a mini-review on in situ enzymatic fabrication of · pdf filea mini-review on in situ...

A mini-review on in situ enzymatic fabrication of hydrogels

Hao Liu1,2, Han Zhang2, Xiaomin Zhao1, Jianliang Sun1, Shiyu Fu1 and Shao-Yuan Leu*,2 1State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong,

China 2Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

Polymeric hydrogels have attracted considerable attention in biomedical material research over the past decades. Enzyme-triggered hydrogelation offers significant advantages over the conventional chemical processes, such as superior biocompatibility and process controllability under physiological conditions. A number of oxidases (e.g. horseradish peroxidase and tyrosinase) have been successfully used to drive the crosslinking of gel precursors by oxidizing the phenolic or acrylic moieties to free radicals. Several transferases and hydrolases catalyze elongation of biopolymer chains which gradually self-assemble into hydrogels. Enzymes can also participate in hydrogel formation by releasing gelation factors such as precursors, carboxylic acids, and ammonia. Meanwhile, post-modification inside or on surface by enzymes could promote desired properties of the hydrogel products. This chapter reviews the latest findings on hydrogel fabrication through the enzymatic routes. Perspectives and challengesare also discussed for future consideration.

Keywords: hydrogel; enzyme; crosslinking; biocompatibility

1. Introduction

Hydrogels are insoluble water-swollen networks formed by physical and/or chemical crosslinking between the polymeric constituents. For many decades, hydrogels have attracted much attention as injectable drug delivery vehicles, as scaffolds for tissue engineering and as matrix materials for regenerative medical therapy [1]. For these applications, it is desired to develop green formation technology that allows in situ hydrogelation under physiological conditions. The resultant hydrogels should meet some general requirements including tunable gel properties, injectable properties, appropriate degradability, acceptable cytobiocompatibility, and anti-infection capacity, etc. [2-4]. Enzymatic approach offers interesting opportunities for in situ production of the functional hydrogels with high biosafety and biocompatability. Enzyme process offers some more advantages such as (1) high specificity which minimizes the generation of unwanted structures, (2) mild working conditions, and (3) feasibility of process control. Dozens of enzymes have been investigated for supporting hydrogel fabrication. The commonly used enzymes as well as their catalytic reactions are summarized in Table 1.

A certain enzyme can exhibit multifunctions in mediating hydrogelation (See Table 1). Briefly, direct crosslinks of gel precursors are triggered by oxidoreductases via oxidative coupling, 1,4-Michael addition, and/or Schiff base formation, and by specific hydrolysases or transferases via chain elongation. Particular enzymatic reactions have been designed for indirectly inspiring hydrogelation by releasing metal ions or basic molecules. Several enzyme technologies have also been developed to post-modify the hydrogel networks for constructing sophisticated hydrogel products with multifunctional properties. The rapidly growing interest in enzymatic hydrogel technologies has even stimulated the discovery and application of novel enzymatic reaction routes, e.g. a self-supported peroxidase reaction [2, 5]. However, there is still lack of a review that systematically summarizes all the major enzymatic approaches for hydrogel synthesis. Previous articles either discussed certain categories of enzymes (e.g. peptide-related enzymes) [6] or concluded a special type of enzymatic reactions (e.g. crosslinking) [7]. In this chapter, we focus on the enzyme functions and broadly describe the enzymatic reactions that contribute to hydrogel formation. Enzymes extensively reported in recent ten years are discussed while those infrequently used or commercially unavailable enzymes are briefly mentioned.

2. Oxidases-induced crosslinking for hydrogelation

2.1 Horseradish peroxidase

Horseradish peroxidase (HRP, EC1.11.1.7), a calcium-containing, extensively glycosylated, stable haemoprotein, catalyzes the oxidation and polymerization of aromatic compounds in the presence of H2O2. HRP is one of the most commonly used enzymes for biosynthesis of hydrogels. As early as 1968, HRP was studied to form insoluble gels from soluble collagens by crosslinking the tyrosine moiety to dimers [8]. Then in 1990, Izydorczyk et al. and Moore et al. separately described the enzymatic gelation process of water-soluble wheat pentosans and their fractions, arabinoxylan and arabinogalactan [9, 10]. HRP crosslinked the feruloyl groups in these polysaccharide chains, inducing gelation at a much lower rate than ammonium persulfate [9].

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

1

Table1 Enzymes extensively studied for hydrogel fabrication

Enzyme Catalytic reaction Functions in hydrogel fabrication

Horseradish peroxidase (HRP)

Oxidation and polymerization of aromatic compound in the presence of H2O2

(1) Oxidative coupling of phenolic moieties in presence of H2O2, or GOX, or thiol; (2) Initiating radical polymerization in presence of acetylacetone

Glucose oxidase (GOX)

Oxidation of β-D-glucose to gluconic acid with simultaneous release of H2O2 from oxygen reduction

(1) Supplying H2O2 for HRP; (2) Generating hydroxyl radicals with Fe2+; (3) Reducing specific substrates to be radical initiators; (4) Releasing Ca2+ by acidifying CaCO3 for alginate gelation; (5) Interfacial hydrogel coating

Tyrosinase (TYR) Conversion of phenolic substances to the corresponding quinones

Crosslinking through C-C coupling, 1,4-Michael addition, or Schiff base formation

Laccase (LAC) Oxidation of diphenol to free radicals for subsequent non-enzymatic coupling

(1) Crosslinking through C-C coupling, 1,4-Michael addition, or Schiff base formation; (2) Generating Fe3+ from Fe2+ for alginate gelation

Protease Hydrolysis of peptide bonds in a polypeptide chain

(1) Producing precursors via transesterification of acrylate to polysaccharides; (2) One-pot transesterification and crosslinking of polysaccharides; (3) Post-modifying the hydrogel networks

Lipase Hydrolysis of lipids (fats) to produce glycerol and fatty acids

(1) Producing precursors via transesterification of acrylate to polysaccharides; (2) One-pot transesterification and crosslinking of polysaccharides; (3) Producing precursors via esterification

Phosphorylase Addition of a phosphate group from an inorganic phosphate

Gradual chain growth via transesterification and consequent formation via self-assembly

Phosphatase Cleavage of phosphate groups from substrates

(1) Hydrogelation of precursors to desired hydrogelators which form hydrogel via self-assembly; (2) Sol-gel transition

Kinase Transfering a phosphonate group from ATP to an acceptor

(1) Phosphorylation of fibrinogen for thrombin-induced gelation; (2) Sol-gel transition

Transglutaminase (TG)

Formation of an isopeptide bond between the γ-carboxyamide group of glutamine and the ε-amino group of lysine

Generating a permanent network of polypeptides

Urease Hydrolysis of urea into NH3 and CO2 (1) In situ generation of base for hydrogelation; (2) Calcification of hydrogels

Until recently, HRP still drew the attention of researchers in triggering direct gelation of natural polymers [11, 12].

Gellable sugar beet pectin were rapidly crosslinked and gelled (within 1 min) via an oxidative coupling reaction of original feruloyl groups catalyzed with HRP and H2O2. The obtained hydrogels could be gradually degraded over time within 12 days. In situ gelation did not cause damage of encapsulated cells; and implantation did not induce necrosis in the surrounding tissue [11]. In an improved recipe, gelatin was supplied, resulting in more readily degradable hydrogels under simulated physiological conditions [12]. There are abundant of tyrosine residues in silk fibroin (~11 wt%) and gelatin (~20% wt%) [13, 14]. Injectable hydrogel was thus fabricated by Brown et al. based on the HRP-induced crosslinking of silk fibroin. The material showed promise as a tissue bulking agent for cerclage therapy [15]. Typical hydrogelation triggered by HRP is illustrated in Fig. 1.

