Commercial hydrogel product for drug delivery based on route of administration

Hydrogels are hydrophilic, three-dimensional, cross-linked polymers that absorb significant amounts of biological fluids or water. Hydrogels possess several favorable properties, including flexibility, stimulus-responsiveness, versatility, and structural composition. They can be categorized according to their sources, synthesis route, response to stimulus, and application. Controlling the cross-link density matrix and the hydrogels’ attraction to water while they’re swelling makes it easy to change their porous structure, which makes them ideal for drug delivery. Hydrogel in drug delivery can be achieved by various routes involving injectable, oral, buccal, vaginal, ocular, and transdermal administration routes. The hydrogel market is expected to grow from its 2019 valuation of USD 22.1 billion to USD 31.4 billion by 2027. Commercial hydrogels are helpful for various drug delivery applications, such as transdermal patches with controlled release characteristics, stimuli-responsive hydrogels for oral administration, and localized delivery via parenteral means. Here, we are mainly focused on the commercial hydrogel products used for drug delivery based on the described route of administration.

Hydrogels offer remarkable versatility and are utilized in different subjects owing to their unique structures and compatibility with varying conditions of use (Morteza et al., 2016).Hydrogels show outstanding biomimetic properties because of multifunctional qualities, such as flexibility, softness, nontoxicity, biocompatibility, and biodegradability.These properties enable numerous biomedical, pharmaceutical, and other biomedical applications (Caló and Khutoryanskiy, 2015;Ahmad et al., 2019;Volpi et al., 2022).The particular physical characteristics of hydrogels have generated considerable investigation into their prospective uses in drug delivery applications.These biocompatible hydrogels have very advantageous physical characteristics, such as elasticity, that provide controlled release and long-term protection for the encapsulated entities (Gull et al., 2020).The porous structure of hydrogels can be easily modified by controlling the density of crosslinks within the gel matrix and the hydrogels' affinity for the external aqueous environment during swelling (Hoare and Kohane, 2008).The rapid diffusion of drug molecules into and out of swollen hydrogels, which involves drug entrapment and release, enables the utilization of cross-linked polymer networks in a hydrated or dehydrated state as effective carriers for drug delivery via various routes such as vaginal, ocular, oral, buccal, and parenteral administration (Erol et al., 2019).
The use of hydrogels in drug delivery systems has seen an increase in recent years.The current valuation of the hydrogel market stands at USD 22.1 billion as of 2019, with projected growth to reach USD 31.4Billion by the year 2027.This growth is anticipated at a compound annual growth rate (CAGR) of 6.7% from 2020 to 2027.A rise in the utilization of hydrogel products stands as a significant stimulant for expanding the hydrogel market over the next few years (Cascone and Lamberti, 2020).Commercial hydrogels can be obtained for diverse drug delivery applications, including localized delivery through the parenteral route, stimuliresponsive hydrogels for oral delivery, and controlled release properties for transdermal patches (Shi and Li, 2005;Sharpe et al., 2014a;Zielińska et al., 2022).
Research on hydrogel has been ongoing for many years, leading to the development of products that have found numerous applications in the drug delivery system.Therapeutic drugs are now loaded into polymer-based carriers, and the delivery and release of drug molecules is a topic of great interest in many medical fields (Roointan et al., 2018;Farjadian et al., 2019a;Farjadian et al., 2020;Farzanfar et al., 2021;Ghasemi et al., 2022a;Bahmani et al., 2022).The carriers facilitate the transportation of drugs to the specific target location (Entezar-Almahdi et al., 2021;Ghasemi et al., 2022b).Numerous studies have been published on commercial hydrogels for biomedical applications, specifically wound dressings, contact lenses, and cosmetics (Cascone and Lamberti, 2020).Following our previous reports on pharmaceutical applications of precious advanced materials (Farjadian et al., 2019b;Farjadian et al., 2019c;Ahmadi et al., 2020;Entezar-Almahdi et al., 2020;Hoseini-Ghahfarokhi et al., 2020;Zarkesh et al., 2021;Hejabi et al., 2022), in this paper we will discuss various commercial hydrogel products used for drug delivery, focusing on different routes of administration such as injectable, oral, buccal, ophthalmic, vaginal, and transdermal routes (Figure 1).Each section contains a table with an overview of the products on the market, their manufacturer, and their main ingredients.

