In the following sections, the main works on polysaccharide-based in situ self-healing hydrogels are summarized. The classification is made according to the interactions or bonds created after their response to external stimuli that lead to the sol-gel transition. Based on this, different types of in situ hydrogels are shown, such as hydrophobic interaction-based hydrogels, ionically crosslinked hydrogels, hydrogels based on in situ polymerization and dynamic covalent bond-based hydrogels, among others.
3.1. Hydrophobic Self-Healing Hydrogels
One of the main types of hydrophobic interaction-based self-healing hydrogels is the type that responds to an external temperature change. In this field, there are numerous polymers capable of undergoing a sol-gel transition in response to temperature changes [84
]. This transition is induced at a temperature named upper or lower critical solution temperature; UCST and LCST, respectively. In polymers, this thermosensible sol-gel transition is due to a variation in the solubility of the polymer owing to a variation between the interactions of hydrophilic and hydrophobic moieties of the polymer with water. The most interesting situation for biomedical applications is that of the LCST cases, in which the interactions between macromolecule chains and the solvent by H-bonding predominate over hydrophobic interactions at temperatures below LCST. However, when the temperature exceeds LCST, the hydrophilic interactions (polymer hydrophilic moieties-water) are weakened and the hydrophobic interactions lead to precipitated hydrogel. This behavior is known as the hydrophobic effect and offers the possibility of developing a reversible in situ matrix, which can return to solution by changing temperature. This thermal response is a promising feature because no other requirement for chemical or environmental treatment is needed, as temperature is the only external factor that takes part in the gelation process.
The main thermosensible polymers are not polysaccharides. They are poly (N
-isopropylacrylamide) (PNIPAAm) [3
] and poly(ethylene glycol)-poly(propylene oxide)-poly(ethylene glycol) block copolymer (PEO-PPO-PEO), also known as Pluronic F-127®
], which shows an LCST close to body temperature. Different studies have been reported with these two thermosensible polymers but, since they are not biodegradable, they typically appear in combination with polysaccharides, such as chitosan [89
], alginate [35
], cellulose [93
] or hyaluronic acid [95
] in order to obtain a hydrogel matrix with tunable degradation behavior. However, these materials lack the ability to self-repair because gelation is based on hydrophobic interactions that result in hydrogel precipitation.
For example, taking advantage of the thermosensitivity of a synthetic polymer such as Pluronic®
(LCST ≈ physiological temperature), a thermosensitive structure capable of self-repairing by the host-guest mechanism was obtained by host-guest interaction with alginate derived with β-cyclodextrin (β-CD) [97
]. This double-crosslinked network was obtained due to the interaction between host polymer (Pluronic ®
) and guest moieties of alginate-β-CD. In addition, at a temperature around physiological temperature and thanks to the rapid response of Pluronic®
to temperature variation; the second crosslinking was achieved as a consequence of the hydrophobic interactions between polymer chains. Moreover, the mechanical measurements reveal that the shear storage moduli of the hydrogel (30 kPa) at body temperature was maintained even after breaking the hydrogel which demonstrates their potential to be used as scaffold for biomedical applications.
Chitosan has also been successfully employed in the development of in situ hydrogels with the ability to self-repair. This is the case of hydrophobically functionalized chitosan (adding C12 groups) mixed with dodecyltrimethylammonium bromide (DTAB) and a thermosensitive vesicle (5-methyl syalic acid) [98
]. The obtained gel shows good stability at temperatures around 20 °C. However, the increase in temperature favors the gel-sol transition, reverting the gel to its original form (liquid). The sol-gel transition, as in the aforementioned case, is directly related to hydrophobic interactions between the hydrophobically modified chitosan and the vesicles, which in turn play as multiple crosslinking points, leading to supramolecular hydrogels. The self-repairing of these gels has been thoroughly studied by rheology, where a quick repair (almost 10 s) was obtained, which was kept for four cycles. Therefore, thermosensitive vesicles offer the possibility of forming reversible dynamic bonds thanks to the hydrophobic interactions and can be embedded or de-embedded from the vesicle bilayer.
