Introduction

Hydrogels are 3-D networks composed of water-soluble polymer chains linked together by chemical or physical bonds. They are employed as carriers for delivery of bioactive agents, as wound healing films, bio-sensing materials, implants, and scaffolds in tissue engineering, etc. (Ullah et al., 2015; Wang L. et al., 2016; Al Harthi et al., 2019). In the presence of water, instead of dissolution, hydrogels are swollen from a few to several times of their own dry weight depending on the crosslinking degree. Similar to water-soluble polymers, based on their chemical structures, hydrogels can be divided into anionic (Ullah et al., 2015), cationic (Qi et al., 2018), non-ionic (Golabdar et al., 2019), and zwitterionic (Vatankhah-Varnosfaderani et al., 2018).

Anionic polymers, which are essentially poly(acid)s, show globule to coil transition upon pH increment and/or reduction of ionic strength (Abu-Thabit and Hamdy, 2016; Meka et al., 2017). This behavior is reflected in hydrogels as swelling when the polymer is cross-linked (Varaprasad et al., 2017). Aside from polysaccharide-based anionic polymers, most of anionic hydrogels are not biodegradable, thereby posing environmental problems and creating pollution challenges in the long-term (Guilherme et al., 2015; Pakdel and Peighambardoust, 2018). Therefore, seeking a suitable substitute that is biodegradable and non-toxic is of outmost importance.

Poly(aspartic acid) (PASP), a synthetic poly(amino acid) with a protein-like amide bond in its backbone, and a carboxylic acid as a pendant group in each repeating unit, has drawn a great deal of attention and so the demand for its production has significantly grown. The former bond provides PASP with degradability (Nakato et al., 1998; Tabata et al., 2000, 2001), while the latter groups gives the polymer acidic properties and negative charge (Yang et al., 2011; Sattari et al., 2018). Various enzymes such as trypsin (Zhang C. et al., 2017), chymotrypsin (Wei et al., 2015), dispase and collagenase I (Juriga et al., 2016), as well as different media such as activated sludge (Alford et al., 1994) and river water (Tabata et al., 2000) have been examined for biodegradation of PASP-based hydrogels and polymers. Depending on the condition (e.g., enzyme concentration and temperature), complete degradation varies from a few days to one month.

PASP hydrogels typically exhibit the same response to pH and ionic strength as other anionic ones, such that higher swelling ratio can be achieved by increasing pH or lowering ionic strength (Zhao et al., 2005; Sharma et al., 2014). This property, which is called polyelectrolyte effect, stems from ionization of carboxylic groups. Ionization (or de-protonation) creates negative charges along the chain/network, causing extended chain conformation and globule to coil transition. On the other hand, high ion concentration shields the network charge and lowers swelling (Yang et al., 2006). Additionally, by changing crosslinking density, mechanical properties and swelling could be tuned (Vatankhah-Varnoosfaderani et al., 2016). More importantly, PASP hydrogels can be readily modified with a wide variety of species to meet the need of any given application. This feature arises from highly reactive imide rings in the intermediate poly(succinimide) (PSI), which allow grafting of different molecules bearing primary amine group under mild conditions without using any catalyst. Therefore, considering facile modification, biocompatibility, and biodegradability, PASP-based hydrogels offers potential advantages over conventional anionic hydrogels [e.g., poly(acrylic acid)] and can be considered as a promising choice for hydrogel preparation in diverse applications.

In the light of the aforementioned features, in this paper we provide a review on the synthesis, gelation process, cross-linking agents, and recent applications of hydrogels based on PASP.

Synthesis of Polyaspartic Acid

PASP homopolymer is generally synthesized through poly-condensation of aspartic acid (ASP) monomer or polymerization of maleamic acid which is produced from maleic anhydride and a nitrogen source like ammonia or urea as shown in Figure 1. Whatever the method is, the reaction yields the intermediate poly(anhydroaspartic acid), i.e., poly(succinimide) (PSI). The subsequent alkaline hydrolysis of PSI leads to imide ring opening through either carbonyl groups, resulting in a mixture of α and β. Also, as mentioned, PSI can easily undergo a nucleophilic reaction with primary and secondary amines without catalyst even at room temperature to yield poly(aspartamide) derivatives, allowing one to tailor-make PASP to be exploited as a versatile and multi-functional hydrogels (Feng et al., 2014; Nayunigari et al., 2014; Zhang S. et al., 2017).

 

Figure 1

Synthesis procedure of PSI, PASP, and poly(aspartamide). PSI can either be obtained through poly-condensation (usually at elevated temperature >160°C using an acid as the catalyst) of ASP or malic acid (synthesized by maleic anhydride and a nitrogen source such as urea or ammonia). PSI can be hydrolysed under alkali media to yield PASP or reacted with primary amines (without catalyst at room temperature) to yield poly(aspartamide) derivatives. The ring opening of succinimde groups occurs both at α and β sites in amidation and hydrolysis reaction.