Classification of hydrogel products
Hydrogels are cross-linked networks of hydrophilic polymeric chains.Hydrogels absorb about 70%-99% of their weight in water and can be used to formulate semi-solid delivery systems for labile and hydrophilic drugs (Narayanaswamy and Torchilin, 2020).Hydrogels can be classified in multiple ways (Figure 2).Based on their source, hydrogels are categorized as follows: natural hydrogels are biodegradable and biocompatible; synthetic hydrogels are nontoxic and compatible; and hybrid hydrogels, which combine synthetic and natural polymers for use in biomedical applications, are available (Malpure et al., 2018).Based on the synthesis route, hydrogels can be categorized into homopolymers, copolymers, and multipolymers.Hydrogels can exhibit different charge characteristics on the bound groups, leading to their classification as cationic, anionic, ampholytic, or electrically neutral, depending on the ionic charges present (Mahinroosta et al., 2018).Hydrogels, as complex polymeric structures, can display swelling behavior in response to various external stimuli.These stimuli can include alterations in pH, temperature, electromagnetic radiation, ionic strength, and other similar factors (Farjadian et al., 2023).The swelling of a polymeric chain occurs due to the hydration of hydrophilic and polar groups.The polar moieties expand, exposing hydrophobic groups that interact with water molecules.The polymeric network absorbs more water through an osmotic force, leading to infinite dilution (Holback et al., 2011).Hydrogels, according to the type of cross-linking, can be classified into two distinct categories: chemically cross-linked networks, characterized by the presence of permanent junctions, and physical networks, characterized by the formation of temporary junctions through polymer chain entanglements or physical interactions (Malpure et al., 2018).From the initial studies on the pharmaceutical applications of hydrogels, an extensive range of drug delivery systems were designed to prolong therapeutic efficacy and accomplish targeted delivery to particular tissues or organs.Hydrogels are divided into four groups concerning the mechanism controlling the drug release: 1) swelling-controlled, 2) diffusion-controlled, 3) chemically-controlled, and 4) environmentresponsive systems (Mohite and Adhav, 2017).

Drug delivery commercial product
Hydrogels are frequently utilized as drug delivery vehicles.The commercial hydrogel-based drug delivery products are discussed in the following section according to the administration route.

Injectable drug delivery product
There has been a growing focus on injectable hydrogels compared to conventional gels in recent years.This is mainly due to their minimally invasive nature during surgery and ability to change shape in real time (Tan and Marra, 2010).Injectable hydrogels can be implanted through a minimally invasive procedure, significantly reducing patient discomfort and pain and decreasing healing time.Furthermore, this approach is cost-effective and applicable to hard-to-reach tissue sites (Rizzo and Kehr, 2021).Injectable hydrogels with multiple functions can be utilized to treat  cancer, diabetes, and gene therapy by enabling the effective delivery of numerous pharmaceuticals and other substances, including proteins (Huang and Brazel, 2001).The main types of injectable hydrogels are synthesized by chemical or physical cross-linking.
Physically cross-linked hydrogels, developed by non-covalent secondary forces, are preferable for a sustained delivery system.In contrast, injectable hydrogels created by chemical cross-linking offer superior long-term stability and mechanical qualities but may cause adverse effects due to toxic crosslinkers (Yu and Ding, 2008;Mathew et al., 2018).Novel types of hydrogels, like dual cross-linking hydrogels, nanocomposite hydrogels, and slide-ring hydrogels, are developed to enhance mechanical properties (Figure 3) (Katsuno et al., 2013;Zhao et al., 2016).
Temperature-responsive hydrogels are as attractive as physical gels because they can be customized for specific purposes.These materials possess a biologically compatible in situ setting process, allowing for the incorporation of cells, bioactive compounds, and even intricate 3-D structures (Han et al., 2018).Hydrogel has been  extensively investigated for localized therapy, but few have been commercialized because of its complicated delivery system, difficulties in large-scale production, failure to pass clinical trials, and related side effects (Mishra and Singh, 2021).Table 1 contains a list of several commercial products.