Fig. 1 Scheme of typical oxidative crosslinking in hydrogel fabrication triggered by HRP/H2O2 or HRP/GOX

HRP/H2O2 or HRP/GOX


2

The available species of natural polymers suitable for hydrogel fabrication are rather limited. Researchers now pay more attention to the monomer-crosslinker conjugates. For example in an early article, Polyethylene glycol (PEG)-Dopa conjugates were enzymatically crosslinked to form gel for resembling melanogenesis[16, 17]. Some more findings during the past five years were summarized in Table 2. In general, the monomer can be a natural polymer (e.g. dextran), or a synthetic polymer (e.g. PEG), or a small molecule (e.g. hyaluronic acid) depending on the objective properties. Tyramine and its derivatives are most frequently selected as the crosslinker. The presence of primary amines allows tyramine readily coupling with carboxyl monomers through the EDC/NHS reactions. Gelation initiated by HRP/H2O2 was very fast, usually within seconds or minutes, according to the literature (See Table 2). The major physical properties of hydrogel such as swelling, porosity, mechanical strength could be feasibly controlled by tuning the dosages of enzyme and H2O2. For potential biomedical applications, the biocompatibility of hydrogels always need to be concerned, including cell toxicity, antimicrobial properties, in vitro degradability, drug release and delivery capacities.

Table.2Hydrogelation initiated by HRP in recent years (2012~2016) Precursor Catalysts Properties and applications Ref.

Epsilon-poly-L-lysine conjugated with hydroxypropylacrylate (HPA)

HRP/H2O2 Rapid gelation within several seconds. Adjustable gelation rate,mechanical strength and degradation behavior. Adhesiveness10~35kPa. Excellent cytocompatibility. Good anti-bacteria and anti-infection activity.

[18]

Carboxymethyl pullulan-tyramine conjugated with chondroitinsulfate-tyramine

HRP/H2O2 Gelation time 36~287 s. Feasible modulation of physicochemical properties. Good cytocompatibility. Facilitating chondrogenesis. Acceptable tissue compatibility.

[19]

PEG-b-poly(L-glutamate-g-tyramine)

HRP/H2O2 Hydrogel nanoparticles with diameter of 125 nm. pH-sensitive protein release. Non-toxic up to 2.0 mgmL−1. Efficiently delivering and releasing proteins into HeLa cells.

[3]

Periodate-oxidized chitosan–PEG–tyramine

HRP/H2O2 Hydrogelation within a few seconds.Cytobiocompatible. Good tissue-adhesive ability and wound healing properties.

[4]

poly(γ-glutamicacid)–tyramineconjugates

HRP/H2O2 Gelation time 25 s ~ 5 min. Storage modulus40 to over 1100 Pa. Swelling ratio 110~470. Controlled release of bovine serum albumin (BSA). 68~90% release in 60h depending on crosslink density and mesh size.

[20]

Tyramine conjugated high methoxyl content gum tragacanth

HRP/H2O2 Rapid formation of gel within 1 min. Equilibrium swelling degree 10~20. Storage modulus less than 100 Pa. Tunable rheological properties. Non-toxic up to 0.05%(w/v). Good BSA release profile with limited burst release.

[21]

Ph-chitosan conjugated with Poly(vinyl alcohol) (PVA)-tyramine

HRP/H2O2 Gelation time 27~37 s. Swelling ratio 1~2. Controllable cell adhesion. Antibacterial activity depending on chitosan percentage.

[22]

Tyramine grafted poly(L-glutamic acid) conjugated with PEG

HRP/H2O2 Gelation time 20~160 s. Injectable hydrogel with pore size of 20~120 µm depending on H2O2 concentration. Controllable storage modulus of 1600 to 2300 Pa. Swelling ratio 30~50. High viability towards L929 fibroblast cells. Persisting for up to 10 weeks in vitro.

[23]

Tyramine–tetronic–grafted chitosan

HRP/H2O2 Gelation time 4~60 s. Highly porous structure controlled by H2O2 concentration. Swelling ratio around 40. High cytocompatibility. Good cell adhesion. No inflammation after two weeks and one day of the in vivo injection.

[24]

Poly(amidoamine) copolymers containing tyramine residues

HRP/H2O2 Gelationtime50~350s. Completedegradation within 6~8 days underphysiological conditions. Storage modulus 2500 to 4100 Pa. Cyto-biocompatible. Effective release of methylene blue and IgG protein.

[25]

Alginic–hyaluronic acid composite

HRP/H2O2 Gelation time < 10 s. Swelling ratio 6. Storage modulus 6 kPa. Absence of burst release and low release of BSA. Degraded in body. High cell viability.

[26]

PVA-tyramine conjugates HRP/GOX Hydrogelation within 20 s at normal blood glucose concentration. Good compression and stretching properties. Potential use in wound healing.

[27]

Alginate-tyramine conjugates HRP/GOX Gelation time 60~800s. Good mechanical properties. Potential use as wound healing material.

[28]

Bionanocomposites and PEGMA*

HRP/GOX/AcAc

Desirable mechanical properties. Stable activity of self-immobilized HRP/GOX.

[29]

Magnetic Fe3O4 nanoparticles, PEGMA and PEGDA*

HRP/GOX/AcAc

Magnetic core–shell microgelsfor glucose detection. Good thermal stability and reusability

[30]

Thiol-functionalized propylene glycol

HRP Gelation time 110 ~ 250 min. Storage modulus 1~10 kPa. Good cell encapsulation. Redox sensitivity

[5]

Thiolated PEG and tyramine HRP Gelation time 27 ~ 60 min. Good cytocompatibility. Mild degradation. [2]

*Note: PEGMA= Polyethyleneglycol monomethylether methacrylate; PEGDA=Poly(ethylene glycol) diacrylate

Many authors have suggested the use of glucose oxidase (GOX) to gradually generate H2O2 and thereby form a mild initiation system with HRP, which could avoidthe detrimental effect of excess H2O2[29]. Similar with HRP, GOX also


3

functions under physiological temperature and pH ranges. The results of some representative studies were summarized in Table 2. It was worth to note that replacement of exogeneousH2O2with enzymatic supply can improve the mechanical properties of resultant hydrogel. For instance, PVA-tyramine hydrogel and alginate-tyramine hydrogel obtained through the co-enzymatic process all showed significantly larger compression stress than that obtained through direct addition of H2O2[27, 28]. A probable reason is that inactivation of HRP by H2O2was suppressed, rendering the hydrogel high cross-linking density[5].

Another typical system using HRP/H2O2 can initiate radical polymerization by introducing acetylacetone (AcAc) to form an AcAc-HRP-H2O2 ternary system. HRP catalyzes H2O2 reacting with the enol form of ACAC to generate carbon-centered AcAcradicals. The radicals could then initiate the polymerization of a precursor (e.g. PEGMA).H2O2can also be yielded from the catalysis of GOX. Wang’s group recently developed a self-immobilized HRP/GOX hydrogel comprising inorganic nanoparticles, AcAc and PEGMA. The hydrogel can be fabricated in forms of layer or microsphere and have shown potential application in colorimetric glucose detection[29, 30].

A novel HRP-mediated polymerization without GOX or exogenous H2O2was developed by Groll and coworkers in 2013. By the auto-oxidation of thiol substrates in presence of O2, H2O2 was generated for HRP-catalyzed disulfide-cross-linking. Ahydrogel was finally obtained which showed redox-sensitive degradation behavior in reductive cytosol-like environments[2]. This gelation system is very simple, but requires a basic pH (pH 8.5) and a long gelation time (>110 min). Moriyama et al. improved the process by adding to the tyramine reaction. A shorter gelation time of ∼30 min was observed under physiological conditions[5].