3.2. Ionically Induced Self-Healing Hydrogels
Ionically crosslinked hydrogels are obtained from a mixture of two or more polyelectrolytes in a suitable pH which leads to the protonation or deprotonation of the ionizable moieties which are through the structure. Typically, natural polymers with pendant ionizable groups, such as alginate [33
], chitosan [36
], hyaluronic acid [38
] and cellulose have been extensively used in the development of ionically crosslinked systems and ionically induced self-healed hydrogels. As there are several pH changes in physiological medium, such as upper stomach (4.0–6.5), lower stomach (1.5–4.0) or saliva (6.5–7.5), hydrogels that are able to be crosslinked due to pH changes are promising materials for biomedical applications, e.g., as sensors and actuators, as scaffolds for mimicking different microenvironments for 3D culture or as bioactive molecular carriers [99
The most typical polysaccharide within ionically crosslinked self-healing hydrogels is alginate. Alginate is an anionic polysaccharide, thanks to the carboxylic groups that are repeated throughout its structure. This biopolymer shows an ability to create gels when cations, such as calcium (Ca2+
), zinc (Zn2+
) or magnesium (Mg2+
) are added [33
]. Those cations, at a specific pH ≈ 4.5 (pKaalginate
= 3.5) interact with the carboxylate groups of the mannuronic acid and guluronic acid units and an ionic hydrogel with self-healing by dynamic and reversible ionic bonds is formed. The alginate/CaCl2
system has been used for several applications in biomedicine. However, the main drawback of these materials under physiological conditions is their poor mechanical stability. For this reason, chemical crosslinking is normally added to these hydrogels by modifying the biopolymer with acrylic or vinyl groups, among others, to improve their mechanical properties [101
]. Often, these double crosslinking systems do not present total degradation or an efficient ability to self-repair. Consequently, interest in combining oppositely charged polysaccharides to obtain biodegradable and biocompatible matrices with self-healing properties by ionic bonds has grown over the last years.
This is the case of Ren et al. [102
] who developed a reversible in situ hydrogel with self-healing ability based on the oppositely charged polysaccharides, alginate and chitosan. However, the number of interactions between both polysaccharides is extremely high and the obtained hydrogels resulted so strong, that the self-healing process was hindered. Therefore, chitosan was substituted by 2-hydroxypropyltrimethylammonium chitosan chloride (HACC), because the large size of the quaternary ammonium cationic groups leads to an increase in the distance with alginate and, as a result, the ionic interactions became weaker, favoring the self-healing process which takes place in 7 h without any external stimuli (Figure 2
). Isothermal titration calorimetry (ITC) reveals that the electrostatic interactions between alginate and HACC were endothermic, this is, the hydrogel formation is entropically driven due to the formation of positive peaks. The polyelectrolyte-based hydrogel exhibits promising properties such as muco-adhesion, shear-thinning and cell compatibility which would be useful for bioapplications.
A few works have focused on hyaluronic acid and chitosan-based polycomplexes, which provide the possibility of in situ gel formation as well as rapid self-healing process thanks to ionic interactions. In this case, the mixture of chitosan and hyaluronic acid in adequate pH (~4) leads to the formation of a hydrogel with ability to self-repair in 2 min (Figure 3
]. In order to measure the mechanical response of the synthesized hydrogels, different cut-recovery cycles (2) were applied and the results showed that after the self-healing process a new material can be obtain with properties nearly as good as the original hydrogel, which opens a new research line with promising opportunities in the biological field.
Another study that has revealed the interest in combining oppositely charged polysaccharides [103
] used chitosan, guanidine hydrochloride and poly (acrylic acid) in order to obtain a ionically crosslinked matrix with pH-responsive ability and self-healing. When the pH of the system increases (pH ~ 8), the carboxylic acid moieties of poly (acrylic acid), the hydroxyl groups of guanidine hydrochloride and amine groups of chitosan became deprotonated and the ionic interactions between the three compounds are switched on. The self-healing study revealed that the hydrogel was able to self-heal itself in a few hours by recovering its initial shape owing to ionic interactions (Figure 4
). Moreover, the gelation of the hydrogel seems to be suitable for drug delivery or 3D cell culture applications as the pH induced a rapid sol-gel transition.
