the market is unavailable[20].

3.8 Poly Acrylic Acid

A polyacrylic acid-based gel was made by combining certain accessible chemicals with natural ingredients that do not harm the patient’s skin. Chemicals that were utilized include polyacrylic acid, tri-ethanolamine, food-grade color, glycerine, sodium benzoate, and the natural ingredient utilized was aloe vera extract, extracted through ethanol extraction. Aloe vera extract, due to its synergistic characteristics, displays strong activity against inflammation issues, irritation, and allergies. The prepared gel displayed a pH of 6.8 and confirmed that the formulation was compatible with the secretions of the skin. Bacterial growth did not occur throughout the incubation phase of the prepared ultrasound gel. Furthermore, the stability study showed that the gel was stable. Ultrasound gel generally acts as a conductive medium, and information regarding the quality of images is given by the conductivity results. The prepared gel had a conductivity of 4.05μS/cm. With the application of this ultrasound gel formulation in ultrasonography, a clear and good-quality image was obtained. The overall findings indicate that this formulation could be regarded as effective with good safety profilr on the skin in comparison to the synthetic one [11].

Here, a polyacrylamide (PAA) hydrogel has been reported as a potential acoustic coupling medium for high-intensity focused ultrasound systems. Over the last 30 years, the properties and structure of PAA have been investigated extensively[135,136]. The gel was easily prepared at room temperature and contained very high water content (70% to 95% by weight). The high water content of the gel gives suitable acoustic properties to the gel, such as low speed of sound, attenuation, and similar impedance as water. The physical characteristics of the gel like stiffness and density, could be altered straightforwardly by varying the quantity of the acrylamide. Furthermore, PAA has been employed in various biomedical treatments and its good biocompatibility has been presented in numerous researches[137,138]. This research covers the functional testing and material characterization of polyacrylamide gel as an acoustic coupler. This study concentrated on the particular application of an intraoperative hemostasis device that focuses on the areas of bleeding near tissue surfaces, such as the spleen and liver[24].

The PAA gel, with acrylamide concentrations in the range of 10% to 20%, showed an impedance, attenuation coefficient, and sound speed similar to water. These acoustic properties increased as a linear function of an increase in the concentration of acrylamide. Due to this property, the impedance of the coupler can be tuned to match the impedance of a specific type of tissue [139].

Approximate values of attenuation coefficient at 1MHz, acoustic impedance, and SoS for water and PAA gel at 25°C is given in Table 3.

Table 3. Approximate Values of Attenuation Coefficient at 1MHz, Acoustic Impedance, and SoS for Water and PAA gel at 25°C[24,140].

The values of specific heat capacity and thermal conductivity of PAA were reported to be greater than those of water, and these values did not depend upon the concentration of acrylamide. Such properties are considered significant in the examination of the device's heating effects. Heat is produced at the crystal of the transducer and in the coupling gel throughout the HIFU treatment. High temperatures of the transducer can decrease its performance and, in the long run, cause permanent crystal damage. PAA is a comparatively poor heat conductor, despite its thermal conductivity being approximately twice that of many plastics. Furthermore, it doesn’t facilitate convection as water does. According to preliminary measurements, heat loss from the surface of the transducer was relatively slow for a PAA coupling medium in comparison to a coupler filled with water. During HIFU operations, the cooling of the probe could be a serious problem. The absorption of US energy will cause further heating of the gel. Thankfully, PAA has a larger heat capacity than many plastics and metals. As a result, the gel's temperature will not increase as quickly as it does in other materials. Heating could cause variations in coupling efficiency because the acoustic characteristics of the material were found to be dependent on temperature. The PAA coupler was shown to have medium power transmission efficiency due to the low attenuation and its reduction as the concentration of acrylamide increased. Nevertheless, the loss of output power is mostly due to the attenuation of the gel[24].