2.2 Glucose oxidase

Glucose oxidase (EC 1.1.2.3.4, GOX) is a flavoprotein that catalyzes the oxidation of β-D-glucose to gluconic acid with simultaneous release of H2O2 from oxygen reduction. GOX was most frequently used as a source of H2O2 in hydrogel formation, supporting the polymerization catalyzed by peroxidases as discussed above. In presence of Fe2+, hydroxyl radicals are generated through reduction of H2O2 which is capable of initiating radical polymerization to form crosslinked hydrogel. A pioneering work by Iwata et al. verified that oxygen played a crucial role in GOX/Fe2+ initiation system although oxygen strongly prohibits radical polymerization (See Fig. 2) [31]. In a following report on HEA/PEGDA copolymerization, the use of GOX/Fe2+ allows hydrogel formation within minutes without the need of prior purge of oxygen [32]. The polymerization rate was increased with increasing glucose concentration (<1 mM) and Fe2+ concentration (0.1~0.5 mM) suggesting the process was kinetically controllable. Excess Fe2+ reduced final acrylate conversion; and such irons’ inhibitory can be minimized by maintaining a high glucose/iron concentration ratio[32]. Zavada et al. reported accordant results from the thiol-ene hydrogelation initiated by GOX/Fe2+[33]. Complete reaction required 25~30min, converting 90% of monomers into hydrogel. As a comparison, the use of HRP instead of Fe2+

significantly accelerated the process, refraining from the inhibitory by iron. But concerning the cost and availability, GOX/Fe2+ is no doubt an excellent candidate that polymerizes upon exposure to aerobic conditions [33]. (A) (B)

Fig. 2 Scheme of polymerization induced by (A) GOX/Fe2+ and (B) HRP/GOX

GOX/glucose system can readily induce the reduction of many N-hydroxyimide, quinones and indophenols compounds to be carbon-centered free radicals. These radicalsthereafter initiate the polymerization of acrylamide in an aqueous media. It was as early as 1992, Wilson and Turner summaried the appropriate electron acceptors (oxidizing substrates) for GOX in addition to O2[34]. Later, Trivic et al. discussed the reduction mechanism of several xenobiotic compounds by glucose under catalysis of GOX [35]. Free radicals were detected from the enzymatic reduction of p-nitroso-N,N-dimethylaniline, methyl-1,4-benzoquinone, and 7,7,8,8-tetracyano-quinodimethane [35]. This reaction has been attached great importance recently. Su et al. created enzyme-responsive hydrogels by using an enzyme-mediated redox initiation system involving GOX, N-hydroxyimide–heparin conjugate (radical source), PEGDA (precursor) and glucose[36]. Hydrogels were obtained within 30 min in absence of O2. Indeed, dissolved O2 in the gelation media could compete for electrons to generate H2O2, thereby inhibiting the polymerization[36]. The specific enzyme-responsive properties depend on the species for conjugation with N-hydroxyimide.


4

2.3 Tyrosinase

Tyrosinase (EC 1.14.18.1, TYR) is a group of bicopper monooxygenase that converts phenolic substances to the corresponding quinones. TYR in vivo catalyzes its natural substrate tyrosine to be melanin via (1) enzymatic hydroxylation of tyrosine into 3,4-dihydroxyphenylalanine, or Dopa for short, (2) enzymatic conversion of Dopa to Dopa quinone which spontaneously converts into dopachrome and consequently into melanin [37]. There are abundant of tyrosine residues in silk fibroin (~11 wt%) and gelatin (~20% wt%) [13, 14]. These tyrosine residues are oxidized by TYR/O2 to be o-quinone structures which could subsequently crosslink with nucleophile groups through C-C coupling, 1,4-Michael addition, or Schiff base formation (detailed scheme is shown in Fig. 3)[38]. In a well-developed silk fibroin physical gelation system, the addition of TYR could save the time from 65 h to 38 h, and further to 28 h when incorporated with catechol [38]. However, the introduction of TYR led to unwanted coloration of hydrogel products; the more TRY used, the deeper color obtained [38]. In a work by Das et al., TYR together with live cells were dispersed in mixed solutions of silk fibroin and gelatin for printing 3D tissue constructs[39]. Compared with sonic treatment, enzymatic crosslinking required more time for gelation and resulted in colorful products. But the incorporation of TYR in the process significantly improved the bioactivity and stability of 3D hydrogels [39]. Besides, peptides containing tyrosine units, e.g. 2-NapGFFY, were also used as precursor for enzymatic polymerization [40]. The model peptide 2-NapGFFY tended to self-assemble into fibrous networks driven by the stacking of the N-terminal 2-naphthalenyl group (2-Nap) as well as the diphenylalanine motif. The addition of silica nanospheres induced the physically crosslinked peptide chains to form hydrogel. Further treatment with TYR oxidized the tyrosine residue into Dopa, creating dopa-based crosslinking and additional interactions between the fibrils and silica nanoparticles. The supramolecular hydrogels via TYR-triggered crosslinking showed an enhanced mechanical stability by more than 3,000 times [40].

Fig.3 Schematic illustration of TYR/O2– mediated polymerization for hydrogel synthesis Hydrogelation of polymer-tyramine conjugates have also been studied on basis of TYR catalysis. Jin et al. reported TYR-mediated crosslinking of chondroitin sulfate-tyramine conjugates under physiological conditions [41]. They found the gelation times ranged from 2.3~129 min depending on the polymer concentration, substitution degree of tyramine and enzyme dosage. The hydrogel showed well-interconnected porosity, good biodegradability, high elasticity and excellent biocompatibility [41]. TYR/O2 exhibited lower efficiency than HRP/H2O2 according to Jin’s results [41, 42]. Comparison of the two catalytic systems for crosslinking chitosan-glycolic acid/tyrosine conjugates showed that gelation by HRP/H2O2 was much faster, but hydrogels from TYR/O2 catalysis had a lower cytotoxicity, more suitable

TYR/O2 TYR/O2 NH2

Schiff base

Michael addition

Coupling


5

for biomedical applications[42]. Another comparison of TYR/O2 with NaIO4 for synthesis of fluorescent hydrogels suggested that TYR had more excellent specificity to target moieties with little damage to the polymer networks [1].

2.4 Laccase

Laccase (EC 1.10.3.2, LAC) is another group of copper-containing phenol-oxidizing enzymes. LAC oxidizes a broad range of substrates in presence of O2 including monolignols, diphenols, polyphenols, aromatic amines as well as some inorganics [43]. Although showing an overlapping substrate range with TYR, most LACs do not oxidize tyrosine while TYR has no activity towards syringaldazine [44]. Therefore, applications of LAC in hydrogel formation primarily incorperated plant-derived phenolic crosslinkers rather than tyrosine derivatives. Polymerization of LAC starts from the generation of free radicals from phenolic substrates. Subsequent non-enzymatic coupling results in the formation of C-C and C-O bonds between monomers[45]. An example is that ferulated arabinoxylans were cross-linked by LAC leading to the gelation via dimerization and trimerization of ferulic residues [46]. A more recent study demonstrated that the microgels electrostatically formed by gelatin and pectin were strengthened and stabilized after the oxidative crosslinking of pectin ferulic residues [47]. Phenolic radicals from LAC catalysis can also non-enzymatically react with nucleophiles such as amino groups from chitosan and gelatin by Schiff base formation or 1,4-Michael addition (also See Fig. 3)[48]. Rocasalbas et al. studied the crosslinking of chitosan and gelatin with plant polyphenols in presence of LAC. Hydrogels with antibacterial and antioxidant properties were obtained within 1~2 h [48]. Our research suggests that catechol can be used as crosslinker instead of polyphenol. However, no hydrogel could be generated from the control experiment without addition of LAC or catechol. In addition to polyphenols, acrylic derivatives can also serve as crosslinker for LAC-induced polymerization. But except acrylamide, other acrylates required the need of mediators (e.g. β-diketones) [49]. Nieto et al. polymerized PEGDA with LAC and a block-copolymer mediator (F68) to generate bioactive hydrogels with macroporous structures. Although O2 inhibits radical polymerizations, it is a mandatory co-substrate for laccase catalysis. Simultaneous addition of GOX and catalase increased the yield of hydrogel from 48% to 60%, suggesting the crucial role of oxygen balance [50]. Dual initiating/inhibiting roles of oxygen can be a major reason for the relatively low catalytic efficiency of LAC/O2 as compared with HRP/H2O2[51]. There are some othe oxidases which could catalyze crosslinking for hydrogel fabrication include manganese peroxidase [52], lysyl oxidase [53], xanthine oxidase [54]. These enzymes were not discussed in this review because they are commercially unavailable and thereby less of a concern in recent years.