Oral drug delivery product
The oral route is a popular drug delivery method owing to its numerous benefits, including sustained and controlled drug delivery, ease of administration, feasibility of solid formulation, and high level of patient compliance (Homayun et al., 2019).The gastrointestinal tract (GIT) has a complex structure and functions that notably impact the release, dissolution, and absorption of orally administered dosage forms.This impact is primarily attributed to factors such as enzyme content, bile salts, acidity, and the mucosal absorptive surface of the GIT (Shah et al., 2017).Hydrogels are employed in smart oral delivery systems for hydrophobic biological molecules, facilitating site-specific dispersion of medicinal compounds within the intricate GIT.Traditional methods encounter obstacles such as limited permeability of the epithelial membrane and denaturation of drugs (Carr et al., 2010).Stimuliresponsive hydrogels are vital in oral delivery as they can respond to environmental changes.These hydrogels can be triggered by various physical and chemical stimuli, like pH, light, ionic strength, solvent composition, temperature, and electric field (Peppas et al., 2000;Farjadian et al., 2022).Drug delivery systems can be designed to administer therapeutic substances to specific organs in a regulated and predictable manner (Gull et al., 2019a).In the complicated environment of the GI tract, hydrogels protect therapeutic agents and allow targeted delivery by taking advantage of basic physiological changes (Figure 4) (Sharpe et al., 2014b).There have been studies of controlled release systems used in various pH ranges within the body, including the oral, intestines, gastric, and periodontal areas (Gull et al., 2019b).Hydrogels were extensively studied for their potential application in oral drug delivery of insulin, trying to solve the challenges associated with parenteral insulin administration.However, despite numerous advances in this field, progress has been limited, and there is now no commercially successful oral insulin product available for human use (Chaturvedi et al., 2013).Biopolymers such as alginate, hydroxypropyl cellulose, soybean protein, pectin, and cellulose acetate have been the subject of significant investigation in the field of GIT drug delivery (Vashist et al., 2014).Researchers are now working on the development of cost-effective and efficient drug delivery methods utilizing hydrogel materials.Numerous hydrogel systems have been designed to achieve controlled drug release through the utilization of various mechanisms that arise during the administration of medications through the oral route.Despite the challenges encountered by oral drug delivery, these systems have been designed to successfully solve these obstacles (Farrukh et al., 2023).Some examples of these commercial products are included in Table 2.

Buccal drug delivery product
The buccal mucosa is highly accessible and characterized by a wide area of smooth muscle and generally static mucosa, making it an ideal site for administering retentive dosage forms.The internal jugular vein bypasses the hepatic first-pass metabolic process to allow direct entry of medicines into the systemic circulation, resulting in increased bioavailability.Additional benefits include minimal enzymatic activity, compatibility with medications or excipients that cause only minor and reversible mucosal damage or irritation, administration without pain, and convenient drug withdrawal (Sudhakar et al., 2006).Drug administration through the buccal route provides a viable alternative for drug delivery to the systemic circulation.In the study of Hu et al., a mucoadhesive film inspired by mussels and containing polydopamine (DOPA) nanoparticles has been shown to have more significant advantages for transporting drugs over the mucosal barrier, as well as increased drug bioavailability and therapeutic efficacy in models of oral mucositis (Hu et al., 2021).The synthesis of PVA-DOPA polymers involved the modification of poly (vinyl alcohol) (PVA) using the mussel adhesive protein DOPA.Then, different PLGA (poly (D,L-lactide-co-glycolide) nanoparticles were integrated into the PVA-DOPA film to create a combined buccal drug delivery system with dexamethasone (Dex) known as the PVA-DOPA@NPs-Dex film (Figure 5).The nanoparticles are slowly released from the film, then move through the mucus layer and pass through the epithelium.This results in sustained drug release and increased therapeutic efficacy in the treatment of oral mucositis.
Various commercial buccal medication administration dosage forms are available on the market, including buccal tablets, sprays, mucoadhesive formulations, sublingual lozenges, chewing gums, films, and solutions (Nagai and Machida, 1993).There are numerous disadvantages associated with buccal drug delivery.Firstly, the buccal mucosa acts like a barrier that may limit the permeability of certain drugs.Secondly, the presence of saliva can potentially dilute the drug, thereby affecting its absorption.Lastly, the oral cavity experiences a variable environment due to factors such as food consumption and other daily activities.To address this challenge, many strategies can be employed for buccal drug delivery, including physical penetration enhancers like iontophoresis, chemical penetration enhancers like (1) preserving the drug in a low-pH environment.(2) Hydrogen bonds between polymer chains cause the carrier to become complex.
surfactants, and formulation technologies like polymeric and mucoadhesive dosage forms (Wanasathop et al., 2021).Their bioadhesive qualities significantly influence the choice of hydrogel delivery methods.When a hydrogel has high adhesion to the epithelium, it can extend the system's retention at a target site, delivering enough drug doses for the intended therapeutic effect.This is especially crucial for buccal delivery (Peppas and Sahlin, 1996).Specific polymers, like poly (acrylic acid) and chitosan, have been observed to possess mucoadhesive properties.Poly (acrylic acid) can establish hydrogen bonds with the mucosa, whereas chitosan, which carries a positive charge, can engage in electrostatic interactions with negatively charged surfaces of tissues and cells (Li and Mooney, 2016).Hydrogel carriers for drug delivery contain the potential to control the release rate of drugs based on the hydration state.This hydration level determines the hydrogel's swelling ability (Narayanaswamy and Torchilin, 2020).In the field of drug delivery, a variety of materialbased hydrogels have been commercially applied.Some commonly used hydrogels include hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyacrylic resins, carboxymethyl cellulose, and hydroxypropyl methylcellulose (Cascone and Lamberti, 2020).Some of these commercial products are listed in Table 3.