About 20 years back, gelatin-based substances having acoustic values close to the soft tissues of some mammals were initially identified[141]. Later then, numerous tissue-mimicking substances for the US were created and investigators have highlighted the importance of these tissue-mimicking materials in ultrasonic bioeffects and biomedical US research, mostly for US imaging purposes[24,141,142]. Although a lot of tissue-mimicking materials have been created for application in US imaging, none has been developed for investigation of the high-pressure and temperature regimes associated with HIFU till Lafon et al.[143] (2001) formed a PAA gel comprising bovine serum slbumin (BSA) that was transparent optically and had similar acoustic and physical characteristics to that of soft tissue.

A polyacrylamide gel comprising egg white was utilized in research as a new product for irradiation studies in HIFU. The model was identical to and had similar advantages as a BSA protein-containing gel, but the egg is inexpensive as compared to BSA. The gel was prepared from polyacrylamide and different concentrations of egg white (10-40%) with some other ingredients by polymerization in 10 to 20min. The acoustic characteristics of the gel were analogous to those of soft tissues.

Preliminary experiments determined that excessive egg white leads to an opaque gel and less egg white leads to poor visual contrast and sensitivity. A novel model of a gastric submucosal tumor was created in this study by injecting a solution of PAA gel including egg white into the stomach of a rabbit. Then it was subjected for 10 seconds to a frequency of 2.19-MHz with 100-Watts of electric power. The stomach mucosa remained unaffected, and the coagulated lesion’s color shift was identifiable readily. Its features include low cost and ability to develop a rigid shape quickly and easily at room temperature, enabling it an appealing choice for single-use acoustic material ideal for HIFU applications. It could be an effective experimental design for HIFU therapy in terms of visualizing the rise in temperature that can cause organic tissues to coagulate[144].

3.9 Carbopol Polymer

Carbomers are acrylic acid synthetic polymers with a large molecular weight that are cross-linked with polyalcohols or alkenyl ethers of sugars. The monomer unit of the carbomer is shown in Figure 7. Based on the level of cross-linking, mode of synthesis, and rheological properties, various carbomer grades existed in the market, such as Carbopol (CAR) 980, CAR 934, CAR 981, and CAR 940[145-147]. Carbopols can be grouped as hom*opolymers or copolymers depending on crosslinking density. Carbopol hom*opolymers are acrylic acid polymers that are crosslinked with allyl pentaerythritol or allyl sucrose. Carbopol 974P NF is a hom*opolymer, which is a crosslinked acrylic acid polymer with allyl pentaerythritol. C10-C30 alkyl acrylate and acrylic acid polymer undergo crosslinking with allyl pentaerythritol to form carbopol copolymers[148].

Figure 7. Chemical structure of a monomeric unit of acrylic acid in a carbomer polymer.

.

When carbopol polymer is subjected to a pH of 4.0 to 6.0, it swells 1000 times more than its initial volume to produce a gel. For that reason, a neutralizing reagent is used to jellify CAR molecules in various liquids or their mixtures with water[149,150]. It is generally used as a stabilizer and thickener in a wide range of cosmetics, pharmaceutical, and personal care products because of its high efficacy, ease of handling, and very good transparency[52,151]. Carbomer polymeric gels are often used to make lotions, creams, gels, as well as drug carriers in a variety of treatment modalities[147,149]. For example, carbopol 934 has been used as a thickening agent in wound healing gel made up of spidroin protein (the main protein component of spider silk)[152]. The use of carbopol 934[14,52] and carbopol 980 in ultrasound gel formulation has been reported[153]. Carbopol 940 caused some allergic reactions to the skin[55].

For gel purposes, a 0.5-1% concentration of carbopol was found to be suitable. When the concentration was greater than 1%, molecules of carbopol became closely packed and behaved like elastic solids. Greater concentrations were also found to be susceptible to air entrapment. Moreover, the solution of carbopol in water shows high viscosity at neutral pH as it completely swells at neutral pH. The inverse effect of temperature was observed on the viscosity of carbopol solution[151,153].