3. Enzymatic chain elongation for hydrogel formation

3.1 Protease

Protease refers to any enzyme that catalyzes proteolysis, the hydrolysis of peptide bonds in a polypeptide chain. There are a large number of subcategories of protease due to the protein diversity of the biosphere. Bacillus subtilis protease (EC 3.4.21.62), cormercially named as Proleather FG-F, has been successfully used to activate polysaccharides for hydrogel fabrication. In Ferreira’s pioneering work, FG-F catalyzed the transesterification of vinyl acrylate (VA) to dextran and inulin in dimethylformamide [55, 56]. Then two differentstrategies were studied to obtained dextran-based hydrogels. Selected enzymes allow a one-pot transesterification and crosslinking of dextran with divinyladipate (DVA) in neat dimethylsulfoxide (DMSO) (see Fig. 4). Both Proleather FG-F and two lipases yielded >58% conversion of the dextran and finally resulted in the formation of a gel [57]. In the other method, the enzymatically activated dextran ester was crosslinked using enzymatic means, e.g. the addition of tetramethylethylenediamine (TEMED) and ammonium persulfate (APS) [56]. Another polysaccharide, starch-derived dextrin, has also been enzymatically grafted with acrylate groups and crosslinked to gel through radical polymerization[58, 59].One-pot enzymatic reactions spent quite a long time for hydrogelation, probably due to the use of neat polar organic solvents. The proleather protease and lipase were found not fully dissolved in DMSO; moreover the activities were strongly inhibited although there was still activity remained after 12 h[56]. It deserves to screen novel green solvents, e.g. ionic liquids, for supporting both protease catalysis and hydrogel formation.

Fig. 4 Scheme of one-pot transesterification and crosslinking of dextran with DVA catalyzed by protease or lipase

DVA Hydrogel

Enzyme in DMSO, 50°C +

Dextran


6

3.2 Lipase

Lipase is a subcategory of esterases catalyzing the hydrolysis of lipids (fats). Bacterial lipases (EC 3.1.1.3) were formerly used in transesterification of monosaccharides with vinyl acrylate in pyridine to generate the 6-acryloyl esters. The esters were polymerized with AIBN and crosslinked by ethylene glycol dimethacrylate to give a hydrogel (see Fig. 4)[60]. Recently, a novel hydrogel precursor, hyaluronic acid vinyl esters, was synthesized through the lipase-catalyzed transesterification. 3D hydrogel constructs were subsequently fabricated with addition of a photo initiator [61]. As described above, lipases produced by Candida rugosa and Pseudomonas cepacia could also inspire the one-pot dextran activation and crosslinking. The two lipases followed a similar reaction pathway as protease but resulted in a lower monomer conversion [56]. Lipase-mediated esterification has recently been more concerned for producing the precursors or hydrogelators[62]. Under catalysis of lipase, graft copolymerization of acrylic acid onto gum tragacanth was carried out. When glutaraldehyde was previously mixed in the polymerization system, a swelled hydrogel was finally obtained which had excellent water-holding and urea-releasing capacities for soil amelioration [62]. Lipase also supports the chain growth of peptides. Tripeptides have been prepared by the peptide bond formation between 9-fluorenylmethoxycarbonyl-phenylalanine (Fmoc–Phe) and the dipeptide diphenylalanine using lipase as catalyst. After 48 h, hydrogels formed from the self-assembly of the Fmoc–tripeptides under physiological conditions. The self-assembly was driven by π–π stacking interactions of the Fmoc groups[63]. Lipase-catalyzed inclusion of p-hydroxybenzylalcohol to peptide bolaamphiphiles formed an activated diester building block that self-assembled to produce nanofibrillar thixotropichydrogels. After sophisticated post-modification steps, thixotropic nanofibrillar hydrogel was obtained and could serves as a scaffold for stem-cell proliferation[64].

3.3 Phosphorylase, kinase and phosphatase

The term phosphotransferasebroadly refers to a transferase that catalyzes the addition of a phosphate group to an acceptor. In particular, a subgroup that catalyzes the addition of a phosphate group from an inorganic phosphate is defined as phosphorylase. Another group that transfers a phosphonate group from ATP to an acceptor is denoted as kinase. Phosphatase belongs to hydrolysase, which removes phosphate groups from substrates. Althoughacting by different mechanisms, the three enzymes can all support hydrogel formation. Phosphorylases from different sources have been studied for the chain elongation of carbohydrates for many years. A typical synthesis involves glucose 1-phosphate (Glc-1-P), glycogen and the enzyme, e.g. glucan phosphorylase (EC 2.4.1.4). New α-(1→4)-D-glucosidic linkages can be constructed by the phosphorylase-catalyzed glycosylation using Glc-1-P as a glycosyl donor, leading to gradual chain growth and consequent formation of hydrogel [65]. The publications (before 2013) on phosphorylase-catalyzed enzymatic polymerization have been reviewed by Kadokawa et al. [66, 67]. Their latest work described the phosphorylase-catalyzed successive reactions to prepare pH-responsive amphoteric glycogen hydrogels[68]. They used a thermostable α-glucan phosphorylase to synthesize an amphoteric glycogen first; and then incubated the amphoteric glycogen with Glc-1-P and phosphorylase. The resultant hydrogel is solublized in base due to the dissociation of amylose double helical conformation. By adjusting the pH to 9, amylose chains reformed double helices, which served as the crosslinking spots for re-hydrogelation[68]. Phosphatase has been applied to transform a subtrate-derived precursor to a hydrogelator that subsequently forms three-dimensional fibrous networks via a self-assembly process. A photosensitive precursor was designed by Li et al. containinga short peptide motif,an azobenzene, and a tyrosine phosphate residue[69]. Alkaline phosphatase triggered the hydrogelation of the precursor to the desired hydrogelator from which hydrogels formed after 6 h. Such a mild process allows the hydrogels to respond to the expressions of functional molecules; for example, the photosensitivity corresponded to the trans–cis isomerization of azobenzene induced by light[69]. A similar work was reported by Wang et al. A synthetic precursor, folic acid- phosphorylated tripeptide (GpHK)-Taxol, was converted by phosphatase to the hydrogelator, FA-GYK-Taxol, which finally self-assembled to hydrogel nanosphere[70]. Kinases-assisted hydrogelation has also been reported by several authors. Martin et al. reported in 1991 the thrombin-induced gelation of fibrinogen phosphorylated by kinases [71]. Then Suket al. investigated the synergistic stimulation of fibrinogen gelation by casein kinase II in the presence of polycationic compounds [72]. Later, Yang et al. developed an interesting kinase/phosphatase switch to regulate a supramolecular hydrogel. A gel-sol phase transition was achieved by adding a kinase and adenosine triphosphates (ATP) to convert tyrosine residue into hydrophilic tyrosine phosphate [73].

