**4.1 Chitosan based chemically crosslinked hydrogels**

Chitosan (CHI) is a linear polysaccharide formed by arbitrarily allocated β-(1 → 4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine

#### **Figure 1.**

*Left: Image of a hydrogel based on crosslinked hyaluronic acid. Right: Scanning electron microscopy (SEM) picture of the hydrogel. Figure produced by the authors.*

(acetylated unit). Chitosan is one of the most versatile biopolymers due to its unique properties: biodegradability, biocompatibility, non-toxicity, antioxidant, anti-inflammatory, antifungal, and antibacterial "contact killing" [33]. Therefore the applicability of this polysaccharide extends to a wide range of various biomedical areas, such as cosmetics, drug delivery, and tissue engineering, among others [34].

In this regard a covalently crosslinked chitosan hydrogel was produced Diels Alder reaction of furan and maleimide functionalized CHI. The resulting biopolymer held the typical pH sensitivity and antibacterial properties of non-functionalized CHI. The drug delivery capabilities of this system were evaluated with model drug antibiotic chloramphenicol (ClPh). Drug release experiment did not show an initial burst, which indicated that the ClPh was successfully encapsulated, whereas it displayed a sustained delivery of the drug with a complete release of the total amount of drug loaded (2.61 ± 0.036 mg ClPh/g hydrogel) after 4 hours [35].

CHI was also crosslinked with genipin (GP) to obtain biocompatible, antibacterial and anti-inflammatory hydrogels with wound healing properties. Sustained release of acetylsalicylic acid (ASA), cefuroxime (CFX), tetracycline (TCN) and amoxicillin (AMX) from the hydrogels displayed a Pharmacologic Half Life t1/2 values of 88 h, 62 h, 135 h, and 240 h for ASA, CFX, TCN and AMX respectively. These antibiotic releases generated antibacterial activity against *Staphylococcus aureus* and *Escherichia coli* that reached almost 100% bacterial reduction and an antibacterial efficacy R > 2. The synergistic anti-inflammatory activity was confirmed by the reduction in the amount of pro-inflammatory cytokines when ASA was mixed with CFX (5.39 ± 0.81 ng·mL−1 TNF-α), TCN (4.70 ± 0.21 ng·mL−1 TNF-α and 49.06 ± 9.64 ng·mL−1 IL-8), and AMX (2.28 ± 0.36 ng·mL−1 TNF-α, 14.84 ± 5.57 ng·mL−1 IL-8, and total IL-6 removal) [36].

Moreover, dialdehyde-β-cyclodextrin (DA-β-CD) crosslinked carboxymethyl chitosan (CMCS) hydrogels were prepared from carboxymethyl chitosan (CMCS) and periodate oxidized β-CD. Phenolphthalein (PhP), a formerly used laxative agent, [16] was selected as a model molecule to investigate the drug loading and sustained release capabilities of such hydrogels. PhP release results show that increasing crosslinking rate between DA-β-CD and CMCS delays the drug liberation process. On the other hand, DA-β-CD/CMCS system displays faster releases, with a 50% release in 2 h

#### *Sustained Drug Release from Biopolymer-Based Hydrogels and Hydrogel Coatings DOI: http://dx.doi.org/10.5772/intechopen.103946*

and about 90% within 12 h, compared to CMCS crosslinked with glyoxal dialdehyde which only releases 19% of PhP after 24 h [37].

CHI based hydrogels (N-succinyl chitosan-g-Poly(acrylamide-co-acrylic acid) were synthesized by free radical mediated cross-linking of N-succinyl chitosan, acrylamide and acrylic acid [38]. Drug delivery capabilities of the system were tested by encapsulation of theophylline, a phosphodiesterase inhibiting drug used for the treatment of respiratory diseases. The drug release experiments showed a pH dependent behavior. In this regard, at pH 1.2 the theophylline released rate was found to be between 14 and 24% whereas at pH 7.4 the release of the drug reached 67–93%. CHI itself has been used as a cross-linking agent for poly(acrylic acid). The resulting hydrogels display pH sensitive properties that have been exploited to control the release of antibiotic amoxicillin and anti-inflammatory drug meloxicam. Concerning this, the release rates of these molecules rise with increasing pH due to the disruption of hydrogen bonds between the hydrogel components and the drugs. As a result 30%, ∼60% and ∼80% of amoxicillin is released after 800 min at pH 1.2, 6.8 and 7.4, respectively. The corresponding release data for meloxicam are ∼20%, ∼70% and ∼90% at pH 1.2, 6.8 and 7.4, respectively [39].