Crosslinking

Commonly, hydrogels based on PASP are prepared either via crosslinking of PSI followed by alkali hydrolysis, or by crosslinking of PASP itself. Various types of crosslinking agents can be used. Because of the simplicity, the agent is generally introduced by PSI modification or the gelation process itself is carried out on PSI followed by alkali hydrolysis.

Hydrogels Based on Disulfide Bond

Crosslinking through disulfide or thiol containing agents endows an interesting feature to the PASP-based hydrogels. The reaction of thiol to disulfide can be carried out under application of a reducing agent. This reaction can be reversed in the presence of an oxidizing agent. Therefore, PSI is generally modified with thiol groups (cysteamine or cystamine) for the preparation of reducing/oxidizing-responsive PASP hydrogels (Molnar et al., 2014). In order to maintain structural integrity in different media, a permanent linker such as a diamine can be employed (Figure 3A; Zrinyi et al., 2013; Krisch et al., 2018). Recently, such dual cross-linked hydrogels have drawn a great deal of attention due to swelling under reductive state. For instance, Zrinyi et al. (2013) synthesized PASP with diaminobutane (DAB), and cystamine (CYS) as permanent and cleavable crosslinkers, respectively. They showed that disulfide bonds arising from the latter is broken by the addition of a reducing agent, leading to an increase in swelling and a decrease in modulus. Likewise, redox- and pH-responsive PASP hydrogels were prepared by dual crosslinking using cysteamine, and 1,4-diaminobutane which creates reversible and irreversible bonds, respectively (Gyarmati et al., 2014). It was indicated that swelling degree of hydrogel and elastic modulus can be tuned by reducing/oxidizing agents without hydrogel disintegration/dissolution. Swelling increased as pH increased both under oxidized and reduced states. However, under the latter condition, swelling was higher. The hydrogels maintained their mechanical stability under repeated redox cycles for at least three cycles and the reversibility was shown to be independent of initial redox state of PASP (reduced or oxidized) (Figures 2A,B). Krisch et al. (2018) employed poly(ethylene glycol) diglycidyl ether (PEGDGE) for crosslinking thiolated PASP in order to secure structural integrity of the hydrogels in reducing media. A part of thiol groups were reacted with the former to establish a non-cleavable gel junction while the remaining ones were oxidized into breakable disulfide bonds. It should be noted that the epoxide groups with thiol groups form unbreakable S-C bonds.

 

Figure 2

PASP hydrogels based on disulfide bonds. (A) Swelling of PASP hydrogels cross-linked with cysteamine in reduced and oxidized states as a function of pH shows conventional behavior of anionic hydrogels, swelling under reduced condition is higher. (B) Elastic modulus of the corresponding hydrogels shows reversible increase and decrease upon oxidation/reduction. Reproduced from Gyarmati et al. (2014) with permission from The Royal Society of Chemistry.

Hydrogels Based on Dopamine

Catechol moieties in dopamine exhibit a multifunctional characteristic for the design of mussel-inspired coatings (Ryu et al., 2015, 2018; Saiz-Poseu et al., 2019). Complex formation of catechol with boron and/or iron ions (Fe³⁺) can be employed for hydrogel preparation (Vatankhah-Varnoosfaderani et al., 2014; Krogsgaard et al., 2016). Injectable dopamine modified PASP hydrogels with superior adhesive character were synthesized by complexation with Fe³⁺ ions (gelation time around 1 min; Figure 3B; Gong et al., 2017). It was suggested that the resulting crosslinking are composed of both Fe3⁺ coordination as well as covalent quinone-quinone bonds. Boric acid was also shown to crosslink dopamine-modified PASP and yield hydrogels due to boron–catechol coordination (Wang B. et al., 2016). The prepared hydrogels had autonomous self-healing feature due to such a coordination.

 

Figure 3

PASP hydrogels prepared by different cross-linkers. (A) PASP hydrogel with a permanent and a cleavable cysteamine cross-linker exhibited response to oxidation/reduction without the gel dissolution. (B) Modification of PSI with dopamine led to PASP with catechol pendant groups which can establish complex with Fe³⁺ ions, and form hydrogel as a results; adapted from Gong et al. (2017) with permission from Wiley. (C) Synthesis of injectable PASP hydrogels by introduction of aldehyde and amine groups on PASP. Reproduced from Lu et al. (2014) with permission from Wiley. (D) Allyl amine as a monomer that is polymerized via radical polymerization is grafted onto PASP; the subsequent polymerization of allyl amine gives rise to PASP-based hydrogel. (E) Application of gamma-irradiation for crosslinking of PASP. (F) Grafting of APTS as an aminosilane on PSI backbone followed by its hydrolysis.