Double physical crosslinking has been also exploited for the development of in situ hydrogels with self-healing properties with improved consistency. For instance, chitosan, a copolymer of acrylamide and acrylic acid, was mixed in the presence of iron (III) salt. The authors [90
] worked at an specific (slightly acidic) pH where the primary amine groups of the chitosan were protonated (-NH3+
) and, at the same time, the -COOH groups of the acrylic acid and the -CONH2
of the polyacrylamide were also in an ionized state (-COO−
, respectively). The results of the gelation process analyzed by FTIR revealed that the trivalent cation of iron (Fe3+
) interacts ionically with the anion formed by the deprotonation of the carboxylic groups. In turn, the free amino groups of chitosan and the -COONH3+
group of the copolymer also interacted with -COO−
. Furthermore, as the ionization is not total, the amino and amide groups as well as the carboxylic group are capable of establishing hydrogen bonds, leading to a double-crosslinked network. Besides, synthesized hydrogels were able to achieve good self-healing capacity due to the reversible ionic and hydrogen bonds. The results of the tensile stress before and after the self-repair process demonstrated that the hydrogel maintains almost totally the mechanical stability.
Other types of ionically induced hydrogels are those based on metal-ligand interactions. This class of smart materials are composed of a ligand (electron donor, catechol for instance) and a metal ion that results in a hydrogel formed by self-assembly in a supramolecular network. Metal-ligand bonds are reversible in nature and have become the point of interest of many researchers due to the ability to rapid self-repair. Since chitosan offers suitable possibility to obtain hydrogels with excellent biocompatibility and versatility to be functionalized, its derived-based hydrogels have attached huge attention in the field of self-repair materials. Chitosan-catechol, which was formed by the modification of the polysaccharide with dihydrobenzaldehyde, was able to self-assemble by establishing coordinative interactions between catechol moiety and iron (III). These in situ hydrogels that formed in no more than 5 min also have an excellent ability to self-heal thanks to the reversible coordination bonds, which gives them the possibility of being injected subcutaneously. The results obtained from these gels demonstrate that the self-healing process occurs even after loading drugs, such as doxorubicin hydrochloride or docexatel. Therefore, this material is presented as a promising alternative to be used for administration and controlled release of drugs, for example, in cancer therapies.
Not only chitosan but also other polysaccharides have also been modified with catechol groups so as to obtain self-healable hydrogel by coordinative interactions. This is the case of modified alginate with dopamine (Alg-DA). A recent study compares the self-healing of the traditional ionic gel composed of unmodified alginate/Ca2+
and two gels formed in situ by coordinative interactions between modified Alg-DA and Fe (III) and polymerized-catechol Alg-DA and Fe (III) [104
]. The results demonstrated that although all hydrogels were capable of self-healing, only dopamine-modified gels were capable of establishing bridges strong enough not to break under vigorous mechanical agitation (Figure 5
). This behavior is due to the fact that the self-healing of dopamine-modified gels are able to bind strongly with iron (III) ions, thus giving the hydrogel more consistency after breaking.
3.3. In Situ Polymerization and Self-Healing Hydrogels
In situ hydrogels that are able to be self-repaired often do not show good mechanical stability and, in addition, they have poor self-repair ability, which strongly limits their application. Thus, as physical interactions typically used in the development of in situ hydrogels with self-healing due to their reversible nature promote the lack of mechanical stability, the interest in incorporating covalent bonds by in situ polymerization in physically crosslinked networks has increased. Most of the hydrogels based on in situ polymerization are based on a double crosslinking network. In fact, in situ polymerization generates a covalent network with improved mechanical stability. Since in most cases this network is not biodegradable, they frequently appear in combination with natural polysaccharides and form hydrogels based on reversible physical interactions that offer the possibility of self-repair. Related to this, usually this kind of material needs external stimuli such as temperature to complete self-healing process. This is the case, for example for double crosslinked hydrogels formed by in situ free radical polymerization and hydrogen bonds based on N
-acryloyl glycinamide [105
]. The hydrogels show good mechanical stability for the desired biomedical applications thanks to the covalent crosslinked and self-healing process being successfully achieved with the help of an external temperature, which favors the H-bonds between the residues of amino acids in the polymer. However, this hydrogel is unsuitable for biomedical applications because although it showed good cytocompatibility and mechanical stability (1400% elongation at break) the nature of the matrix does not present biodegradability.
