Synthesis and characterization of starch ether/alginate hydrogels with reversible and tunable thermoresponsive properties

Utilization of natural polysaccharide polymers for the design of hydrogels has long been a research hotspot in the past several years (Bibi et al 2019, Graham et al 2019). Of the numerous macromolecules that have been used for the synthesis of hydrogels, polysaccharide-based hydrogels exhibit a number of advantages over synthetic polymers. On one hand, polysaccharide-based hydrogels have a three-dimensional network structure that is like that of traditional synthetic polymer hydrogels; this makes the hydrogels insoluble in water and able to maintain a stable structure in water (Wei et al 2017). On the other hand, because of the biocompatibility and biodegradability of polysaccharides (Hubbe et al 2019b), polysaccharide-based hydrogels possess excellent properties and show potential application value in several fields (Stabenfeldt et al 2006, Vulic et al 2015, Hubbe et al 2019a). Because of the superiority of polysaccharide-based hydrogels, particularly, hydrogel prepared from sodium alginate displays outstanding characteristics such as gelling capacity, low toxicity, high availability and low cost (Giri et al 2012, Hernandez-Gonzalez et al 2020), and it can be tailor-made to fit different demands of various practical fields.

Sodium alginate (SA) is a kind of naturally-occurring polysaccharide that is mainly extracted from brown seaweed (Smidsrod and Skjakbraek 1990) and is an anionic copolymer composed of 1,4-linked-α-L-guluronic acid and β-D-mannuronic acid (Wan et al 2018). It tends to ionically cross-link in the presence of divalent cations such as calcium ions (Tan and Ting 2014), and thus, it has been generally used for preparing hydrogel. To leverage the excellent biocompatibility, high availability, nontoxicity, and biodegradability of SA, SA–based hydrogel has been investigated in depth and applied in multiple fields, such as tissue engineering (Reakasame and Boccaccini 2018), wound healing (Aderibigbe and Buyana 2018), drug delivery (Motealleh et al 2019), cell culture (Lee et al 2010), and adsorbents (Ren et al 2016). Recently, the functional properties of SA-based hydrogel have been improved, and the addition of stimuli-sensitive moieties in alginate can lead to 'smart' materials that have stimuli-responsive hydrogel characteristics (Kass et al 2019, Swamy et al 2013, Soledad Lencina et al 2015).

Thermoresponsive hydrogels are a class of smart hydrogels that are able to change their physicochemical properties (for example, volume) with respect to changes in temperature (Kim and Matsunaga 2017, Zhao et al 2019). Specifically, thermoresponsive hydrogels can switch from being in a hydrophilic, water swollen state to being in a hydrophobic, shrunken state when they are heated to a temperature above the volume phase transition temperature (VPTT) (Chang et al 2015). Such hydrogels that exhibit reversible volume transition behavior have received widespread attention because changing the temperature is convenient and cost-effective in potential applications (Jeong et al 2002). Therefore, endowing SA hydrogels with a thermoresponsive property can make it possible to make full use of the potential of SA hydrogels. Scientific studies have focused on hydrogel formation via the grafting of copolymers, which have an alginate backbone and synthetic thermoresponsive polymer side chains. The most typical thermoresponsive grafted copolymer hydrogels that have been prepared involve grafting poly(N-isopropylacrylamide) (PNIPAM) onto an alginate (ALG) backbone (Ciocoiu et al 2018). ALG-g-PNIPAM hydrogels have a VPTT of 32 °C–35 °C, depending on the grafting characteristics (Leal et al 2013). In other words, synthetic thermoresponsive polymer moieties control the VPTT of these hydrogels. However, these thermoresponsive hydrogels have their own limitations: (i) It is difficult to tune the VPTT of these hydrogels because the methods used to change the thermoresponsive nature of a synthetic polymer are complex. (ii) The disadvantages of biomedical potential applications of PNIPAM are that they are nonbiodegradable, synthetic polymers and are mostly cross-linked with toxic chemical components (Haq et al 2017). When these factors are taken into consideration, it is of great interest to substitute polysaccharides in synthetic polymers to prepare thermoresponsive alginate/polysaccharide composite hydrogel. Additionally, it is not surprising that polysaccharide-based thermoresponsive hydrogels will attract more extensive use in many biomedical applications if convenient methods can be used to adjust their VPTT.

Starch is a natural polymer that is low-cost, plentiful, easily modified, and environmentally friendly (Xie et al 2013). Starch Ether is a typical kind of modified starch that can acquire a thermoresponsive property if the hydrophilic and hydrophobic balance is modified (Liu et al 2018). In our previous work (Ju et al 2012, Ju et al 2014), a series of novel thermoresponsive starch ethers with a network structure were synthesized, and they seem to be ideal candidates for preparing thermoresponsive hydrogels. We propose that composite hydrogels consisting of starch ether and SA can possess a thermoresponsive property and macroporosity.

In the present work, starch ether and sodium alginate were used with a hybrid cross-linking agent to prepare a thermoresponsive polysaccharide composite hydrogel. The thermoresponsive property of the composite hydrogel was investigated in terms of the swelling ratio (SR). The obtained HIPS/SA hydrogels were measured using attenuated total reflection infrared spectroscopy (ATR-IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). Swelling and deswelling kinetics were investigated. The VPTT was tuned as a result of changes in the HIPS concentration to systematically investigate the NaCl concentration and organic solvent.