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

Bioprinting for bone tissue engineering

The shape transformation characteristics of four-dimensional (4D)-printed bone structures can meet the individual bone regeneration needs, while their structure can be programmed to cross-link or reassemble by stimulating responsive materials. At the same time, it can be used to design vascularized bone structures that help establish a bionic microenvironment, thus influencing cellular behavior and enhancing stem cell differentiation in the postprinting phase. These developments significantly improve conventional three-dimensional (3D)-printed bone structures with enhanced functional adaptability, providing theoretical support to fabricate bone structures to adapt to defective areas dynamically. The printing inks used are stimulus-responsive materials that enable spatiotemporal distribution, maintenance of bioactivity and cellular release for bone, vascular and neural tissue regeneration. This paper discusses the limitations of current bone defect therapies, 4D printing materials used to stimulate bone tissue engineering (e.g., hydrogels), the printing process, the printing classification and their value for clinical applications. We focus on summarizing the technical challenges faced to provide novel therapeutic implications for bone defect repair.

Hofmeister effect in gelatin-based hydrogels with shape memory properties

The soaking strategy with the Hofmeister effect has been proposed to fabricate gelatin- based hydrogels with excellent properties. However, the modulation mechanism of hydrogels lacks in-depth study. In this work, we studied in detail the effects of Hofmeister ions on the structural, thermal, viscoelastic and mechanical properties of gelatin hydrogels. The results showed that kosmotropic anions (Cit^3-, SO₄^2-, H₂PO₄^- and S₂O₃^2-) enhanced hydrogen bonds and hydrophobic interactions between gelatin molecules, resulting in increases in the length and content of triple helices and thus improving the properties of gelatin hydrogels. In contrast, chaotropic anions (I^- and SCN^-) weakened the interactions between gelatin molecules, and thus attenuated the properties. Based on the Hofmeister effect, we successfully fabricated gelatin poly N-methylolacrylamide (PNMA) double network hydrogels with shape memory properties. The Hofmeister effect provides an excellent route for the rational design and fabrication of functional gelatin-based hydrogels.

Photoswitchable architecture transformation of a DNA-hybrid assembly at the microscopic and macroscopic scale

Molecular recognition-driven self-assembly employing single-stranded DNA (ssDNA) as a template is a promising approach to access complex architectures from simple building blocks. Oligonucleotide-based nanotechnology and soft-materials benefit from the high information storage density, self-correction, and memory function of DNA. Here we control these beneficial properties with light in a photoresponsive biohybrid hydrogel, adding an extra level of function to the system. An ssDNA template was combined with a complementary photo-responsive unit to reversibly switch between various functional states of the supramolecular assembly using a combination of light and heat. We studied the structural response of the hydrogel at both the microscopic and macroscopic scale using a combination of UV-vis absorption and CD spectroscopy, as well as fluorescence, transmission electron, and atomic force microscopy. The hydrogels grown from these supramolecular self-assembly systems show remarkable shape-memory properties and imprinting shape-behavior while the macroscopic shape of the materials obtained can be further manipulated by irradiation.

Engineering shape memory and morphing protein hydrogels based on protein unfolding and folding

Engineering shape memory/morphing materials have achieved considerable progress in polymer-based systems with broad potential applications. However, engineering protein-based shape memory/morphing materials remains challenging and under-explored. Here we report the design of a bilayer protein-based shape memory/morphing hydrogel based on protein folding-unfolding mechanism. We fabricate the protein-bilayer structure using two tandem modular elastomeric proteins (GB1)₈ and (FL)₈. Both protein layers display distinct denaturant-dependent swelling profiles and Young’s moduli. Due to such protein unfolding-folding induced changes in swelling, the bilayer hydrogels display highly tunable and reversible bidirectional bending deformation depending upon the denaturant concentration and layer geometry. Based on these programmable and reversible bending behaviors, we further utilize the protein-bilayer structure as hinge to realize one-dimensional to two-dimensional and two-dimensional to three-dimensional folding transformations of patterned hydrogels. The present work will offer new inspirations for the design and fabrication of novel shape morphing materials.

Engineering shape memory and morphing materials achieved considerable progress in polymer-based systems, but protein-based shape memory and morphing materials remain less investigated. Here, the authors report the engineering of protein-based shape memory and morphing hydrogels using protein folding-unfolding as a general mechanism to trigger shape morphing in protein-bilayer structures.