Nevertheless, multiple coordination bridges established by coordination metals are a promising way to add a second reversible network to covalently crosslink hydrogels, and thus, to gain high mechanical stability as well as an efficient self-repairing ability. Typically, chemically crosslinked hydrogels are obtained by in situ polymerization of poly (acrylic acid), polyacrylamide or their copolymers that are also coordinated with metallic cations like Fe3+
to add physical crosslinking points due to electrostatic interactions to the polymerized networks [106
]. As aforementioned, the use of synthetic polymers for biomedical research is limited and, therefore, in situ hydrogels with self-healing properties based on polysaccharides are increasingly investigated. As aforementioned, chitosan is one of the most used biopolymers for the development of hydrogels based on polysaccharides. For example, double-crosslinked chitosan hydrogels were prepared by combining the in situ free-radical polymerization of acrylic acid monomer and reversible coordinative interactions between water soluble chitosan and Fe3+
ions that provide a hydrogel with good mechanical properties and ability to self-repair [106
]. The results showed that in situ hydrogels based on chitosan had excellent efficiency for self-repair (98%) and were able to achieve total self-repair in just 2.5 h (Figure 6
). Finally, the mechanical properties of the hydrogels after being fractured were compared with the original hydrogel and it was seen that after 2.5 h of self-healing, there were no changes in the mechanical properties, for example, the elongation at break could reach up to 1900% and its maximum tensile strength was about 280 kPa in both cases (Figure 7
As aforementioned, an acid medium is often necessary to dissolve chitosan. However, its primary amino group allows easy functionalization. This is the case of quaternized chitosan which, in addition to possessing a high charge density due to a high degree of substitution, has very good solubility in water. Therefore, quaternized chitosan has been successfully used in the development of hybrid hydrogels based on a double crosslinking [107
]. On the one hand, direct in situ polymerization of acrylic acid (AA) monomers within a concentrated solution of quaternized chitosan and, on the other hand, ionic crosslinking between the positive charges of the quaternized chitosan and the negative charges of poly (acrylic acid) (PAA). The results revealed that these hydrogels, in addition to presenting very good mechanical stability due to the strong electrostatic interactions between the polysaccharide and the PAA (maximum stress 16.1 MPa), also had the capacity to recover their initial shape after rupture in just 1 min after having them immersed in water. The self-repairing process was primarily driven by electrostatic interactions between quaternized chitosan and PAA.
Another type of hybrid hydrogels that could be included in this classification is based on alginate and polyacrylamide [108
]. Thanks to alginate, a reversible non-covalent network is achieved based on ionic interactions between alginate and divalent ions (Ca2+
) through which self-repair is achieved. On the other hand, to provide the gel with good stability and mechanical resistance, a double covalent network is added, based on an in situ photopolymerization between the polyacryamide and the monomer N
-methylenebisacrylamide. The results of the mechanical tests revealed that during elongation the covalent bonds remain stable (they can stretch 20 times their initial length) and it is the ionic bonds that dissipate energy by breaking. These hybrid hydrogels reach up to 9000 J m−2
when other synthetic hydrogels have hardly reached 1000 J m−2
]. Thanks to the ionic bonds, the gel was able to recover its initial shape by up to 74% after one charge-discharge cycle.
3.4. In Situ Hydrogels Formed via Dynamic Covalent Bonds with Self-Healing
Click chemistry is one of the most exploited strategies for the development of smart in situ hydrogels, specifically for the development of self-healing hydrogels. Different polysaccharide-based hydrogels were obtained in situ by click chemistry with capacity to self-repair. For instance, alginate has been successfully used so as to obtain scaffolds with enhanced biodegradability and biocompatibility [43
]. As has been commented above, physically crosslinked hydrogels usually do not have good mechanical stability so, in order to obtain more consistent hydrogel, some researchers added cellulose nanocrystals (CNCs). Initially, polysaccharides and the CNCs, to get the aldehyde groups necessary for the Schiff reaction, were oxidized and subsequently, vinyl monomers modified with amine groups were introduced to the CNC surface and backbone of the alginate using the Schiff-base reaction. Hydrogels with a homogeneous chemical structure were formed in a few minutes with self-heal ability in 3 h at room temperature thanks to the hydrogen bonding and chain entanglements (Figure 8
Other polysaccharides, such as gelatin, have also been employed in the development of in situ self-healable hydrogels. Vahedi et al. [112
] developed a new gelatin-based in situ hydrogel prepared by Schiff-base reaction. Gelatin is a natural, biocompatible and biodegradable polysaccharide that presents amino groups in its backbone which are suitable for reacting with poly(ethylene glycol) di-benzaldehyde without the need for borax or any other type of chemical agent. The hydrogels are created in no more than 20 s and show high capacity to be injected, as well as quick self-repair (10 min) without the need of any external factor (Figure 9
) thanks to the dynamic imine bonds. This rapid self-healing and ability to be injected confers to the hydrogel the opportunity to be extruded through a syringe which, as shown in Figure 9
, the mixture of half of the hydrogels merged perfectly due to the combination of both colors.