Ocular drug delivery product
The human eye is a complex, spherical organ.The structure can be anatomically separated into two compartments, namely, the anterior and posterior segments.Ophthalmic drugs administered to the anterior section of the eye encounter challenges from dynamic and static barriers.The main barriers are the blood-queous barrier, corneal epithelium, corneal stroma, lymph flow, conjunctival blood flow, and tear drainage.These factors play a critical role in the formulation and development of ophthalmic therapies (Torres-Luna et al., 2020).Drug contact with ocular surface tissues is brief, often lasting between 1 and 2 min.This is due to the continuous generation of tears, which range from 0.5 to 2.2 μL per minute, as well as the turnover of tears (Sánchez-López et al., 2017).Hydrogels possess the capability to effectively resist the rapid blinking and flushing actions of tears, thereby extending the duration of drug presence on the eye surface.This prolonged drug resident time facilitates enhanced drug efficacy regarding localized therapeutic action on mucosal surfaces or more profound penetration into eye tissues.Hydrogels can extend the duration of drug presence on the ocular surface and maintain a controlled release of drugs within the intraocular tissues, including the vitreous cavity and aqueous humor (Cooper and Yang, 2019).The versatility and modifiability of hydrogels make them ideal for efficiently transporting medications to the eye, as they may be designed to take advantage of the specific function and environment for which they are developed (Torres-Luna et al., 2020;MohammadSadeghi et al., 2021).Figure 6 illustrates the diverse range of potential applications of hydrogels in therapeutic ophthalmic.
Hydrogels can augment medication retention on the ocular surface by elevating solution viscosity, facilitating their swelling in water or aqueous solvents.Chitosan, hydroxypropyl cellulose, dextran, hydroxypropyl methylcellulose, and poly (acrylic acid) derivatives, including polycarbophil and carbomer 934, have been identified in studies that are most appropriate for bioadhesive polymers for ocular medication administration (Fathi et al., 2015).Table 4 is a compilation of several commercial products.