3.4 Transglutaminase

Transglutaminase (EC 2.3.2.13) catalyzes the formation of an isopeptide bond between the γ-carboxyamide group of glutamine and the ε-amino group of lysine [74]. Microbial transglutaminase (MTG) has been widely investigated for generating a permanent network of polypeptides as a tissue engineering scaffold. Potential precursors include gelatin, rationally synthesized peptides, peptide-PEG conjugates, casein-polysaccharide hybrids, etc.[74-77]. The gelation time was generally determined by the precursor functionalities, initial enzyme loadings and substrate kinetics[78]. MTG maintains a high activity level over a broad range of working conditions (50% at 37°C, max. at 50°C; 90% pH 5 ~ 8), which makes it attractive for drug releasing and tissue engineering applications[74]. Studies on swelling ratio of the


7

gelatin-based hydrogels suggested that swelling kinetics fits the Schott’s equation, but the swelling ratio was dramatically increased when heated from 25°C to 30°C [79].More specific details about MTG are availble in a latest review by Domeradzka et al., as well as some other bioactive molecules for peptide hydrogelation[7].

4. Release of gelation factors from enzyme catalysis

4.1 In situ generation of metal ions

Divalent and multivalent metal ions crosslink the carboxylate groups on polysaccharide chains, forming insoluble network junctions. A cascade reaction initiate with GOX was accordingly designed to fabricate calcium alginate hydrogels. Glucose was firstly oxidized to gluconic acid which subsequently reacted with calcium carbonate to release calcium ions. The resulting alginate hydrogels have found extensive applications as matrices for drugrelease or cell immobilisation [80]. In a similar work, the ferroxidase activity of laccase was utilized to oxidize Fe2+ into Fe3+ which binds to alginate polyanions. Monodisperse hydrogel nanoparticles were formed as a result of cross-linking between alginate chains via the newly formed trivalent cations. The growth of hydrogel nanoparticles is tunable at nanoscale, for example, from a diamter of 2.5 nm to 100 nm, by controlling the rate of enzymatic reaction [81].

4.2 In situ generation of base

In low-pH-induced gelation, the concentration of bases determines the crosslinking rate. Gradual in situ generation of soluble base is desired for constructing homogeneous intra networks. Urease, hydrolyzing urea into ammonium and carbon dioxide, allows in situ pH modulation with minimized gradients. Chenite et al. reported that the pH rose from 5.3 to 6.2 after incubation with urease and urea for 15 min [82]. Then the G’ of reactants started to increase with time following the first order kinetics. A higher temperature (e.g. 45°C) was suggested for obtaining maximal catalytic activity as well as good substance diffusion. Gelation time could be decreased to 2~3 min when sufficient enzyme and substrate participated in [82]. A more recent work discussed the potential role of urease/urea in triggering the base-catalyzed Michael addition of a soluble trithiol to PEGDA [83]. These two starting chemicals were polymerized to be propagating fronts via autocatalytic reactions at base pH. Therefore, a time lapse of frontal polymerization and gelation occurred until the pH was switched to 7 by urease catalysis. Accumulated bases subsequently degraded the hydrogels, which enabled convenient tuning of the gelation rate and the gel lifetime by changing the concentration of enzyme or substrate [83].

5. Post-modification of hydrogel by enzymes

5.1 Interfacial coating

Formation of multi-layered hydrogel is significant in manufacturing smart microdevices for controlled drug delivery and in engineering cartilage and bone tissues [84]. Post-modification of hydrogel substrates with interfacial gel coatings can be an effective way to obtain desired characteristics, e.g., physical strength, water permeability, size exclusion, cell type sensitivity [85]. The GOX/Fe2+ system has been investigated by Bowman’s group to catalyze interfacial polymerization on a PEGDA hydrogel surface [85, 86]. The core hydrogel obtain via photopolymerization in presence of glucose was immersed in the precursor media containing GOX, Fe2+ and appropriate acrylate monomers. A conformal, micrometer-scale, uniform hydrogel layer could thus form within 15~120s and keep stable in 4 months. Sequential multilayer coatings could be obtained by reswelling the layered hydrogel with glucose following with exposure to the precursor media [85]. Shenoy and Bowman studied the kinetics of such interfacial polymerization and found positive corelations between the layer thickness and the major biocatalysis parameters including glucose content, Fe2+ concentration and reaction time [86]. When coating with the second layer, a few seconds were enough for the monolayered hydrogel immersed with fresh GOX; meanwhile, hours were required for the reference without immersion to form a layer with a comparable thickness. This suggests the GOX trapped in the polymerization front had a significantly lower activity compared with that in the bulk media [86]; in another word, the diffusion of trapped GOX and some other target proteins was rather limited.

5.2 Re-assembly of networks

In addition to the case of enzymes-switched sol-gel transition, many studies on the development of hydrogel networks by protease have been reported. A protease-assisted photolithography technology was established by Gu and Tang to modify inert hydrogels with arbitrary large-area patterns and functional sites [87]. The original PEG hydrogel film contained a bisacrylated peptide crosslinker with an amino acid sequence digestible to protease. The peptide was caged by a photo labile moiety and could be decaged under UV light at a patterned area. The protease triggered the proteolysis of peptide following the patterns, resulting in fresh nucleophilic amine groups for further functionalization [87]. The


8

cleavage of original peptide network by protease could lead to the transition of chemical crosslinking to physical crosslinking as reported by Ayyub and Kofinas. In the protease-modified hydrogel, the newly formed secondary, physically cross-linked network had a 12-fold increase in storage modulus than the original one [88]. The enzymatic hydrogel development strategy can also be applied in preparation of biomedical materials. In a recent research, collagenase and collagen microspheres pre-seeded with cells wereencapsulated into the photocrosslinked alginate hydrogels. Collagenase degraded collagen microspheres and thus enlarged the space for cell spreading and migration. The scaffold properties were feasibly tuned by changing the amount of collagenase and microcarriers [89].

5.3 Calcification of hydrogels

Mineralization of hydrogels with calcium salts (or simplified as calcification) is an important step in biomimetic synthesis of bone-like composites and nacre-like materials [90, 91]. Conventional strategies such as soaking and the Kitano method address the drawbacks that precipitating calcium compounds blocked the diffusion pathways inside hydrogel matrices which caused the hydrogel was calcified mostly at the surface[92]. Enzyme-directed calcification has been recently developed to overcome these problems. Alkaline phosphatase (ALP) releases inorganic phosphate groups from hydrolysis of organic phosphoesters resulting in local deposition of carbonated apatites[90]. Spoerke et al. described the biomimetic hydroxyapatite calcification in bone templated with an amphiphilic nanofiber hydrogel that also contained CaCl2. After incubated with β-glycerolphosphate and ALP for 8 days, calcification was visibly apparent throughout the volume of the gel. Gradual enzymatic harvesting of phosphate groups minimized the unwanted precipitates during the mineral crystal growth in hydrogel [93].

Urease action produces carbonate ions from the cleavage of urea, which has been used to induce the crystallization of Ca2+[91, 94]. Rauner and his coworkers immobilized urease onto chemically-crosslinked hydrogel films and mimicked the calcification by immersing the films in CaCl2/urea solutions. Various forms of CaCO3 crystals wereselective formedby controlling the enzymatic reactions. The growth of crystals led to a dense, nonporous inner structure and strong interaction between crystals and network (e.g. stiffness was increased by up to 700 folds)[94]. Their following research showed a calcification degree of up to 94 wt% was obtained from the urease-directed mineralization of poly(2-hydroxyethylacrylate)/triethylene glycol dimethacrylate co-networked hydrogels. The introduction of CaCO3 crystals improved the Young’s moduli of the composites (e.g. 40~300 MPa) but did not increase the stiffness [91].