In one study, different formulas with different carbopol 934 concentrations were prepared with other ingredients (propylene glycol, methylparaben, and triethanolamine) and evaluated for different physicochemical characteristics. A comparison analysis was also conducted by using a commercial ultrasonic gel as a reference. One formulation with a 0.3% polymer concentration was deemed the best of all the developed formulations because of its great transparency, free from allergenic chemicals, low cost, excellent uniformity, and pH of 6.8 which is close to the pH of the skin. In all evaluations, this formulation was quite close to CG except for the viscosity, which was larger than the viscosity of CG, but the low concentration of CAR 934 could be used to match the viscosity with CG. Microbiological tests revealed that the addition of methylparaben was enough to inhibit the germs from growing in the gel. Skin irritation assessments conducted on both humans and animals revealed no detrimental impact on the patient’s skin. There was no substantial distinction between prepared formula and CG, according to three veterinary radiologists. Ultimately, the findings indicate that the best-chosen formulation was prepared by a simple method and could be utilized as an inexpensive and good substitute for the commercially available imported ultrasound gel[52].

Myasar et al.[14], 2019 also prepared the ultrasound gel with a different method using the same ingredients as Al-Nima et al., 2019 used with the addition of propylparaben. The prepared gel showed the following characteristics: pH6.7, excellent viscosity, no irritation (redness or edema), good spreadability, ease of removal, and excellent stability after 26 weeks of storing at ambient temperature. A new ultrasound gel formulation was evaluated in this study by employing ultrasonography and evaluating the images received from two gels: the newly produced gel and the commercially available gel. The gel was utilized for imaging various structures such as gall bladder and liver, urinary bladder and kidney, uterus and fetus, pancreas and spleen, thyroid gland, joints, and other body organs. The images obtained by applying this gel were of excellent quality, with good detailing and resolution. There was no major distinction between the commercial gel and the prepared formulation, according to the results. In some areas, the selected formulation performed better than CG, whereas, in other situations, the CG appeared to be superior. However, in both situations, the difference was not significant statistically. In conclusion, it was recommended that the manufactured gel could be utilized as a replacement for the expensive and imported gel. The versatility of the investigated organs and the huge number of cases lead to robust results of this study[14].

3.10 Skin Conditioning Agents

It performs many functions in gel composition. It provides a barrier to water evaporation, imparts soothing properties to gel composition when applied to the skin, restores moisture to the skin, and provides lubricity to the skin to help the movement of the transducer on the skin, hold water and make it available to the skin. It also increases the water content of the skin, which makes the skin more pliable and softer.

Occlusives and humectants are two types of conventional skin-conditioning agents that are commonly used in gel composition. Occlusive are the skin-conditioning agents which create a layer over the skin that decreases the evaporation rate. Humectants are hygroscopic and nonocclusive materials that hold water and make it accessible to the skin. It may also work by increasing the skin’s lubricity.

Suitable skin-conditioning agents include glycerin, long-chain fatty acids (C12-C22), propylene glycol, polyethylene glycol, etc.[7]. The solvents like propylene glycol, polyethylene glycol, glycerin, and glycerol could change the hydrogen bond properties between the solvent, polymer, and water, thus disturbing polymer swelling[153,154]. Glycerin also served as a viscosity protector when ultrasound gel was exposed to gamma radiation for sterilization[148]. Propylene glycol (1,2-propanediol) is commonly employed in various pharmaceutical preparations as a humectant, solvent or cosolvent, and preservative. It is usually considered to be a noncorrosive and comparatively nontoxic substance. It is also utilized widely in cosmetics and foods[145,155].

3.11 Anti-inflammatory Agent

Anti-inflammatory or analgesic agents could be introduced to ultrasound gel to increase the efficiency of treatment[156,157]. According to the US procedure, ultrasound waves caused transitory changes to the skin, so that drug ingredients could be taken by the skin through absorption, pointing to an inflammatory zone, and relieving the muscular pain.