Vaginal drug delivery product
The vagina is an essential reproductive organ made of a 7.50cm-long muscular canal located between the urethra, rectum, and bladder.The vaginal membrane is composed of three different layers, including the muscular coat, epithelial layer, and tunica adventitia.The thickness of the vaginal epithelium is dependent upon various factors, including age, hormonal activity, and life phases.The vaginal branch of the uterine artery is responsible for providing blood circulation to the vagina (Valenta, 2005).The vaginal route of drug administration is thought to be highly advantageous owing to its extensive blood flow, capacity to elude the first-pass effect, and significant permeability to many pharmacological compounds, including peptides and proteins (Aka- Any-Grah et al., 2010).A variety of pharmacological groups, including antimicrobials, labor inducers, sexual hormones, and spermicides, have been delivered via the vaginal mucosa.The majority of these drugs are employed for localized conditions, while a few of them can achieve serum concentrations that are adequate for producing systemic effects (Alexander et al., 2004).Vaginal drug delivery methods use natural or synthetic polymers to facilitate drug interaction with the target site and achieve controlled, reproducible, and predictable drug release.Various systems are presently being utilized or under examination (Osmałek et al., 2021).The term "mucoadhesion" pertains to when a substance with biological origin is bound to the mucosa surface for a prolonged period due to interfacial forces (Das Neves and Bahia, 2006).The mucoadhesive system possesses the capacity to regulate the rate of drug release from the vaginal canal and prolong its residence time.Bioadhesion has the potential to enhance the level of contact and extend the residence time of dosage forms across different administration routes.These formulations have the potential to include pharmaceutical substances or function in combination with moisturizing agents to serve as a means of managing vaginal dryness (de Araujo Pereira and Bruschi, 2012).A variety of bioadhesive polymers were evaluated to establish a vaginal delivery system.These polymers The synthesis and biomedical application of PVA-DOPA@NPs-Dex mucoadhesive film enhances mucoadhesion for buccal drug delivery.PVA: poly (vinyl alcohol), DOPA: 3,4-dihydroxy-D-phenylalanine, NPs: nanoparticles, Dex: dexamethasone.Adopted from (Hu et al., 2021).
included sodium alginate, polycarbophil, guar gum and xanthan, Carbopol ® , sodium carboxymethylcellulose, hydroxypropyl methylcellulose, and hydroxypropyl cellulose (Veiga et al., 2018).Mucoadhesive vaginal drug delivery systems include gels, tablets, films, emulsion-type systems, and suppositories, but gels are the primary mucoadhesive vaginal drug delivery methods in current usage.Vaginal formulations are characterized by their ease of manufacturing, comfort, and efficacy in establishing intimate The application of hydrogels in various ocular regions.
contact with the vaginal mucosa.These compounds contain a significant amount of water and exhibit rheological qualities, hence offering hydrating and lubricating effects, particularly in cases of dry vaginal mucosa.The utilization of mucoadhesive polymers has been shown to enhance the duration of contact, reduce formulation loss, and prolong the therapeutic impact (Das Neves and Bahia, 2006;de Araujo Pereira and Bruschi, 2012).Table 5 presents a comprehensive overview of several proprietary vaginal compositions now accessible in the marketplace.

Transdermal drug delivery product
The skin, which is the largest and most external organ within the human body, covers an approximate surface area of 1.8 square meters and constitutes approximately 20% of the whole body weight of an average individual (Brown and Williams, 2019).The utilization of the skin as a way of drug delivery presents advantages such as sustained release, greater compliance, and reduced rates of adverse effects in comparison to oral and parenteral administration methods.The transdermal delivery method is considered ideal due to its large surface area, minimal potential for enzymedependent degradation, and extended duration of application (Akbarian et al., 2022;Dahri et al., 2023).The stratum corneum offers an important challenge in the transdermal administration of active substances due to its role as the initial barrier of the skin.This barrier restricts drug absorption and limits the range of medications that can be effectively given (Al-Japairai et al., 2020).Polymers are of significant importance in the formulation of skin preparations since they act as a matrix for delivering the active ingredient to the desired application site or target organ.This intricate system distinguishes between active and inactive components, achieving the desired result.Polymers possess diverse applications, serving as gelating agents in gel systems, consistency excipients in emulsions and creams, matrix materials in patches, and skin adhesives in transdermal systems.Their diverse range of applications makes them useful in numerous formulations for skin preparations (Valenta and Auner, 2004).Hydrogels, which are derived from hydrophilic polymers, exhibit favorable characteristics for drug delivery purposes owing to their capacity to hold large amounts  (2018) of water, thus enhancing skin elasticity and moisturization (Viyoch et al., 2005).Hydrogel-based patches offer the capacity to deliver drugs in a controlled manner over a specified duration, rendering them more favorable compared to conventional methods.Additionally, these systems possess the advantage of being quickly packaged and transported while also enabling specific drug delivery to the intended site (Boriwanwattanarak et al., 2008).The Scopolamine patch, which represents the first marketed transdermal delivery patch, presents a revolutionary approach to preventing the effects of motion sickness (Prausnitz et al., 2012).Transdermal patches are available in various types, such as single-layer, multi-layer, reservoir, matrix, and vapor patches.In a single-layer patch, the adhesive layer contains and releases the drug.The multi-layer patch consists of two distinct layers designed for immediate and controlled drug release, which are effectively separated by a membrane.The reservoir transdermal system includes different layers, including a drug layer and a liquid compartment containing a drug suspension or solution, separated by an adhesive layer.The release rate follows zero-order kinetics (Pastore et al., 2015).The matrix system consists of a partially overlaid adhesive layer and a semi-solid matrix, which includes a drug solution or suspension.The noveltype vapor patch is designed to deliver essential oils continuously for over 6 h.Its primary purposes include decongestion and enhancing sleep quality (Lee et al., 2001).Hydrogels are utilized for transdermal delivery in patches or creams, as they effectively enhance drug permeation by promoting skin hydration.
Moreover, hydrogels are well-suited for topical applications.Furthermore, investigations have been conducted on their potential to improve the stability and efficacy of transdermal delivery systems such as liposomes, micelles, and nanoparticles (Kong et al., 2016).Table 6 presents examples of commercially available transdermal hydrogels for drug delivery.

