Adhesive Catechol-Conjugated Hyaluronic Acid for Biomedical Applications: A Mini Review

Abstract

Recently, catechol-containing polymers have been extensively developed as promising materials for surgical tissue adhesives, wound dressing, drug delivery depots, and tissue engineering scaffolds. Catechol conjugation to the polymer backbone provides adhesive properties to the tissue and does not significantly affect the intrinsic properties of the polymers. An example of a catecholic polymer is catechol-conjugated hyaluronic acid. In general, hyaluronic acid shows excellent biocompatibility and biodegradability; thus, it is used in various medical applications. However, hyaluronic acid alone has poor mechanical and tissue adhesion properties. Catechol modification considerably increases the mechanical and underwater adhesive properties of hyaluronic acid, while maintaining its biocompatibility and biodegradability and enabling its use in several biomedical applications. In this review, we briefly describe the synthesis and characteristics of catechol-modified hyaluronic acid, with a specific focus on catechol-involving reactions. Finally, we discuss the basic concepts and therapeutic effects of catechol-conjugated hyaluronic acid for biomedical applications.

1. Introduction

Hyaluronic acid (HA) is a naturally occurring polysaccharide composed of d-glucuronic acid and N-acetyl-d-glucosamine, which is used in various biomedical applications, such as wound healing, viscosupplementation for wrinkle fillers, post-surgical antiadhesive barriers, drug delivery carriers, and tissue engineering scaffolds [1,2,3,4,5,6,7,8,9,10,11,12,13]. HA is abundantly found in connective tissues and fluids, particularly in articular cartilage and synovial fluids [1,2,3,4,5]. The roles of HA in biological functions are diverse, including the hydration of the extracellular matrix, the regulation of tissue homeostasis, receptor-mediated regulation of cell proliferation, migration, and differentiation, and the lubrication of cartilage and of the eyes [14,15]. For instance, HA in synovial fluids is responsible for the lubrication of cartilage in the synovium owing to its viscoelasticity and biocompatibility with synovial fluids [16]. In addition, HA is fully degraded by hyaluronidase in the body [17]. As a result, HA is widely used for drug delivery depots and tissue engineering scaffolds. However, there are difficulties in using unmodified HA in various medical settings due to its poor mechanical properties. Therefore, chemical modifications of the amine, thiol, and catechol functional groups on the HA backbone can address this drawback.

Marine mussels strongly adhere to various organic and inorganic surfaces (e.g., metal, metal oxides, ceramics, and polymer surfaces) by the secretion of mussel adhesive proteins via catechol-based reactions in wet environments (i.e., seawater) [18,19,20,21,22]. These involve the catechol side chains of 3,4-dihydroxy-l-phenylalanine (DOPA), an uncommon amino acid abundantly found in mussel adhesive proteins [18,19,20,21,22]. The catechol groups are oxidized at basic pH and are transformed into catechol quinones, which can react with various functional groups, including thiols, amines, and other catechols [23,24,25]. In addition to forming covalent bonds, the catechol groups interact with various substrates via electrostatic interactions, hydrogen bonds, π–π stacking, and π–cation interactions [25,26]. Inspired by the mussel adhesion behavior, catechol-conjugated polymers have been synthesized to improve adhesion of fluids [27,28,29,30,31]. For instance, catechol-conjugated chitosan has excellent solubility at neutral pH, excellent adhesive properties in tissues, and superior hemostatic capability compared to unmodified chitosan [32,33,34,35,36]. Therefore, the introduction of catechol groups into the HA backbone can increase HA mechanical properties and adhesiveness in wet environments.

In this review, we briefly describe the methodologies used for the preparation and characterization of HA–catechol. In addition, we focus on the biomedical uses of HA–catechol, which are categorized into the following four subsections: antifouling, wound healing, drug/gene delivery carriers, and tissue engineering scaffolds. Furthermore, we discuss the current challenges and opportunities for technologies using HA–catechol, which require further fundamental research, and its use in translational medicine.