### **4.2 Hyaluronic acid based chemically cross-linked hydrogels**

Hyaluronic acid (HA) is a non-sulfated glycosaminoglycan constituted by repeating disaccharide β-1,4-D-glucuronic acid–β-1,3 N-acetyl-D-glucosamine units that form hydrogels in aqueous solutions. This naturally occurring polysaccharide is found in connective tissues, skin, and synovial joint fluids of the human body. HA displays bio-functionality, biocompatibility, and physicochemical properties, such as viscoelasticity and high-water retention. As a result hyaluronic acid is used for the treatment of dry eye disease, dermatological conditions as well as a as a viscosupplement for the treatment of osteoarthritis [5].

Biocompatible antibacterial hydrogels of HA were synthetized by crosslinking HA solution with divinyl sulfone (DVS) followed by loading with antibiotic molecules. This way cefuroxime (CFX), tetracycline (TCN) and amoxicillin (AMX) loaded hydrogels displayed in vitro antibacterial activity against S. aureus. The antibacterial properties of the hydrogels were synergically enhanced by merging antibiotics with anti-inflammatory agent acetyl salicylic acid (ASA). Consequently it was observed an increase in the log10 reduction value (R) from 3.2, in the absence of ASA, to R 5.55 when TCN or CFX were combined with ASA [40].

Hyaluronic acid was crosslinked with 1,4-butanediol diglycidyl ether (BDDE) and loaded with quetiapine (QTP), an antipsychotic drug, and quercetin (QCT), a hyaluronidase (HAase) inhibitor that decreases the biodegradation of HA. Subcutaneous injection in rats of the system showed that the cHA hydrogel with QCT exhibited a lower maximum QTP concentration (Cmax. 782.6 ± 174.4 ng/mL) and longer halflife (t1/2 23.5 ± 2.7 h) and mean residence time values (MRT 30.9 ± 3.9 h) compared to the hydrogel without QCT (Cmax. 1827.6 ± 481.3 ng/mL, t1/2 13.4 ± 4.9 h, MRT 14.3 ± 4.8 h). These results demonstrated that HAase containing HA hydrogels are suitable systems for sustained drug delivery applications [41].

A thiol functionalized hyaluronic acid HA-SH was used, together with DMSO, for the fabrication of HA-SS-HA hydrogels. This system was loaded with antitumoral drugs such as doxorubicin (DOX), zinc phthalocyanine (ZnPc), and indocyanine green ICG, for implant post peritumoral administration. In vivo experiments validated that drug loaded hydrogel implant possessed satisfactory biocompatibility and

succeeded in long term sustained release of drugs. As a result the system to ensured high tumor aggregation efficiency and adequate tumor suppression [42]. Hyaluronic acid (HA) functionalized with thiol and hydrazide moieties has been combined with oxidized sodium alginate (ALG)to produced cross-linked hydrogels (HA/ALG). These materials display tunable physicochemical properties and drug release behavior as a function of the HA/ALG precursor concentration. In this regard for HA2/ALG2 (2% w/v), HA3/ALG3 (3% w/v) and HA4/ALG4 (4% w/v) the yield stress of hydrogels were 1724, 4349 and 5306 Pa, and the degradation percentage were about 64%, 51%, and 42% after 35 days incubation, respectively. Thus, in vitro cumulative release of Bovine serum albumin (BSA) for HA2/ALG2, HA3/ALG3 and HA4/ALG4 were 79%, 72%, and 69% respectively for a 20 day release assay [43].

Near-infrared (NIR) light-triggered and reactive oxygen species (ROS) degradable hyaluronic acid hydrogels (HPTG) were synthesized through the formation of dynamic covalent acylhydrazone bonds. Such system was loaded with photosensitizer protophorphyrin IX (PpIX) and anticancer drug doxorubicin (DOX), to obtain a with light-tunable on-demand drug release for chemo-photodynamic therapy. In this regard NIR light irradiation generated ROS that induced the required degradation of hydrogel and subsequent on-demand DOX release for cascaded chemotherapy. In vivo imaging-guided antitumor study using 4 T1 tumor- mouse model demonstrated that the treatment of DOX-loaded HPTG with laser irradiation nearly accomplished the suppression of tumor growth without noticeable regrowth [44].