Advantages and disadvantages
Hydrogels provide numerous advantages that make them ideal for biomedical applications, particularly drug delivery.These benefits include sustained action, decreasing administration doses, ease of modification, reduced side effects, drug targeting capabilities to specific locations, and the ability to respond to stimuli.Nevertheless, hydrogels exhibit several disadvantages, including hypoxia, dehydration, as well as limited mechanical strength, challenging manipulability, and high cost.
Oxygen is a vital factor in the existence of cellular organisms.Some hydrogels can induce hypoxia as a state of lack of oxygen.This phenomenon can be considered as a limitation or advantage of hydrogel application in therapy.Hypoxia-induced hydrogels in surrounding tissues can stimulate the invasion of blood vessels and activate hypoxia-regulated pathways to regenerate tissue in neovascularization (Park and Gerecht, 2014).One drawback of hydrogel utilization is dehydration, which can cause stiffening and locking their dynamic behavior.This can be observed in thermos-responsive hydrogel after reaching LCST and can be considered as a limiting factor in their application in therapy.(Zhang et al., 2023).Hydrogels have low mechanical strength due to two characteristics: high solution and low friction between chains (Lin et al., 2022).This problem has arisen by designing composite structures by integrating heterogeneous elements such as silica in the hydrogel structure (Xu et al., 2022).Overall, the advantages of hydrogels exceed their disadvantages in the field of drug delivery.Considering positive factors, hydrogel commercial administration in drug delivery will be vast.

Conclusion and future perspectives
Drug carriers represent revolutionary delivery systems within the science of medicine.Numerous studies have shown various polymers employed in carrier synthesis.Among these, hydrogel-based systems received significant attention due to their cost-effectiveness, ease of production, and remarkable capacity to carry diverse drug types.Utilizing cross-linked polymeric networks for enhancing therapeutic efficacy presents a novel way for drug delivery applications.Still, despite the numerous capabilities and advantages of hydrogels in the field of drug delivery, there remains an essential requirement for further research and development to efficiently and swiftly introduce more hydrogel-based formulations to the market.Novel features of hydrogels will continue to play a crucial function in drug delivery and enable the development of a vast array of drugs, peptides, proteins, and delivery systems.
Commercial hydrogel products for drug delivery based on different routes of administration represent a rapidly growing market with significant potential.Continued research and development in hydrogels hold promising prospects for the future.Advancements in drug delivery and polymer-based carrier systems will facilitate targeted and efficient transportation of therapeutic drugs to specific locations within the body.Continued research, technological innovations, and collaborative efforts can fully realize the potential of hydrogel drug delivery in improving therapeutic outcomes.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

FIGURE 1
FIGURE 1Application of hydrogel for drug delivery based on route of administration.

FIGURE 2
FIGURE 2Classification of hydrogel products based on different parameters.

TABLE 1
Some of the injectable commercial hydrogels.

TABLE 2
Some of the commercial oral hydrogels.

TABLE 3
Some of the transdermal commercial hydrogels.

TABLE 4
Some of the ocular commercial hydrogels.

TABLE 5
Some of the vaginal commercial hydrogels.

TABLE 6
Some of the transdermal commercial hydrogels [data from the Food and Drug Administration (FDA)].