2. Methodologies for the Synthesis and Characterization of Hyaluronic Acid–Catechol

Carbodiimide chemistry is commonly used to prepare HA–catechol [37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81]. It involves the formation of an amide bond between the carboxylic group of HA and the primary amine of dopamine (Figure 1a). The important reaction conditions for HA–catechol synthesis include maintaining the pH between 4 and 6 to prevent oxidation of the catechol groups [37]. The reaction solution turns black at a pH above 6, indicating the oxidation of the catechol groups. The degree of catechol substitution (DS) in HA is 10–35%, identified using standard carbodiimide chemistry. Similar approaches have been used to prepare catechol-conjugated negatively charged polymers (i.e., alginate–catechol [82,83], heparin–catechol [84], and polyacrylic acid–catechol [85]). In a previous study, a Schiff’s base reaction between aldehyde-modified HA and dopamine was used to prepare HA–catechol [86]. HA was first oxidized using sodium periodate to prepare dialdehyde-functionalized HA. The formation of the Schiff base was achieved by the reaction between dialdehyde groups in oxidized HA and the primary amine groups in dopamine. The DS (25–45%) obtained using the Schiff base reaction was higher than that obtained using the carbodiimide coupling reaction (~10%), probably due to the dialdehyde groups of the oxidized HA [86].

Synthesized HA–catechols can be characterized using ¹H-NMR and UV–Vis spectroscopy. Figure 1b shows the ¹H-NMR spectrum of HA–catechol. HA peaks have been previously assigned as follows: the N-acetyl peak at 2.0 ppm, and multiplets of disaccharide units and anomeric protons at 2.8–3.7 ppm [38,39,40,41,42,43]. In addition, the catechol protons were assigned to the peaks at 6.5–7.0 ppm [38,39,40,41,42,43]. The degree of catechol substitution was calculated by comparing the integrated area of the catechol protons (3H, 6.5–7.0 ppm, D₂O) and acetyl groups (3H, –COCH₃, 2.0 ppm, D₂O). In UV–Vis spectroscopic studies, catechol groups show maximum absorption at a wavelength of 280 nm (Figure 1c). Catechol content can be quantified using standard curves of catechol concentrations constructed using a standard solution of dopamine [37,38,44,45]. Fourier transform infrared spectroscopy is also used to confirm the catechol conjugation of HA–catechol [38]. The peaks at 1720, 1630, and 1410 cm⁻¹ are assigned to C=O, amine bonds, and C–O stretching in HA–catechol, respectively. In addition, C–H vibration (2850 cm⁻¹) and C=C ring stretching (1520 cm⁻¹) indicate catechol conjugation on the HA backbone.

3. General Properties of Hyaluronic Acid–Catechol

3.1. Biocompatibility

High levels of HA are present in tissues and cartilage, particularly in those of the musculoskeletal system. In addition, HA is widely used in medical applications mainly due to its biocompatibility. The conjugation of molecules to HA does not significantly affect its biocompatibility. For instance, injectable tyramine-conjugated HA hydrogels prepared by the oxidative crosslinking of phenolic groups maintain high cell viability (>95%) of human mesenchymal stem cells (hMSCs) for up to 14 days [87]. In addition, the viability of MSCs in an HA hydrogel crosslinked with polyethylene glycol (HA–PEG gel) was ~100% when 10⁶ cells mL⁻¹ of MSCs were treated with the hydrogel for three days [88].

Similarly, HA–catechol is also non-cytotoxic and biocompatible. Treatment of cells at a density of 1.0 × 10⁶ cells/100 μL with a pre-gelation HA–catechol solution preserves cell viability (~100%) [46]. Treatment with extracts of HA–catechol hydrogels maintains ~83% viability in L929 cells compared to that obtained using fresh media. In addition, the viability of human hepatocytes (hHEPs) encapsulated in HA–catechol hydrogels prepared by the addition of NaIO₄ was ~70.9%, which is far higher than that observed using HA–methacrylate hydrogels (~52.2%) on day 14 of treatment [46,89]. Furthermore, HA–catechol hydrogels incorporated with basic fibroblast growth factor exhibit improved hHEPs cell viability to ~81.4% after 14 days [37]. The viability of human neural stem cells (hNSCs) in an HA–catechol hydrogel after seven days was approximately 70%, which is two-fold that observed using an HA–methacrylate hydrogel (less than 30%).