6. Conclusions and future perspectives

There are many enzymatic reactions that contribute to hydrogel formation. HRP, TYR and LAC primarily drive the C-C coupling of gel precursors via an oxidation of phenolic moieties. Besides HRP/GOX/AcAc, GOX/Fe2+ and LAC/O2 can initiate free radical polymerization. Several transferases and hydrolases such as protease, lipase and phosphorylase catalyze elongationof polysaccharide chains which gradually self-assemble into hydrogels. Enzymes can also participate in hydrogel formation by releasing gelation factors such as carboxylic acids, ammonia,etc. Post-modification such as interfacial coating, network development, mineralization by enzymes could promote desired properties of hydrogel products. These enzymatic approaches offer opportunities for in situ production of functional hydrogels with high biosafety and biocompatability. There are several exciting developments in prospect for the future of enzymatic hydrogelation. (1) Synergistic actions of dual enzymes are promising in constructing multifunctional hydrogel materials (e.g. HRP/MTG bienzymatic crosslinking approach [95]) although the process control is still challenging. It requires intensive studies on finding the mutual optimal conditions for enzymes with different properties. (2) Biomimetic enzyme nanocomplexes in particular magnetic nanoparticles specifically functionalized are attracting in this field due to the feasibility to rationally tune the catalyst features. Their biosafety and biocompatibility should be extensively evaluated for practical use in biomedicine and health care. (3) Novel green media are investigated for supporting enzyme activity in polymerization. The candidates include microemulsions, ionic liquids, and deep eutectic solvents. These solvents may offer more advantages than expected. For instance, ionic liquids that have excellent dissolution capacities are desired for homogeneous synthesis hydrogels from recalcitrant natural polymers (e.g. lignin, cellulose).

Acknowledgements The financial supports of Fundamental Research Funds for the Central Universities (No. x2qsD2142050), Hong Kong General Research Fund - Early Career Scheme (No.25201114) and Guangdong-Hong Kong jointed innovation program (No. 2014B050505019) are gratefully acknowledged.

References

[1] Jonker AM, Borrmann A, van Eck ER, van Delft FL, Löwik DW, van Hest J. A fast and activatable crosslinking strategy for hydrogel formation.Advanced Materials. 2015;27(7):1235-40.

[2] Singh S, Topuz F, Hahn K, Albrecht K, Groll J. Embedding of active proteins and living cells in redoxsensitive hydrogels and nanogels through enzymatic crosslinking. Angewandte Chemie International Edition. 2013;52(10):3000-3.


9

[3] Zeng Z, She Y, Peng Z, Wei J, He X. Enzyme-mediated in situ formation of pH-sensitive nanogels for proteins delivery. RSC Advances. 2016;6(10):8032-42.

[4] Phuong NT, Ho VA, Nguyen DH, Khoa NC, Quyen TN, Lee Y, et al. Enzyme-mediated fabrication of an oxidized chitosan hydrogel as a tissue sealant. Journal of Bioactive and Compatible Polymers: Biomedical Applications. 2015:0883911515578760.

[5] Moriyama K, Minamihata K, Wakabayashi R, Goto M, Kamiya N. Enzymatic preparation of a redox-responsive hydrogel for encapsulating and releasing living cells. Chemical Communications. 2014;50(44):5895-8.

[6] Teixeira LSM, Feijen J, van Blitterswijk CA, Dijkstra PJ, Karperien M. Enzyme-catalyzed crosslinkable hydrogels: emerging strategies for tissue engineering. Biomaterials. 2012;33(5):1281-90.

[7] Domeradzka NE, Werten MW, de Wolf FA, de Vries R. Protein cross-linking tools for the construction of nanomaterials. Current opinion in biotechnology. 2016;39:61-7.

[8] LaBella F, Waykole P, Queen G. Formation of insoluble gels and dityrosine by the action of peroxidase on soluble collagens. Biochemical and biophysical research communications. 1968;30(4):333-8.

[9] Izydorczyk M, Biliaderis C, Bushuk W. Oxidative gelation studies of water-soluble pentosans from wheat. Journal of Cereal Science. 1990;11(2):153-69.

[10] Moore A, Martinez-Munoz I, Hoseney R. Factors affecting the oxidative gelation of wheat water-solubles. Cereal Chem. 1990;67(1):81-4.

[11] Takei T, Sugihara K, Ijima H, Kawakami K. In situ gellable sugar beet pectin via enzyme-catalyzed coupling reaction of feruloyl groups for biomedical applications. Journal of bioscience and bioengineering. 2011;112(5):491-4.

[12] Takei T, Sugihara K, Yoshida M, Kawakami K. Injectable and biodegradable sugar beet pectin/gelatin hydrogels for biomedical applications. Journal of Biomaterials Science, Polymer Edition. 2013;24(11):1333-42.

[13] Wang Z, Chen W, Cui Z, He K, Yu W. Studies on photoyellowing of silk fibroin and alteration of its tyrosine content. The Journal of The Textile Institute. 2015:1-7.

[14] Chen T, Embree HD, Wu LQ, Payne GF. In vitro protein–polysaccharide conjugation: Tyrosinasecatalyzed conjugation of gelatin and chitosan. Biopolymers. 2002;64(6):292-302.

[15] Brown JE, Partlow BP, Berman AM, House MD, Kaplan DL. Injectable silk-based biomaterials for cervical tissue augmentation: An in vitro study. American journal of obstetrics and gynecology. 2016;214(1):118. e1-. e9.

[16] Lee BP, Dalsin JL, Messersmith PB. Synthesis and gelation of DOPA-modified poly (ethylene glycol) hydrogels.Biomacromolecules. 2002;3(5):1038-47.

[17] Lee BP, Dalsin JL, Messersmith P. Enzymatic and non-enzymatic pathways to formation of DOPA-modified PEG hydrogels. POLYMER PREPRINTS-AMERICA-. 2001;42(2):151-2.

[18] Wang R, Xu D-l, Liang L, Xu T-t, Liu W, Ouyang P-k, et al. Enzymatically crosslinked epsilon-poly-L-lysine hydrogels with inherent antibacterial properties for wound infection prevention. RSC Advances. 2016;6(11):8620-7.

[19] Chen F, Yu S, Liu B, Ni Y, Yu C, Su Y, et al. An Injectable Enzymatically crosslinked carboxymethylated pullulan/chondroitin sulfate hydrogel for cartilage tissue engineering.Scientific reports. 2016;6.

[20] Peng Z, She Y, Chen L. Synthesis of poly (glutamic acid)-tyramine hydrogel by enzyme-mediated gelation for controlled release of proteins.Journal of Biomaterials Science, Polymer Edition. 2015;26(2):111-27.

[21] Tavakol M, Vasheghani-Farahani E, Mohammadifar MA, Soleimani M, Hashemi-Najafabadi S. Synthesis and characterization of an in situ forming hydrogel using tyramine conjugated high methoxyl gum tragacanth. Journal of biomaterials applications. 2015:0885328215608983.

[22] Sakai S, Khanmohammadi M, Khoshfetrat AB, Taya M. Horseradish peroxidase-catalyzed formation of hydrogels from chitosan and poly (vinyl alcohol) derivatives both possessing phenolic hydroxyl groups. Carbohydrate polymers. 2014;111:404-9.

[23] Ren K, He C, Cheng Y, Li G, Chen X. Injectable enzymatically crosslinked hydrogels based on a poly (L-glutamic acid) graft copolymer. Polymer Chemistry. 2014;5(17):5069-76.

[24] Nguyen DH, Tran NQ, Nguyen CK. Tetronic-grafted chitosan hydrogel as an injectable and biocompatible scaffold for biomedical applications. Journal of Biomaterials Science, Polymer Edition. 2013;24(14):1636-48.

[25] Sun Y, Deng Z, Tian Y, Lin C. Horseradish peroxidasemediated in situ forming hydrogels from degradable tyraminebased poly (amido amine)s. Journal of Applied Polymer Science. 2013;127(1):40-8.

[26] Ganesh N, Hanna C, Nair SV, Nair LS. Enzymatically cross-linked alginic–hyaluronic acid composite hydrogels as cell delivery vehicles.International journal of biological macromolecules. 2013;55:289-94.