Chitosan has also been selected on numerous occasions for the formation of dynamic bonds by the Schiff-base reaction. Glycol chitosan (GCH), due to its improved solubility compared to traditional chitosan, has been used together with telechelic difunctional poly(ethylene glycol) (DF-PEG) to develop in situ hydrogels capable of self-healing [56
]. Thanks to the -NH2
groups of the polysaccharide chain and to the benzaldehyde groups of DF-PEG, dynamic Schiff-base covalent bonds were satisfactorily established, resulting in the gelation of the material in just 1 min. As discussed previously, due to the quasi-covalent nature of Schiff-base mediated bonds, hydrogels were expected to self-repair. That is why to verify it, the traditional study of cutting the hydrogel in two halves was left aside and studying the self-healing process after being injected was chosen, that is, it was expected that when adding pressure and extruding the material, the links break and when the pressure stops the links form again. To do this, they stained the hydrogel (GCH-DF-PEG) with blue and put it in a syringe. Meanwhile, in another syringe they put a traditional gelatin gel stained with rhodamine B, thus acquiring the pink color seen in Figure 10
. As can be seen in the aforementioned figure, the evidence of the self-repair process is clear. After injection, only the blue gel (GCH-DF-PEG) was able to regenerate again. That process lasted around 30 min and without the help of any external stimulation thanks to the reversible Schiff-base bonds.
Other studies have also relied on GCH-DF-PEG hydrogels for biomedical applications so as to obtain in situ self-healing hydrogels [113
]. In the case of Figure 11
, the self-healing process is studied in a different way from that shown in Figure 10
. In this case, the GCH-DF-PEG gels were able to regenerate a hole in the center of the material while the alginate hydrogels used in this case as a control could not regenerate. Once again, it is clear that the Schiff-base bonds are pseudo-covalent which makes them capable of re-establishing after fracture. More recently, hyaluronic acid has been included to GCH-DF-PEG hydrogels leading to semi-interpenetrating polymer network with improved injectability and differentiation of in vitro loaded neural stem cells [114
Another derivative of chitosan, N
-succinyl chitosan (SC), has also been used in the development of in situ hydrogels capable of self-regeneration in combination with chondroitin sulfate multiple aldehyde. Schiff-base bonds between aldehyde groups of chondroitin sulphate and amine group of chitosan caused the hydrogel to form in time ranging from 34 to 41 s depending on the molar ratios between the polysaccharide and the chondroitin sulfate. As in the previous cases, the self-healing process took place satisfactorily in just 2 h without the use of any external stimulus, and the results can be seen in Figure 12
. Thanks to the cytotoxicity tests which show favorable results, this material, unlike the previous study, was tested in vivo on mice. Thanks to the fact that they are injectable, they could easily inject subcutaneously into the mice and none showed signs of local irritation. As time passes, the size of the hydrogel in the mouse’s skin decreased, making it clear that the biodegradation process was taking place (Figure 13
As indicated before, in situ self-healing hydrogels with pH-responsiveness present as promising materials for bioapplications, in particular for those related with controlled release of drugs. Chitosan-derived N
-carboxymetil chitosan was used combined with dibenzaldehyde-terminated poly(ethylene glycol) (PEG-DA) for the research of hepatocellular carcinoma [36
]. The biocompatible hydrogel was synthesized thanks to dynamic covalent Schiff-base bonds between primary amine groups of chitosan and benzaldehyde groups of PEG-DA. The self-healing process was studied macroscopically and rheologically. As shown in Figure 14
, the different color parts come together perfectly and there are even certain areas that acquire a bluish color, indicating that there is indeed molecular movement between the different parts of the hydrogel. This movement is given thanks to Schiff-base pseudo-covalent bonds that, as has been shown in previous studies, allow the hydrogel to self-repair. The hydrolytic degradation given due to pH sensitivity is explained by chitosan. As previously reported, the microenvironments in the tumor areas acquire a certain acidic pH that, considering the pKa of the chitosan is more or less 6.5, it would be protonated, positively charged. Therefore, the Schiff-base reaction between the primary amine group of chitosan and the aldehyde group of PEG-DA weakens and decomposition of the hydrogel takes place.