Aloe vera has anti-inflammatory, antimicrobial, antioxidant, antifungal, and anti-aging characteristics and also promotes the healing of wounds[158]. Additionally, it prevents the formation of stretch marks[159], and has synergistic activity due to its 75 active ingredients[11]. Ultrasound gels for the ultrasound procedure have been prepared using fresh aloe vera gel and an ethanol extract of aloe vera gel. It didn’t irritate the skin even for people with sensitive skin[11,55]. It was studied that formulation of ultrasound gel with 10% and 20% aloe vera gel and other ingredients showed good pain relief effect in physiotherapy treatment as compared to menthol or methyl salicylate. Menthol or methyl salicylate or their combination is used to produce a cool-burning sensation in physiotherapy treatment[55]. Most of the topical analgesic preparation contains menthol, producing a cool sensation, and methyl salicylate, producing a hot sensation. Hill and Sumida 2002, reported that warm and cool sensations would decline the pain felt by an individual[160]. Aloe vera gel is commonly used as a soothing agent in most skincare products due to its well-known cooling effect. Specific herbs that contain essential fatty acids for example alpha-linolenic acid and linolenic acid could also use as an anti-inflammatory agent[7].

Plai oil obtained from Zingiber cassumunar Roxb. rhizome comprised of an active component, (E)-1-(3,4-dimethoxy phenyl) butadiene (DMPBD) which had been shown to have anti-inflammatory properties[161]. Ultrasound therapy was used to test the combination of diclofenac emulgel as an anti-inflammatory drug. Some herbal drugs, for example, plai emulgel, could be used instead of diclofenac emulsion in ultrasound treatment to decrease the cost of treatment for patients[162]. In this study, plai emulsion gel was prepared with the Zingiber cassumunar’s oil that could be utilized as an alternate medication in combination with ultrasound physiotherapy to treat chronic or acute muscular inflammation. Different types and amounts of gel-forming reagents, such as CAR 121, CAR 934, CAR 940, hydroxypropyl cellulose, methylcellulose, and hydroxypropylmethylcellulose, were combined with other ingredients to prepare 18 plai emulgel formulations. The best formulation that was suitable for use with ultrasound transfer through a gel was prepared with 1% carbopol 934 (gel-forming agent), 20% propylene glycol (solvent), 5% plai oil, 0.2% EDTA (chelating agent), 0.2% paraben concentrate (preservative) and triethanolamine (pH adjusting agent). This formulation results in good spreadability, smooth texture with good appearances. By calculating the level of the marker component, DMPBD, the stability of this formulation was also investigated. After 180 days, the remaining percentage of the (DMPBD) in the formulation, stored at 15°C, was higher than 95%, whereas it was less than 80% in the formulation that was stored at 30°C. As a result, to preserve the active component, DMPBD, this plai emulgel formulation should be stored at 15°C[163].

3.12 Chelating Agent

The chelating agent commonly sequesters any metal ions that may be present in the gel composition. For example, it may be naturally present in minor amounts in the vehicle or introduced via other components upon the formation of the gel composition to improve its stability in air.The use of a chelating agent might help the preservatives to control the growth of bacteria by binding with metal ions, which help bacteria to grow their cell walls, e.g., Cu ions, and prevent the deterioration of the product. Suitable chelating agents include salts of alkali or/and alkaline earth metals, for example, ethylene diamine tetraacetic acid and/or nitrilotriacetate. Specific species include sodium EDTA, disodium EDTA, tetrasodium EDTA, monosodium nitrilotriacetate, disodium nitrolotriacetate, etc. Combinations of different chelating agents may be utilized together. It was found to be present in the gel composition in a catalytic amount and mostly it was disodium EDTA or EDTA (structure is shown in Figure 8)[7,164].

Figure 8. Structure of EDTA (left) and disodium EDTA (right).

3.13 Preservatives

The major portion of the gel is aqueous that is more vulnerable to contamination by bacteria and fungus. Preservatives are used to prevent the growth of bacteria, yeast, and mold. Parabens (esters of p-hydroxybenzoic acid) were found to be mostly utilized as preservatives in many pharmaceutical, personal care, and cosmetic items. Parabens are stable and resistant to hydrolysis.