[27] Sakai S, Tsumura M, Inoue M, Koga Y, Fukano K, Taya M. Polyvinyl alcohol-based hydrogel dressing gellable on-wound via a co-enzymatic reaction triggered by glucose in the wound exudate. Journal of Materials Chemistry B. 2013;1(38):5067-75.

[28] Sakai S, Komatani K, Taya M. Glucose-triggered co-enzymatic hydrogelation of aqueous polymer solutions. RSC Advances. 2012;2(4):1502-7.

[29] Liao CA, Wu Q, Wei QC, Wang QG. Bioinorganic Nanocomposite Hydrogels formed by HRP–GOXcascadecatalyzed polymerization and exfoliation of the layered composites.Chemistry–A European Journal. 2015;21(36):12620-6.

[30] Wu Q, Wang X, Liao C, Wei Q, Wang Q. Microgel coating of magnetic nanoparticles via bienzyme-mediated free-radical polymerization for colorimetric detection of glucose. Nanoscale. 2015;7(40):16578-82.

[31] Iwata H, Hata Y, Matsuda T, Ikada Y. Initiation of radical polymerization by glucose oxidase utilizing dissolved oxygen. Journal of Polymer Science Part A: Polymer Chemistry. 1991;29(8):1217-8.

[32] Johnson LM, Fairbanks BD, Anseth KS, Bowman CN. Enzyme-mediated redox initiation for hydrogel generation and cellular encapsulation.Biomacromolecules. 2009;10(11):3114-21.

[33] Zavada S, McHardy N, Scott T. Oxygen-mediated enzymatic polymerization of thiol–ene hydrogels. Journal of Materials Chemistry B. 2014;2(17):2598-605.

[34] Wilson R, Turner A. Glucose oxidase: an ideal enzyme. Biosensors and Bioelectronics. 1992;7(3):165-85.


10

[35] Trivić S, Leskovac V, Zeremski J, Vrvić M, Winston GW. Bioorganic mechanisms of the formation of free radicals catalyzed by glucose oxidase.Bioorganic chemistry. 2002;30(2):95-106.

[36] Su T, Tang Z, He H, Li W, Wang X, Liao C, et al. Glucose oxidase triggers gelation of N-hydroxyimide–heparin conjugates to form enzyme-responsive hydrogels for cell-specific drug delivery. Chemical Science. 2014;5(11):4204-9.

[37] Korner A, Pawelek J. Mammalian tyrosinasecatalyzes three reactions in the biosynthesis of melanin. Science. 1982;217(4565):1163-5.

[38] Park J-H, Jeong L, Park W-H. Effect of tyrosinase and polyphenol compounds on hydrogelation of silk fibroin.Textile Science and Engineering. 2009;46(2):55-61.

[39] Das S, Pati F, Choi Y-J, Rijal G, Shim J-H, Kim SW, et al. Bioprintable, cell-laden silk fibroin–gelatin hydrogel supporting multilineage differentiation of stem cells for fabrication of three-dimensional tissue constructs. Actabiomaterialia. 2015;11:233-46.

[40] Li Y, Ding Y, Qin M, Cao Y, Wang W. An enzyme-assisted nanoparticle crosslinking approach to enhance the mechanical strength of peptide-based supramolecular hydrogels.Chemical Communications. 2013;49(77):8653-5.

[41] Jin R, Lou B, Lin C. Tyrosinase mediated in situ forming hydrogels from biodegradable chondroitin sulfate–tyramine conjugates. Polymer International. 2013;62(3):353-61.

[42] Jin R, Lin C, Cao A. Enzyme-mediated fast injectable hydrogels based on chitosan–glycolic acid/tyrosine: Preparation, characterization, and chondrocyte culture. Polymer Chemistry. 2014;5(2):391-8.

[43] Strong P, Claus H. Laccase: a review of its past and its future in bioremediation. Critical Reviews in Environmental Science and Technology. 2011;41(4):373-434.

[44] Durán N, Rosa MA, D’Annibale A, Gianfreda L. Applications of laccases and tyrosinases (phenoloxidases) immobilized on different supports: a review. Enzyme and Microbial Technology. 2002;31(7):907-31.

[45] Bollag J-M. Enzymes catalyzing oxidative coupling reactions of pollutants.MetIonsBiolSyst. 1992;28:205-17. [46] Berlanga-Reyes CM, Carvajal-Millán E, Juvera GC, Rascón-Chu A, Marquez-Escalante JA, Martínez-López AL. Laccase

induced maize bran arabinoxylan gels. Food Science and Biotechnology. 2009;18(4):1027-9. [47] Azarikia F, Wu B-c, Abbasi S, McClements DJ. Stabilization of biopolymer microgels formed by electrostatic complexation:

Influence of enzyme (laccase) cross-linking on pH, thermal, and mechanical stability. Food Research International. 2015;78:18-26.

[48] Rocasalbas G, Francesko A, Touriño S, Fernández-Francos X, Guebitz GM, Tzanov T. Laccase-assisted formation of bioactive chitosan/gelatin hydrogel stabilized with plant polyphenols. Carbohydrate polymers. 2013;92(2):989-96.

[49] Hollmann F, Gumulya Y, Tölle C, Liese A, Thum O. Evaluation of the laccase from Myceliophthora thermophila as industrial biocatalyst for polymerization reactions. Macromolecules. 2008;41(22):8520-4.

[50] Nieto M, Nardecchia S, Peinado C, Catalina F, Abrusci C, Gutiérrez MC, et al. Enzyme-induced graft polymerization for preparation of hydrogels: synergetic effect of laccase-immobilized-cryogels for pollutants adsorption. Soft Matter. 2010;6(15):3533-40.

[51] Zavada SR, Battsengel T, Scott TF. Radical-mediated enzymatic polymerizations.International journal of molecular sciences. 2016;17(2):195.

[52] FigueroaEspinoza MC, Morel MH, Surget A, Rouau X. Oxidative cross linking of wheat arabinoxylans by manganese peroxidase. Comparison with laccase and horseradish peroxidase.Effect of cysteine and tyrosine on gelation.Journal of the Science of Food and Agriculture. 1999;79(3):460-3.

[53] Lau Y-KI, Gobin AM, West JL. Overexpression of lysyl oxidase to increase matrix crosslinking and improve tissue strength in dermal wound healing.Annals of biomedical engineering. 2006;34(8):1239-46.

[54] Girotti AW, Thomas JP, Jordan JE. Xanthine oxidase-catalyzed crosslinking of cell membrane proteins.Archives of biochemistry and biophysics. 1986;251(2):639-53.

[55] Ferreira L, Carvalho R, Gil MH, Dordick JS. Enzymatic synthesis of inulin-containing hydrogels.Biomacromolecules. 2002;3(2):333-41.

[56] Ferreira L, Gil MH, Dordick JS. Enzymatic synthesis of dextran-containing hydrogels.Biomaterials. 2002;23(19):3957-67. [57] Ferreira L, Gil MH, Cabrita AM, Dordick JS. Biocatalytic synthesis of highly ordered degradable dextran-based

hydrogels.Biomaterials. 2005;26(23):4707-16. [58] Ramos R, Carvalho V, Gama M. Novel hydrogel obtained by chitosan and dextrin-VA co-polymerization. Biotechnology letters.

2006;28(16):1279-84. [59] Carvalho J, Gonçalves C, Gil AM, Gama FM. Production and characterization of a new dextrin based hydrogel. European

Polymer Journal. 2007;43(7):3050-9. [60] Martin BD, Ampofo SA, Linhardt RJ, Dordick JS. Biocatalytic synthesis of sugar-containing polyacrylate-based

hydrogels.Macromolecules. 1992;25(26):7081-5. [61] Qin X-H, Gruber P, Markovic M, Plochberger B, Klotzsch E, Stampfl J, et al. Enzymatic synthesis of hyaluronic acid vinyl

esters for two-photon microfabrication of biocompatible and biodegradable hydrogel constructs. Polymer Chemistry. 2014;5(22):6523-33.