3.2. Tissue Adhesive Properties

HA has anti-adhesive properties; therefore, HA-based physical barriers have been developed to prevent post-surgical adhesion. However, the conjugation of catechol groups to HA significantly increases the adhesiveness of HA–catechol to tissues. As mentioned, the oxidized catechol groups (i.e., catechol quinone) can covalently react with the amine or thiol groups that are abundantly found in native tissues, including proteoglycan structures, resulting in superior tissue adhesive properties. The tissue adhesive properties of HA–catechol are commonly measured with a lap shear test using animal skin, such as chicken or porcine skin (Figure 2a). In addition, post-crosslinking of HA–catechol adhesives is carried out prior to its use on skin or dermal tissues to improve tissues’ adhesion properties in general. The adhesive stress withstood by joint chicken skins in the presence of HA–catechol hydrogels is approximately 10 kPa, which is far greater than that bore when using methacrylated HA hydrogels (Figure 2b) [52]. In addition, tissue adhesive properties are improved by the addition of an oxidative intermolecular crosslink agent (NaIO₄) and by an increase of DS in HA–catechol. The maximum adhesive strength is 90.0 ± 6.7 kPa when using HA–catechol with high DS (0.45) at a molar ratio of 1:2 (catechol/NaIO₄). Moreover, the addition of Fe³⁺ ions to the hydrogels increases the adhesive stress of HA–catechol. The detachment stress of hydrogels containing HA–catechol/Fe³⁺ complexes is 14.8 ± 0.9 kPa, while that of an HA–catechol solution is 0.9 ± 0.5 kPa, measured using mouse subcutaneous tissues [47].

HA–catechol-based composite materials also show excellent adhesive properties. For instance, HA–catechol and Pluronic F127 (16 wt%) composite hydrogels show excellent stability with approximately 10-fold greater adhesive strength (~7.2 kPa) than HA–catechol alone (~1.7 kPa) [48]. HA–catechol/PVA (Polyvinyl alcohol) hydrogel composite films can be used as mucoadhesive films for the treatment of oral candidiasis [49]. HA–catechol/PVA hydrogels have a high detachment force of approximately 0.23 N, which was measured using a bovine buccal membrane. HA–catechol/PLL (poly-l-lysine) hydrogels enzymatically crosslinked with horseradish peroxidase (HRP) with H₂O₂ showed a significant improvement in adhesive strength, from 20.8 kPa to 33.8 kPa [39]. Therefore, HA–catechol is useful for long-term tissue adhesion and may have great potential for use in biomedical applications. Furthermore, the tissue adhesiveness of HA–catechol may prevent the malposition or movement of the HA-based materials on the targeted tissue surfaces.

3.3. Catechol-Involving Reactions

The catechol groups in HA backbones can undergo chemical and physical reactions during the manufacture of formulations (i.e., hydrogels, thin films, and nanoparticles) and help them adhere to various substrates (i.e., tissues, metal/metal oxide, polymer surfaces). For example, hydrogels can be prepared by the covalent and/or non-covalent crosslinking of polymers via catechol-involving reactions (Figure 3) [37,39,43,46,47,48,50,51,52,53,54,55,56,57,58]. Catechol groups in basic pH conditions transform into catechol quinone, which is highly reactive towards amines, thiols, and other catechol groups because of their pKa values (pKa1: 9.25; pKa2: 13.0) [90]. Oxidants (e.g., sodium periodate, NaIO₄) are used to achieve the rapid gelling of HA–catechol hydrogels [37,46,51,52,53,54]. HA–catechol achieves a crosslinked gel state with instantaneous solidification upon the addition of NaIO₄. The optimized stoichiometric ratio of NaIO₄/catechol for the preparation of HA–catechol hydrogels is 1.5:1, which results in rapid gelation within 10 s [37]. Additionally, oxidoreductase has previously been used to prepare HA–catechol hydrogels [39,57]. When H₂O₂ (0.01% v/v) and HRP (0.05 mg/mL and 0.1 mg/mL) were added to an HA–catechol solution, the gelation time significantly decreased to 12.5 s for 0.05 mg/mL of HRP and to 5.2 s for 0.1 mg/mL of HRP [57]. In addition to covalent crosslinking, HA–catechol hydrogels are prepared by non-covalent reactions of polymers. In general, the catechol groups can bind to various molecules and substrates by hydrogen bonds, π–π stacking, cation–π interactions, and metal coordination. In particular, catechol groups strongly bind to Fe³⁺ ions, as previously reported [91,92,93,94,95,96]. The rapid gelling of HA–catechol hydrogels is achieved by the addition of Fe³⁺ ions to a solution of HA–catechol [47,59]. The elastic modulus value of Fe³⁺ ion-mediated HA–catechol hydrogels at a concentration of 10 mM FeCl₃ was ~4.6 kPa, which is far higher than that of the HA–catechol solution (~1.7 Pa) [47]. In addition, the gelation time could be controlled by changing the Fe³⁺–catechol stoichiometric ratio. For instance, the gelation time decreased by increasing the Fe³⁺–catechol ratio from 1:1 (6.5 mM Fe³⁺ and catechol) to 1:3 [59]. Therefore, catechol-involving reactions of HA–catechol can be utilized to prepare injectable hydrogels, drug delivery depots, and tissue engineering scaffolds.