It was studied that parabens were more resistant to the growth of molds and yeast. The antibacterial property was observed to increase with the increase in the alkyl group, but solubility decreased. So, the use of a combination of preservatives was found to be better than using them alone[165,166]. The use of methylparaben alone and its combination with propylparaben had been reported in different studies (structure is shown in Figure 9)[14,52,119].

Figure 9. Structure of methylparaben (left) and propylparaben (right).

The use of silver nanoparticles (AgNP) as a bactericidal agent in clinical ultrasound gel has been reported, and it helps to prevent infections from contaminated ultrasound gel. The findings demonstrated that AgNP had strong bactericidal action on Escherichia coli and Staphylococcus aureus, and also weak antibacterial properties towards gram-positive bacteria, with a dose-dependent impact when compared with gram-negative bacteria. Hence, developing a bactericidal medical ultrasonic gel for protection against cross infections[167].

Unluckily, certain chemicals were found to be used for the formulation of ultrasound gel that caused allergic reactions to the skin, such as isothiazolinones, methylisothiazolinones, and phenoxyethanol. These chemicals were utilized as preservatives in several cosmetics and industrial products[1,168]. Furthermore, women, during pregnancy, might also develop sudden allergic reactions to these chemicals, which presumably due to the hormonal changes they go through during this period[11].

3.14 Neutralizer

Neutralizers were used to neutralize the acidic components of the gel, for example, acrylic acid polymers, or they might be utilized for modifying the pH of the gel. It affected the gel's viscosity as well[7]. The selection of a neutralizer is very important as the wrong neutralizer could make the carbopol precipitate out. For carbopol polymer in the hydrogel system, 99% triethanolamine, 18% NaOH, and 18% KOH were used. For hydroalcoholic systems, concentrations of neutralizer might vary[151].

In topical gel compositions, pH is the most vital factor because of the three zones: the impact of pH on skin, stability, and solubility. Any ultrasonic gel composition should have a pH that does not irritate the patient and ensure the formulation's stability. For that purpose, ultrasound gel composition should possess a pH between 5 and 7.4[52].

It is known that hydrogels are commonly utilized as coupling media for ultrasound therapy, imaging, and other ultrasonic-processing devices. A basic component of an ultrasonic gel is to possess acoustic impedance comparable to that of soft tissues. Certain ultrasonic treatments and diagnostic imaging are performed by inserting an ultrasound probe into a body cavity (intracavitary ultrasound procedures) or otherwise internally into a patient. For example, ultrasound dental treatment or intracavitary imaging processes require the application of the gel intra-orally to the patient, which can result in small amounts of gel being ingested. However, there is currently no known ultrasound gel product that has been approved and labeled, especially for intra-oral usage.

In light of this, there remains a need to provide orally and internally compatible ultrasonic gels. For that purpose, the gel should have the following properties: it should be safe for open wounds, long-term repeated ingestion, and excretable by natural pathways; it should also be tasteless, odorless, and slightly unpleasing. The lack of taste or flavor in the mouth reduces the production of saliva, which helps to limit the quantity of gel being wiped away by saliva and possibly swallowed. An increase in the production of saliva occurred due to the mild flavor or fragrance in the mouth, which is unwanted. Furthermore, the lack of taste allows the consumer to bear the gel in their oral cavity. The components of the gel should be based on the FDA’s or the FDA’s Generally Recognized as Safe category of authorized materials. Among the carbomers that are safe for oral or intracavitary use, carbomer 974P has the highest viscosity for any given amount added to water. It means that it requires the least amount of carbomer when using carbomer 974P to achieve the same level of viscosity, which further contributes to safety. If the gel is taken orally, a dental agent could be added to give further dental benefits to the user. Sugar alcohol such as xylitol can be used as a dental agent. The dental agent provides an extra therapeutic benefit to an individual and also prevents respiratory infections and oral bacteria.

