Self-healing, stretchable, and freezing-resistant hydroxypropyl starch-based double-network hydrogels

Highlights

• Double-network hydrogels were prepared by hydroxypropyl starch and sodium alginate.

• The hydrogels formed by hydrogen bonding and metal coordination crosslinking.

• The hydrogels were stretchable, compressible, and self-healing.

• The hydrogels prepared by water and glycerol can maintained gel properties at -30 °C.

Abstract

Starch hydrogel is a biocompatible and biodegradable material. However, due to its poor mechanical properties and fragility, starch hydrogel has limited applications in the food and medicine industries. In this work, we prepared stretchable, compressible, and self-healing double-network (DN) hydrogels using hydroxypropyl starch (HPS) and sodium alginate (SA). By adjusting the amount of sodium alginate added, the DN hydrogels achieved adjustable mechanical properties. The storage modulus of the DN hydrogels with 1% SA increased by nearly 80 times compared with that of pure HPS hydrogels. When prepared in a water/glycerol binary mixed solvent, the DN hydrogels can maintain their gel properties at -30 ºC. These environmentally friendly and biocompatible hydrogels have broad application prospects in the fields of agriculture, food, and medicine.

Introduction

As a hydrophilic material, hydrogel is a three-dimensional (3D) porous network structure composed of polymer chains (Qin, Wang, Qiu, Xu, & Jin, 2019). Due to their various adjustable physical and chemical properties (self-healing, 3D printing, pH response, thermal response, etc.), hydrogels have a wide range of applications in the fields of industry, agriculture, and medicine, such as in water treatment, tissue engineering, drug delivery, biosensors, and intelligent robots (Li et al., 2019). However, most traditional hydrogels have poor mechanical strength and stretchability, which severely limit their applications (Sun et al., 2012). Therefore, enhancing the mechanical strength and stretchability of hydrogels is a great challenge. At present, strategies to improve the mechanical properties of hydrogels mainly include double-network (DN) hydrogels (Zhang et al., 2016), dual cross-linked hydrogels (Lin, Ma, Wang, & Zhou, 2015), nanocomposite hydrogels (Ge et al., 2018), and ion cross-linked hydrogels (Ren, Zhang, Li, Xu, & Liu, 2015), among others.

Synthetic polymers, such as polyacrylamide and polymethacrylate, are usually used to prepare hydrogels that are non-renewable and may cause environmental pollution (Xiao, 2013). In order to reduce environmental pollution and resource wastage, the preparation of high-performance hydrogels using bio-based polymers has elicited intensive attention. Starch is a natural biological macromolecule composed of amylose and amylopectin. It is the second most abundant carbohydrate in nature, except for cellulose, and has the advantages of being safe, cheap, and renewable. Starch molecules contain a large number of hydroxyl groups and have great cross-linking ability, so they are ideal materials for preparing hydrogels (Lima-Tenório et al., 2015). However, natural starch hydrogels generally have the disadvantages of poor mechanical properties, brittleness, and low stretchability, attributes that limit the widespread use of starch hydrogels (Qin et al., 2019). Therefore, improving the performance of starch hydrogels is still a challenging task. According to previous reports, some synthetic polymers or chemical agents are used to enhance the mechanical strength of starch-based hydrogels. For example, it has been reported that 3-(2,3-epoxypropoxy) propyl trimethoxysilane can cross-link the surface of modified starch to obtain high-gel strength starch hydrogels (Amiri et al., 2018). In addition, the mechanical properties of starch-based hydrogels can be enhanced by adding cellulose nanocrystals (Gonzalez et al., 2020). Another way is to use the DN strategy to prepare a polyvinyl alcohol/cornstarch hydrogel with high stretchability (Qin et al., 2019). However, there is a scarcity of reports on the preparation of DN starch-based hydrogels with enhanced mechanical properties using natural biological macromolecules. Recently, whey protein fibrils have been used to prepare potato starch DN hydrogels with improved storage modulus (G′) from 200 Pa to 800 Pa (Chen, Fang, Federici, Campanella, & Jones, 2020; Chen, Bu et al., 2020).

Compared with native starch, modified starch has some special physicochemical properties, so it has more extensive applications in different fields. Hydroxypropyl starch (HPS) is a kind of modified starch that is widely used in the food, paper, and textile fields. It is an etherified starch with the advantages of non-toxicity, high transparency, and good stability (Lin et al., 2019). Therefore, we hypothesized that starch hydrogels prepared from HPS might have better properties, such as self-healing and stretchability, than those prepared from native starch.

As hydrogels with 3D structures are infiltrated with water, when ambient temperature falls below the freezing point, most hydrogels composed of hydrophilic polymers will inevitably freeze and become fragile, thereby losing their original elasticity (Zhu et al., 2019). Therefore, the development of freezing-resistant hydrogels is greatly significant in expanding the applicability of hydrogels under low-temperature circumstances. Currently, two main methods are used to prepare anti-freezing hydrogels. One method is by incorporating lipophilic components into the polymer networks of hydrogels (Sui et al., 2019). For instance, Liu reported an organohydrogel based on a hydrophilic/oleophilic heteronetwork, which could endure -78 °C with enhanced mechanical property (Gao et al., 2017). The other method entails adding a high concentration of solutes (e.g., ethylene glycol, sorbitol) into the precursor solution of the hydrogels to reduce the freezing point of water and achieve the purpose of anti-freezing (Sui et al., 2019). For example, Liu and co-workers synthesized a conductive self-healing hydrogel using H₂O/ethylene glycol binary solvent, which has stable strain sensitivity in the temperature range of -55.0–44.6 °C (Rong et al., 2017).

Therefore, this work aimed to prepare a new type of anti-freeze DN hydrogels based on HPS and sodium alginate (SA). Native starch modification by hydroxypropylation can improve the stretchability of hydrogels. The mechanical properties of starch hydrogels were enhanced by introducing a second network of SA polymer. In addition, the DN starch-based hydrogels were synthesized using a water/glycerol binary solvent that can maintain gelation properties at low temperature. We hope that this stretchable, biodegradable, biocompatible, and freezing-resistant DN hydrogel will have a wide range of applications in food, biomedical, and other fields.