Tyramine functionalized HA solutions were combined silk fibroin (SF) to produce a series of HA/SF hydrogels for application in cartilage tissue engineering an and drug delivery. These hydrogels were loaded with Vanillic acid (VA) or Epimedin C (Epi C), both with anti-catabolic, anti-inflammatory and anabolic effects on human cartilage cells. Hydrogels with HA20/SF80 polymeric ratios displayed the longest and the most sustained release profile with 70.1% release of VA after 60 days of release assay and 54% release of Epi C after 7 days of release. Such behavior makes HA20/SF80 hydrogels a prospective material for the treatment of osteoarthritic joint conditions [45].

Polyethylene glycol (PEG)-HA was modified also with a small biologically active molecule, as dopamine, to fabricate a HD-PEG polymer. This polymer was crosslinked with α -cyclodextrin (α-CD) to afford a polypseudorotaxane supramolecular complex HD-PEG/α-CD. The system was loaded with poly(lactic-co-glycolic acid) (PLGA)/ donepezil microspheres (PDM) in order to evaluate the drug delivery capabilities of the system. The released amounts of donepezil, a drug used for the treatment of mental conditions, reaches 39.9% and 56.7%, after 7 and 14 days respectively. These results demonstrate that the HD-PEG/α-CD/PDM system could be used for the subcutaneous injection of long acting donepezil [46]. Similarly, poly(L-lactide-coglycolide) (PLGA) – dexamethasone (DEX) nanoparticles PLGADEX were combined with crosslinked HA for drug release applications. In this case the chemical crosslinking occurred doubly, by mixing amino-hyaluronic acid and aldehyde-hyaluronic acid in the presence of genipin as a cross-linker agent. Drug delivery experiments showed full DEX release after 2 months for a HPLGADEX hydrogel [47].

Oxidized hyaluronic acid (OHA) was combined with carboxymethyl chitosan (CMC) via Schiff base reaction to fabricate a hydrogel (OHA-CMC) with antibacterial and hemostatic activities. The drug delivery potential of the system was exploited by encapsulating PLGA-PEG nanoparticles of curcumin (CNP) and epidermal growth factor (EGF) that afforded a OHA-CMC/CNP/EGF hydrogel. This system displayed outstanding anti-inflammatory, antioxidant and cell migration-promoting effects *in vitro* and improved wound healing *in vivo* with optimal granulation tissue

#### *Sustained Drug Release from Biopolymer-Based Hydrogels and Hydrogel Coatings DOI: http://dx.doi.org/10.5772/intechopen.103946*

formation, re-epithelialization, and skin appendage regeneration. The cumulative release percentage of CNP reached 55.3% on day 1, 75.5% on day 3 and ~ 90% after 6 days of release experiment. EGF displayed a 28.6% of release on day 1, 51.3% on day 2 and 88.1% in 9 days. These results demonstrate the potential of the hydrogel for the treatment of diabetic wound healing [48].

Finally, HA has been used as well as a biopolymer for the fabrication of a 3D printable dual-network hydrogel with drug delivery capabilities. For that acrylamide-modified HA was synthesized and subsequently mixed with folic acid and Fe3+ to form a physical crosslinking network. Afterwards acrylamide residues were polymerized by ultraviolet radiation affording a material suitable for wound dressing with high elasticity and fatigue resistance. The drug delivery properties were investigated using acetylsalicylic acid (ASA) as a drug model and resulted in a pH responsive hydrogels with the sustained release of ASA over 300 hours [49].

#### **4.3 Other chemically cross-linked biopolymers**

Lignin is a sustainable biopolymer derived from lignol precursors that has been historically related to the paper industry. Hydrogels of hardwood lignin (TCA) have been synthesized through crosslinking with poly(ethylene) glycol diglycidyl ether (PEGDGE) and loaded with paracetamol for drug release applications. Here, decreasing amounts of crosslinker diminishes the interaction paracetamol - hydrogel network and, as a result, the release of paracetamol increases. In this regard, hydrogels produced with a lignin:PEGDGE 1:1 ratio displayed up to 30% of paracetamol release after 120 h assay. The release data follow a pseudo-Fickian behavior of diffusion when fitted to the Korsmeyer-Peppas model [50]. Furthermore, lignin polymers have been mixed with cellulose to generate drug delivery systems. Mechanical and sustained release performances of these gels are tailored by varying the ratio of the precursors: cellulose, hardwood lignin (TCA), and epichlorohydrin (ECH) cross-linker. TCA containing hydrogels display the best release rate (>90%) for drug model paracetamol comparing to the pure cellulose hydrogels (~40%) after 7 hours of release experiment. This behavior is attributed to the lower affinity of paracetamol for lignin compared to cellulose [51].

