Salt-Induced Shape-Memory Effect in Gelatin-Based Hydrogels

Abstract

Hydrophilic biopolymers display a strong tendency for self-organization into stable secondary, tertiary, and quaternary structures in aqueous environments. These structures are sensitive to changes in external conditions, such as temperature, pH or ions/salts, which may lead to molecular and/or macroscopic transitions. Here, we report on biopolymer-based stimuli-sensitive switchable matrices showing a shape-memory function as an output being alternatively switched by two different input signals, such as environmental changes in salt concentration or temperature. This was realized by implementing a shape-memory function in hydrogels based on the coil-to-helix transition of protein chains in gelatin-based networks. The hydrogels exhibited mechanical properties similar to that of soft tissue (storage modulus G′ = 1–100 kPa) and high swelling capabilities (Q = 1000–3000 vol %). In these gelatin-based networks, the covalent netpoints defined the permanent shape while after deformation helicalization of the gelatin acted as reversible stimuli-sensitive switches providing additional crosslinks capable of fixing the deformed temporary shape. By using either chaotropic salts to suppress gelatin helicalization or kosmotropic salts to support conformational changes of gelatin toward a helical orientation, these additional crosslinks could be cleaved or formed. In bending experiments, the strain fixity (R f) and strain recovery ratios (R r) were determined. While R f ranged from 65 to 95% and was depending on the network composition, R r were independent of the hydrogel composition with values about 100%. In addition, R f and R r were independent of the type of chaotropic salt that was used in this study, showing equal R f and R r values for MgCl2, NaSCN, and Mg­(SCN)2.

Thermally-Induced Shape-Memory Behavior of Degradable Gelatin-Based Networks

“…The networks are in the following referred to as G20_OEGy(z), with y = number average molar mass Mn of the OEG crosslinkers and z the thiol:methacrylate molar ratio. The gel content G, which is a measure of network formation, and the volumetric swelling Q of the G20_OEG1000(z) and G20_OEG3400(z) networks has been reported before [22] and is given in detail for the G20_OEG1500(z) in Supporting Information Table S1. As general trend, a reduction of G with increasing length of the crosslinker as well as with increasing z was observed, though there were only small differences between G20_OEGy(0.75) and G20_OEGy(1) samples.…”

“…We recently introduced a gelatin-based covalent network system that is formed by reacting glycidylmethacrylated gelatin (GMA-gelatin) with oligo (ethylene glycol) (OEG)-,-dithiols (Figure 1A) [22]. The approach to crosslink gelatin by a bifunctional crosslinker has in the past been demonstrated to be beneficial for effectively establishing a polymer network.…”

“…by simple polymeranaologous reactions the gelatin-based network could be established. The salt-based control of protein chain conformation in a hydrogel and exploiting it for a material function such as the shape-memory effect [22] is of fundamental interest, however, the required salt concentrations were likely not compatible with a biological environment. It is therefore of interest to investigate whether the shape recovery of gelatin/OEG-based SMH can be initiated by a stimulus compatible with the physiological environment, and whether changes in the composition of the networks influences properties and functions of the SMH.…”

Stimuli-Responsive Polymeric Nanocarriers for Drug Delivery, Imaging, and Theragnosis

“…The presence of gelatin significantly affected the hydrogel, and was responsible for the formation or deformation of a helical shape. Also, the study showed the potential activity of various chaotropic and kosmotropic salts which regulated the gelatin helicalization or conformational alterations [ 324 ].…”

Thermally-Induced Shape-Memory Behavior of Degradable Gelatin-Based Networks

“…We recently introduced a gelatin-based covalent network system that is formed by reacting glycidyl methacrylated gelatin (GMA-gelatin) with oligo(ethylene glycol) (OEG) α,ω-dithiols ( Figure 1 A) [ 22 ]. The approach to crosslink gelatin by a bifunctional crosslinker has in the past been demonstrated to be beneficial for effectively establishing a poly-mer network.…”