4. Biomedical Applications

HA–catechol is widely used in biomedical applications owing to its biocompatibility, adhesiveness, and anti-inflammatory properties. In this section, we describe the various biomedical uses of HA–catechol, focusing on its physicochemical properties and catechol-involving chemical reactions with tissues and/or biomacromolecules.

4.1. Antifouling Materials

HA is a representative antifouling material mainly due to its hydrophilicity and its unique moisture retention and lubrication abilities. The catechol moiety of HA–catechol can form a thin film on substrates (Figure 4a). These HA–catechol coatings on Au surfaces enhance hydrophilicity (11.9° of water contact angle) compared to that measured on bare Au surfaces (81.6°) [40]. In addition, HA–catechol-coated Au substrates have extremely low protein-adsorption levels (<5 ng/cm²) for single proteins (i.e., bovine serum albumin, lysozymes, and β-lactoglobulin). Polyimide (PI), Au, poly(methyl methacrylate) (PMMA), polytetrafluoroethylene (PTFE), and polyurethane (PU) are coated with HA–catechol to prepare antifouling substrates [60]. All substrates coated with HA–catechol exhibit improved hydrophilicity of the substrates (less than 50° of water contact angle) compared to the unmodified substrates, as shown in Figure 4b. The adsorption of bovine serum albumin on HA–catechol-coated substrates was dramatically reduced to 22–46% compared to that on unmodified surfaces (Figure 4c). Therefore, HA–catechol is an excellent coating agent for the antifouling treatment of biomedical devices.

4.2. Wound-Healing Materials

HA–catechol materials have been investigated as wound-healing materials because reactive oxygen species can be scavenged by catechol groups. The incorporation of anti-inflammatory drugs into HA–catechol hydrogels significantly improves wound closure and healing. For instance, arginine derivatives have been incorporated into HA–catechol hydrogels, as shown in Figure 5a [62]. To confirm the antioxidant properties, 2,2-diphenyl-1-picrylhydrazyl (DPPH) and hydroxyl radical (OH) scavenging experiments are generally used. An arginine/HA–catechol composite hydrogel showed strong antioxidant activity with maximum OH and DPPH free-radical scavenging rates of 41.1% and 67.4%, respectively. In addition, complete wound closure and healing could be achieved using these hydrogels after 21 days (Figure 5b). HA–catechol conjugated with chitosan is also used as a wound-healing material [57]. Wound healing can be improved by increasing the concentrations of H₂O₂ and HRP. HA–catechol/chitosan composite hydrogels with the addition of H₂O₂ (0.05%, v/v) and HRP (0.1 mg/mL) showed complete wound healing nine days post-surgery, which depicts a wound closure effect similar to that obtained using surgical suturing, indicating that these hydrogels could replace surgical sutures to achieve wound closure. In addition, a mild acute inflammatory response was observed for six days after the implantation of HA–catechol/chitosan hydrogels, and inflammation receded on the ninth day after surgery. This is similar to the biocompatibility of surgical sutures and enzymatically (HRP) crosslinked hydrogels.