Cellulose itself have been used for the fabrication of hydrogels with drug release properties. In this regard, carboxymethyl cellulose (CMC) functionalized with β-cyclodextrin and nucleic acids have been crosslinked by using of arylazopyrazoles (AAPs) and loaded with anti-cancer molecule Doxorubicin (DOX). The resulting hydrogel behaves as a functional matrix for the UV light mediated ON/OFF release DOX. Irradiation of the matrix provokes the photoisomerization of the trans-AAP to cis-AAP residues and the generation of the low-stiffness hydrogel that releases DOX. Therefore, the liberation of the DOX could be changed between ON and OFF states by oscillating the photoisomerization of the hydrogel by employing UV/Vis irradiation [52].

Xanthan is a heteropolysaccharide produced by fermentation from the bacteria *Xanthomonas campestris* with applications as thickening agent in food industry as well as pharmaceutical aid and release retarding polymer in drug delivery systems [53]. Hydrogels form this biopolymer have been produced by crosslinking oxidized xanthan, with a PEG hydrazine derivative through pH-responsive hydrazone linkages. The drug delivery properties of the hydrogel were assessed by performing release studies of the antitumoral drug Doxorubicin (DOX), at pH 5.5 (tumoral) and 7.4 (physiological). At pH 5.5 the cumulative release of DOX from 3, 4, and 5% hydrogels was 81.06, 61.98, and 41.67% respectively whereas the release at pH 7.4 was 47.43, 37.01, and 35.34% after 30 days of assay. Moreover DOX-loaded hydrogels possessed cytotoxicity against

A549 cells after exposure to DOX containing released media [54]. Curdlan is another example of polysaccharide produced by fermentation and with applications in the food industry. Phosphorylated curdlan (PC) was crosslinked with,4-butanediol diglycidyl ether (BDDE) and loaded with tetracycline (TCN) to fabricate hydrogels with drug delivery applications. Drug release profiles at equilibrium release (3.5 h), pH 6.8, 37°C reached 87% for hydrogels produced exclusively from phosphorylated curdlan (PC), whereas release from curdlan hydrogels achieved 48% of release [55].

Casein is a proline-rich, open-structured protein found in raw milk. It displays high hydrophilicity, good biocompatibility and a lack of toxicity that makes of it a potential candidate for hydrogel development. Casein can be chemically cross-linked with enzymes such as microbial transglutaminase (MTG). This feature was utilized to produced crosslinked casein -γ-polyglutamic acid (PGA) hydrogels in 1/5 and 1/9 ratio. Drug release experiments showed that both composition displayed similar release rate values for aspirin (~ 100% after 10 h), while 1/9 hydrogels possessed a higher release rate for vitamin B12, ~100% after nearly 12 h versus ~20% for 1/5 casein/γ-PGA hydrogels [56].

## **5. Drug release from hydrogel-based bioactive coatings**

Historically, the development of medical implants has been a great concern for biomedical community. Besides, their need has risen dramatically due to the increased number of surgical procedures that are predicted to be even higher in 2030 [57]. Thus, improving the performance of implantable biomaterials has become a high-priority trend, which is reflected in the large number of research realized to successfully meet the upward demand [58].

Implantable medical devices (e. g. coronary stents, cardiac pacemakers, prostheses, insulin pumps) are classified in four main groups: ceramics [59], polymers [60], composites [61], and metals [62]. Among them, metallic biomaterials as titanium and its alloys are of outmost interest thanks to their inert chemical and biological behavior *in vivo* [63]. Even so, their bioactivity can be improved creating coatings by surface modification techniques and thereby, add other beneficial properties [64].