The gel will be safe whether a patient ingests it intentionally or unintentionally since the components would be approved for ingestion and mucous membrane application. The combination of preservative (e.g., potassium sorbate) and sterilization provides extra protection for utilization in oral or other internal applications. Thermal treatment such as autoclaving or some other sterilization techniques could be used to sterilize the gel, for example, gamma irradiation or e-beam. To achieve sterility while maintaining the desired viscosity range, the gel formulation may require additional compounds such as viscosity stabilizing agents or viscosity protection agents for protection from radiation-induced breakdown.

These stabilizing agents include propylene glycol, glycerine (glycerol), or CNC. Glycerol is commonly utilized in personal care products, beverages, and foods because it has less toxic effects when swallowed. Propylene glycol has also very low oral toxicity. Glycerol and propylene glycol also do not damage ultrasound transducers. Different approaches could be used to attain a gel of a specific viscosity. Glycerine, for example, could be added to an initial gel formulation to protect the gel during gamma radiation sterilization. The small amount of glycerol (5%-10%) could be used to enhance gel resistance to break down under larger doses of gamma radiation (40kGy) without significantly sweetening the gel and affecting the acoustic impedance of the gel, which is necessary for ultrasonic wave transmission to reduce the ultrasound reflections at the tissue-gel-transducer boundary. Furthermore, increasing the concentration of the polymer (for example, Carbopol™) in the gel to enhance its viscosity cannot create a safe internal and ingestible ultrasound gel as the higher amount of Carbopol would be potentially ingested by a patient during use, leading to safety concerns.

Another viscosity stabilizing agent is CNC. The term "CNC" refers to Cellulose Nanocrystals or Crystalline Nanocellulose and is also known as Nanocrystalline Cellulose (NCC). CNC has cross-linkage properties and disperses in water. The use of CNC as a viscosity stabilizing agent provides two new and unexpected behaviors. Firstly, the addition of small amounts of CNC can maintain the high viscosity of a carbomer-based ultrasound gel after undergoing gamma radiation sterilization and reduce the concentration of carbomer polymer used in the gel. Secondly, gels made of only CNC (no carbomer) and water increased the gel viscosity after exposure to gamma radiation. However, adding CNC does not appear to change the AI and only protects the gel viscosity. CNC accelerates the effective crosslink density and synthesis of hydrogels. CNC is a strengthening agent for the hydrogel as well as a multifunctional crosslinker for gel formation. Both glycerin and CNC could be added together to an initial formulation for better protection from breakdown due to irradiation[148].

Table 4 shows the different thickening agents, preservatives and their results reported in different researches.

Table 4. Different Thickening Agents, Preservatives and Their Results Reported In Different Researches

The authors acknowledged and were thankful to the faculty and staff of “University of Agriculture, Faisalabad” “National Textile University, Faisalabad, Pakistan” and “Northwestern Polytechnical University, Xi’an, China” for their support and contribution towards the study.

The authors declare that there is no conflict of interest.

Afzal S was responsible for the initial draft writing, design of the article, interpolation of data, data Curation; Zahid M designed the article, and was in charge of project administration; Nimra SS revised the initial draft writing, visualizd the figures; Fatime Z performed the revision of manuscript, and data curation; Shakir HF carried out the conceptualization, writing final manuscript (review & editing); Rehan Z provided resources, co-supervised, and approved the version to be published

AgNP, Silver nanoparticles

AI, Acoustic Impedance

BSA, Bovine serum slbumin

CAR, Carbopol

CFS, Cassava flour slurry

CG, Commercial gel

CNC, Cellulose nanocrystals or crystalline nanocellulose

CT, Computerized tomography

DMPBD, (E)-1-(3,4-dimethoxyphenyl) butadiene

EDTA, Ethylene diamine tetraacetic acid

EKV, High-frame-rate electrocardiogram-gated kilohertz visualization

HIFU, High intensity focused ultrasound

KOH, Potassium hydroxide

MRI, Magnetic resonance imaging

NaOH, Sodium hydroxide

OTC, Over the counter

PAA, Polyacrylamide

RS, Rice starch

SoS, Speed of sound

US, Ultrasound

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