4.3. Drug/Gene Delivery Carriers

A challenge in cancer treatment is targeting anticancer drug delivery to increase the efficiency of drugs. Because HA interacts with CD44, overexpressed in cancer cells, HA-based hydrogels as drug/gene delivery carriers have been developed for anticancer drug therapy. The catechol groups in HA can act as coating and crosslinking moieties to achieve sustained drug release. For instance, HA–catechol/chitosan composite nanoparticles containing the anticancer drug doxorubicin (DOX), with a high loading capacity of 250 μg/mg, exhibited a controlled release of DOX, in comparison to the administration of free DOX [63]. In addition, the nanoparticles showed mucoadhesive properties; over 60% of the nanoparticles remained on the buccal mucosa after washing with artificial saliva, while only 10% remained when dextran was used as a control. In addition, these nanoparticles showed enhanced cellular uptake and accumulation in HN22 cells. Furthermore, treatment using DOX nanoparticles led to extensive apoptosis (22% of the apoptotic rate), which was higher than that observed using free DOX (16%). HA–catechol hydrogels containing amine-functionalized graphene oxide (GO) are prepared by covalent reactions involving the catechol groups of HA and the amine groups of GO [53]. DOX is noncovalently bound to GO in hydrogels by hydrophobic interactions and π–π stacking. In addition, controlled DOX release profiles have been obtained using HA–catechol/GO hydrogels. HA–catechol hollow nanoparticles have also been developed to deliver anticancer drugs. Silica nanoparticles are coated with HA–catechol, and the inner silica particles are removed to fabricate hollow HA–catechol particles to load anticancer drugs, such as DOX [65]. The hollow HA-catechol particles showed drug-loading efficiency (~20 wt%) with different drug-release profiles between pH 5.5 and 7.4. At pH 5.5, 67% of DOX was released from the particles within 8 h, while 25% of DOX release occurred at pH 7.4 after 60 h. Considering the pH of the physiological conditions and the cancer microenvironment, these particles can effectively lead to cancer cell death after pH-triggered DOX release. In addition, HA–catechol-coated Au nanorods have been fabricated for cancer therapy via the interaction of catechol and Au [66]. DOX was covalently conjugated to HA–catechol, which has an acid-labile hydrazone linkage. The release of DOX from the HA–catechol-coated Au nanorods was controlled by two stimuli, namely, pH and NIR (near-infrared) light, and was found to be successful in inhibiting the growth of breast cancer cells.

HA–catechol-based gene delivery has been performed using calcium phosphate (CaP)-containing plasmid DNA or siRNA [45]. HA–catechol has been used as a binder for CaP and cancer-targeting materials. Stabilized CaP/siRNA/HA–catechol nanoparticles are prepared by the simple mixing of solutions and involve specific interactions between calcium and catechol. The nanoparticles deliver siRNA to cancer cells via endocytosis. In the HT29-luc tumor xenograft model, the nanoparticles showed no sign of hepatic toxicity after systemic administration and successfully inhibited cancer growth through the gene silencing effect of siRNA. In addition, CAP nanoparticles stabilized by HA–catechol have been designed to enhance the transfection of hMSCs in a gene delivery system, as shown in Figure 6 [41]. The CAP nanoparticles stabilized by HA–catechol showed efficient gene delivery of both pBMP-2 as a plasmid DNA and mir148b as a microRNA to stem cells via CD44-specific receptor-mediated endocytosis.

4.4. Tissue Engineering Scaffolds

In general, chemical modification of HA-based materials is required for tissue engineering, mainly due to the poor mechanical properties of unmodified HA under physiological conditions. The conjugation of catechol to HA increases its mechanical properties with excellent stability against hyaluronidase. HA–catechol is especially useful to promote osteogenesis and angiogenesis. Vascular endothelial growth factor (VEGF) was previously immobilized on a Ti surface using HA–catechol [50]. The VEGF-immobilized Ti substrate (60 ng/cm²) sustained osteoblast proliferation (70,000 osteoblasts/cm²) after seven days of incubation. In addition, the alkaline phosphatase activity (0.7 μM/mg) of VEGF-immobilized Ti substrate was greater than that of bare Ti surfaces (0.3 μM/mg). Moreover, the VEGF-immobilized Ti substrate using HA–catechol not only allowed osteogenesis for bone formation, but also inhibited bacterial adhesion. In addition, HA–catechol can form thin films with chitosan on a Ti–Nb–Zr alloy (TNZ) [67]. HA–catechol- and chitosan-coated TNZ surfaces improve the proliferation of osteoblast cells, with approximately 35,000 osteoblasts/cm² after seven days, which is comparable with the proliferation levels achieved with bare TNZ (30,000 osteoblasts/cm²). The modification of Ti surfaces with HA–catechol can be used to enhance osteogenesis and angiogenesis. HA–catechol has been used for salivary gland tissue engineering (Figure 7). A polycarbonate (PC) membrane coated with HA–catechol showed enhanced adhesion of embryonic submandibular glands (eSMGs) (5.2-fold increase of for the number of adherent eSMGs) compared to both bare and HA-coated membranes [44]. In addition, branching morphogenesis, including average bud counts, was enhanced by HA–catechol coatings mimicking an HA-rich environment. HA–catechol composite hydrogels with carbon nanotubes (CNTs) and/or polypyrrole (PPy) have been developed for hNSC engineering [54]. The catechol groups bound to CNTs via hydrophobic and π–π interactions promote the polymerization of and. CNT and/or PPy in hydrogels contribute to the improvement of electroconductivity to promote the differentiation of human fetal NSCs and human induced pluripotent stem cell-derived neural progenitor cells. In addition, upregulated calcium channel expression and activation of depolarization have been observed in human stem cells using HA–catechol/CNT hydrogels, and intracellular calcium influx is also enhanced by the stimulation of electrical signals. Therefore, HA–catechol can be used in various tissue-engineering applications, including those involving bone, cartilage, salivary gland, and neural tissue regeneration.