The use of these bioactive coatings entails the development of an improved version of bioactive materials that modulates biological systems response by the establishment of interactions with adjacent tissues and bones [65]. Nevertheless, these coatings require the most suitable physicochemical, mechanical, and biological functionality for a successful implantation and integration so as not to produce any counterproductive disorder in humans [66]. Therefore, it is imperative to develop functional bioactive coatings onto the surface of biomaterials (**Figure 2**, produced by the authors) that combine biocompatibility [67], antibacterial [68], anti-inflammatory [69], self-healing [70], wound healing [71], bone tissue engineering [72], and osseointegration [73] properties.

Such features can be incorporated onto the surface of biomaterials by the use of biopolymer-based coatings, mostly based on hyaluronic acid and chitosan [74]. Moreover, these coatings acquire hydrogel-like three dimensional microstructure after crosslinking for bioactive agents delivery applications (**Figure 3**, produced by the authors). In such manner, bioactive properties that already possess biomaterials can be upgraded or even provide novel outstanding properties [75]. Specifically, hydrogel coatings take advantage of hydrogels peculiar ability of releasing in a controlled space–time manner to the therapeutic target the entrapped bioactive agents (drugs, proteins, peptides, growth factors, inorganic or polymeric nanoparticles, and nucleic acids) through their polymeric network [76].

*Sustained Drug Release from Biopolymer-Based Hydrogels and Hydrogel Coatings DOI: http://dx.doi.org/10.5772/intechopen.103946*

#### **Figure 2.**

*Bioactive properties of functional hydrogel coatings for biomaterials successful implantation. Figure produced by the authors.*

**Figure 3.**

*Bioactive agents loading and controlled release ability from hydrogel-based coatings. Figure produced by the authors.*

Bioactive agents controlled delivery reduces side effects in patients undergoing implant procedures. In addition, highly stable (from hours to months) hydrogel coatings with great loading ability provide a not sudden, uniform, and prolonged

**Figure 4.**

*A) Conventional and B) in situ crosslinking strategies to form hydrogel coating onto the surface of biomaterials. Figure produced by the authors.*

release of specific low- to high-doses [77]. This way, therapeutic effect of bioactive substances is extended, and the over-excessed concentration peaks of conventional methods are diminished. These features endow hydrogel coatings with privileged pharmacokinetic profiles, which can be modulated for personalized therapies [78]. Further, hydrogel coatings do not need to modulate specific linkages to release bioactive agents since their release mechanisms are mainly governed by simple diffusion, swelling, and degradation processes [79].

Nowadays, researchers are focusing their attention is the hinder of hydrogel coatings attachment to surfaces, which occurs due to hydrogels excessively huge swelling and macroscale thickness. One promising alternative approach to create highly adhesive hydrogel coatings with strong and resistant hydrogel-surface attachment is the *in situ* hydrogel crosslinking onto the surface of biomaterials (**Figure 4B**, produced by the authors) [80]. This way, hydrogel crosslinking and hydrogel coating formation occurs almost at the same time, and thereby, all the active groups available in the structure of biopolymers react equitably with crosslinking agent and surface. Conversely, the conventional strategy of first synthesizing hydrogel followed by covalent grafting to the surface (**Figure 4A**, produced by the authors) limits hydrogel-surface adhesion since hydrogel formation consumed almost entirely reactive functional groups and therefore, few of them remain available for the subsequent linkage formation the surface. Additionally, although this method requires purification steps to eliminate toxic unreacted monomers, crosslinkers and initiators, common dialysis processes are used to easily remove these harmful molecules from coatings before their biomedical real-life application [1].

### **6. Conclusions**

The number of works related to the development of novel biopolymer-based hydrogel systems, mainly those synthesized with hyaluronic acid and chitosan,

*Sustained Drug Release from Biopolymer-Based Hydrogels and Hydrogel Coatings DOI: http://dx.doi.org/10.5772/intechopen.103946*

that promote the sustained release of bioactive agents increases year by year. In the current chapter we have summarized the recent accomplishments of biopolymer based physical and chemically crosslinked hydrogels, as well as hydrogel coatings for drug delivery and sustained release applications. The future perspectives in this field involve the development of hydrogel based medicines with specific temporal and spatial controlled release of drugs. Such medicines afford dose control, local delivery and reduced side effects that increase the efficacy and security of the treatment and the adherence of the patient to it. This strategy will lower pharmaceutical costs and improve the quality of life of the patient and the society overall.