[62] Kaith BS, Jindal R, Kapur G. Enzyme-based green approach for the synthesis of gum tragacanth and acrylic acid cross-linked hydrogel: its utilization in controlled fertilizer release and enhancement of water-holding capacity of soil. Iranian Polymer Journal. 2013;22(8):561-70.

[63] Chronopoulou L, Lorenzoni S, Masci G, Dentini M, Togna AR, Togna G, et al. Lipase-supported synthesis of peptidic hydrogels. Soft Matter. 2010;6(11):2525-32.

[64] Das AK, Maity I, Parmar HS, McDonald TO, Konda M. Lipase-catalyzed dissipative self-assembly of a thixotropic peptide bolaamphiphile hydrogel for human umbilical cord stem-cell proliferation. Biomacromolecules. 2015;16(4):1157-68.

[65] Izawa H, Nawaji M, Kaneko Y, Kadokawa Ji. Preparation of glycogenbased polysaccharide materials by phosphorylasecatalyzed chain elongation of glycogen. Macromolecular bioscience. 2009;9(11):1098-104.


11

[66] Kadokawa J-i. Preparation and applications of amylose supramolecules by means of phosphorylase-catalyzed enzymatic polymerization.Polymers. 2012;4(1):116-33.

[67] Kadokawa J-i, Kaneko Y. Phosphorylase-catalyzed enzymatic polymerization. Advances in the Engineering of Polysaccharide Materials: by Phosphorylase-Catalyzed Enzymatic Chain-Elongation. 2013:43.

[68] Takata Y, Yamamoto K, Kadokawa Ji. Preparation of pH responsive amphoteric glycogen hydrogels by α glucan phosphorylasecatalyzed successive enzymatic reactions.Macromolecular Chemistry and Physics. 2015;216(13):1415-20.

[69] Li X, Gao Y, Kuang Y, Xu B. Enzymatic formation of a photoresponsive supramolecular hydrogel. ChemCommun. 2010;46(29):5364-6.

[70] Wang H, Yang C, Wang L, Kong D, Zhang Y, Yang Z. Self-assembled nanospheres as a novel delivery system for taxol: a molecular hydrogel with nanosphere morphology. Chemical Communications. 2011;47(15):4439-41.

[71] Martin SC, Forsberg P-O, Eriksson SD. The effects of in vitro phosphorylation and dephosphorylation on the thrombin-induced gelation and plasmin degradation of fibrinogen.Thrombosis research. 1991;61(3):243-52.

[72] Suk K, Lee JY, Kim SH. Synergistic stimulation of fibrinogen gelation by casein kinase II and polycationic compounds. IUBMB Life. 1997;42(3):487-95.

[73] Yang Z, Liang G, Wang L, Xu B. Using a kinase/phosphatase switch to regulate a supramolecular hydrogel and forming the supramolecular hydrogel in vivo. Journal of the American Chemical Society. 2006;128(9):3038-43.

[74] Yung C, Wu L, Tullman J, Payne G, Bentley W, Barbari T. Transglutaminase crosslinked gelatin as a tissue engineering scaffold. Journal of Biomedical Materials Research Part A. 2007;83(4):1039-46.

[75] Yin W, Su R, Qi W, He Z. A casein-polysaccharide hybrid hydrogel cross-linked by transglutaminase for drug delivery.Journal of Materials Science. 2012;47(4):2045-55.

[76] Sanborn TJ, Messersmith PB, Barron AE. In situ crosslinking of a biomimetic peptide-PEG hydrogel via thermally triggered activation of factor XIII. Biomaterials. 2002;23(13):2703-10.

[77] Hu B-H, Messersmith PB. Rational design of transglutaminase substrate peptides for rapid enzymatic formation of hydrogels.Journal of the American chemical society. 2003;125(47):14298-9.

[78] Sperinde JJ, Griffith LG. Control and prediction of gelation kinetics in enzymatically cross-linked poly (ethylene glycol) hydrogels. Macromolecules. 2000;33(15):5476-80.

[79] Zhu D, Jin L, Wang Y, Ren H. Swelling behavior of gelatin-based hydrogel cross-linked with microbial transglutaminase. J Aqeic. 2012;63(2):11-20.

[80] Liu Y, Javvaji V, Raghavan SR, Bentley WE, Payne GF. Glucose oxidase-mediated gelation: a simple test to detect glucose in food products. Journal of agricultural and food chemistry. 2012;60(36):8963-7.

[81] Bocharova V, Sharp D, Jones A, Cheng S, Griffin PJ, Agapov AL, et al. Enzyme induced formation of monodisperse hydrogel nanoparticles tunable in size. Chemistry of Materials. 2015;27(7):2557-65.

[82] Chenite A, Gori S, Shive M, Desrosiers E, Buschmann M. Monolithic gelation of chitosan solutions via enzymatic hydrolysis of urea. Carbohydrate polymers. 2006;64(3):419-24.

[83] Jee E, Bánsági T, Taylor AF, Pojman JA. Temporal control of gelation and polymerization fronts driven by an autocatalytic enzyme reaction.AngewandteChemie International Edition. 2016.

[84] Luo R, Cao Y, Shi P, Chen CH. Nearinfrared light responsive multicompartmental hydrogel particles synthesized through droplets assembly induced by superhydrophobic surface. Small. 2014;10(23):4886-94.

[85] Johnson LM, DeForest CA, Pendurti A, Anseth KS, Bowman CN. Formation of three-dimensional hydrogel multilayers using enzyme-mediated redox chain initiation. ACS applied materials & interfaces. 2010;2(7):1963-72.

[86] Shenoy R, Bowman CN. Kinetics of interfacial radical polymerization initiated by a glucose-oxidase mediated redox system. Biomaterials. 2012;33(29):6909-14.

[87] GuZ, Tang Y. Enzyme-assisted photolithography for spatial functionalization of hydrogels. Lab on a Chip. 2010;10(15):1946-51.

[88] Ayyub OB, Kofinas P. Enzyme induced stiffening of nanoparticle–hydrogel composites with structural color. ACS nano. 2015;9(8):8004-11.

[89] Zhong M, Sun J, Wei D, Zhu Y, Guo L, Wei Q, et al. Establishing a cell-affinitive interface and spreading space in a 3D hydrogel by introduction of microcarriers and an enzyme. J Mater Chem B. 2014;2(38):6601-10.

[90] Gkioni K, Leeuwenburgh SC, Douglas TE, Mikos AG, Jansen JA. Mineralization of hydrogels for bone regeneration. Tissue Engineering Part B: Reviews. 2010;16(6):577-85.

[91] Rauner N, Buenger L, Schuller S, Tiller JC. Post polymerization of urease induced calcified, polymer hydrogels. macromolecular rapid communications. 2015;36(2):224-30.

[92] Munro NH, Green DW, Dangerfield A, McGrath KM. Biomimetic mineralisation of polymeric scaffolds using a combined soaking and Kitano approach. Dalton Transactions. 2011;40(36):9259-68.

[93] Spoerke ED, Anthony SG, Stupp SI. Enzyme directed templating of artificial bone mineral. Advanced Materials. 2009;21(4):425-30.

[94] Rauner N, Meuris M, Dech S, Godde J, Tiller JC. Urease-induced calcification of segmented polymer hydrogels–A step towards artificial biomineralization.Actabiomaterialia. 2014;10(9):3942-51.

[95] Zhang Y, Fan Z, Xu C, Fang S, Liu X, Li X. Tough biohydrogels with interpenetrating network structure by bienzymatic crosslinking approach. European Polymer Journal. 2015;72:717-25.


12

a mini-review on in situ enzymatic fabrication of · pdf filea mini-review on in situ...

Documents