Despite Schiff-base reaction is the most employed click reaction in the manufacturing of in situ hydrogels, Diels-Alder or Michael additions have been also reported for the synthesis of in situ and self-healable hydrogels. In the work by Ye et al. [112
], a novel in situ polysaccharide-based hydrogel with maleitated chitosan and thiol derivatized sodium alginate was developed via Michael addition between vinyl groups and thiolated anions and ionic interactions between thiol-derivatized sodium alginate and Ca2+
]. This dual-crosslinked hydrogel can self-heal cracks due to disulfide exchange in no more than 12 h at physiological temperature without any external stimuli or chemical factors. Furthermore, the authors proved that the self-healing process could be favored by adding calcium ions, which accelerated the process until the cracks disappeared in 1 min (Figure 15
) since the crosslinking takes place instantaneously. On the other hand, a dual crosslinked polysaccharide-based in situ forming biodegradable hydrogel was developed by substitution of the carboxylic groups of alginate with furan, which leads in the ability of the polysaccharide to react by Diels-Alder click reaction with poly(ethylene glycol) modified with maleimide end groups as a crosslinker. The results showed that the hydrogel could recover its original shape at incubating overnight at 37 °C and it is able to maintain its mechanical stability (70% of recovery) slightly the same as the original hydrogel thanks to the ionic interactions between furan-modified alginate and calcium divalent cations and dynamic covalent bonds between maleimide and furan-modified alginate.
Finally, it is worth noting that although the dynamic covalent bonds shown above are undoubtedly the most widely used in the development of in situ hydrogels capable of healing, the boronic ester bonds that occur when reacting groups of phenyl-boronic acids (PBAs) and 1,2 and 1,3 diols are also an attractive option towards the development of self-healing biomaterials for biomedical applications. This is the case, for example, of a hydrogel based on different derivatives of the polysaccharide hyaluronic acid. Firstly, hyaluronic acid was modified with maltose and with PBA groups. PBA has the ability to react cis-diol units and therefore it was expected that it could modify the polysaccharide easily. However, this did not allow the PBA groups to successfully complex as it happened with other neutral sugars such as glucose and, therefore, different researchers have chosen to add terminal glucose to each repetitive unit of the polysaccharide chain promoting the modification of hyaluronic acid. Once the polysaccharide had been modified with the different groups, the corresponding hydrogel formed in aqueous solution under temperature and physiological pH almost instantaneously. Furthermore, it should be noted that it is commonly accepted that the ideal pH for boronate esterifications is around the pKa of the species that contains these groups. However, in the case reported above it was successfully achieved at neutral pH because hyaluronic acid, being a polyanion, decreased the pKa of the boronic acid units due to the interactions that were established between the carboxylic groups of hyaluronic acid and the PBA. To test the reversibility of these dynamic covalent bonds, the self-healing process of the hydrogels was studied. The gel showed almost immediate self-repair after stressing it out. Furthermore, after five cycles of cut-heal the gel remained stable showing that it did not lose its characteristic mechanical stability, since the elastic modulus values remain intact (~1000 Pa).
Other studies have also corroborated the ability of these pseudo-covalent bonds to develop self-healing hydrogels. For example, alginate has also been used successfully in the synthesis of these type of biomaterials. As in the previous study and taking advantage of the fact that alginate has alcohol groups along its chain, it is modified with boronic acid groups to promote the conjugation between the carboxyl groups of the polysaccharide and the anime groups of the boronic for the creation of dynamic boronic ester bonds. The hydrogel formed in just a few minutes thanks to the links established in the cis-diols of the alginate chain with the modified BA groups in the polysaccharide. Thanks to the reversibility of the dynamic bonds established between the BA and alginate diols, it was possible to self-repair without the help of any external stimulus in just 5 min. On the other hand, it is typical that covalently cross-linked three-dimensional networks suffer fractures at high stresses due to their rigidity. However, this hydrogel could be stretched 23 times in the longitudinal direction (23 cm) and left no evidence of fracture. This behavior, however, was not seen in the traditional alginate hydrogel (alginate/Ca2+).