5. Conclusions

In this review, we have described the synthetic and characteristic methodologies, general properties, and various biomedical applications of HA–catechol, including antifouling, tissue adhesives, wound healing, drug/gene delivery, and tissue engineering. Catechol conjugation onto HA backbones significantly enhances HA mechanical and tissue adhesive properties due to catechol-involving reactions, allowing various biomedical applications. In addition, the intrinsic properties of HA, such as biocompatibility and biodegradability, are still conserved after the conjugation of the catechol groups. We expect that HA–catechol will be exploited as an adhesive and biocompatible material in many clinical settings, opening the doors to translational researches to enhance therapies.

Author Contributions

Conceptualization, J.K., C.L., J.H.R.; writing—original draft preparation, J.K., C.L., J.H.R.; writing—review and editing, J.K., J.H.R.; supervision, J.H.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Wonkwang University in 2018.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figures

Figure 1. (a) Synthesis and chemical structures of hyaluronic acid (HA)–catechol. (b) 1H NMR and (c) UV–Vis spectra of HA–catechol.

Figure 2. (a) Schematic illustration of a lap shear tissue adhesion test. (b) Average adhesive stress (kPa) of methacrylated HA (MeHA) and HA–catechol (HA-Cat) hydrogels (** p < 0.01). Reprinted with permission from [52]. Copyright © 2019 Elsevier.

Figure 3. Schematic illustration of catechol-involving reactions of HA–catechol: (a) covalent bonds, (b) π–π stacking, (c) hydrogen bonds, and (d) metal coordination.

Figure 4. (a) Schematic illustration of HA–catechol (dopamine-conjugated HA: DA–HA) coating on various substrates (i.e., PI (polyimide), gold (Au), poly(methyl methacrylate) (PMMA), polytetrafluoroethylene (PTFE), and polyurethane (PU)) for antifouling applications. (b) Water contact angle measurements of various substrates before and after HA–catechol coating. (c) BSA (bovine serum albumin) adsorption (µg/cm2) on bare and HA–catechol-coated surfaces after treatment with a BSA solution (1 mg/mL) at 37 °C for 6 h (* p < 0.05). Reproduced with permission from [60]. Copyright © 2020 Elsevier.

Figure 5. (a) Schematic illustration of methacrylated HA–catechol (HA–DA) and arginine derivatives (AD) composite hydrogels for wound healing. (b) Photographic images of rat skin wounds after treatments of HA–DA/AD hydrogels with various pore sizes (14.18 ± 1.54 μm for HA–DA/AD1, 13.09 ± 0.79 μm for HA–DA/AD2, 11.08 ± 1.16 μm for HA–DA/AD3, and 9.07 ± 2.17 μm for HA–DA/AD hydrogels). Treatment with PBS was used as a control. Reproduced with permission from [62]. Copyright © 2019 Elsevier.

Figure 6. Schematic representation of an HA–catechol/CaP-based gene delivery system. Reproduced from [41] with permission from MDPI, copyright 2019.

Figure 7. Schematic illustration of mesenchymal HA-mimicking HA–catechol platform for salivary gland tissue engineering. Reprinted with permission from [44]. Copyright © 2020 American Chemical Society.