NSRRC Activity Report 2022

034 NSRRC ACTIVITY REPORT 2022 S oft matter, especially smart soft matter, can be developed as biomedical materials and used to repair human tissues. In 2017, in the article 1 “I will be back: the return of rubber,” Shan-Hui Hsu (National Taiwan University) shared the novel shape memory mechanism enlightened by using small- angle X-ray scattering (SAXS) technique at Taiwan Light Source. Elastomer (or rubber) is a type of soft matter that exhibits elasticity ( i.e. , the ability to deform freely when an external force is applied and to return to its original shape after force removal). Elastomer without shape memory has formed only one permanent shape. Elastomer with shape memory, however, has formed two permanent shapes. In each permanent shape, it can be deformed freely and recovered upon force removal. The switch between the two permanent shapes is often triggered by a temperature change. In the earlier study, 2 the trigger was water at 37°C; a small-shaped elastomer was discovered to expand into a large-shaped elastomer when placed in 37°C water. This indicates that such elastomer is suitable for application in medical devices and can be used for minimally invasive surgery. The SAXS results revealed that the retention of a crystalline orientation during the integration of shape memory materials into elastomer could be considered a novel mechanism for creating such materials. Fig. 1 : Cryogel, a sponge-like soft matter, is generated from a polymer solution at freezing temperatures. This involves the use of a crosslinker to link the polymer chains and to form a network. Ice crystals fill the pores within the network. At room temperature, the ice melts and the remaining porous network forms the cryogel. [Figure drawn by Shan-Hui Hsu] Sponge-Like Cryogels Cellulose Nanofibers are used to overcome the Problem of Shape Fixing and Recovery in Biodegradable Chitosan Cryogel. The present study 3 focused on cryogel, another type of soft matter. Cryogel is also elastic but exhibits properties more similar to those of a sponge ( Fig. 1 ). It can be compressed by external forces and recovered upon force removal. Cryogel does not exhibit shape memory but forms only one permanent shape, as elastomer does. The high porosity of cryogel would be generated after polymer solution froze and then melted. At low temperatures, the polymer molecules form a network in which large pores are generated and left empty after the ice crystals melt within. As a highly compressive sponge, cryogel can be injected through a needle. It will be amazing if one can repair human tissue by injecting a cryogel instead of open- approach surgery. The difficulties of shape memory cryogel served as the biomaterial in minimally invasive surgery application includes (1) the highly compressive cryogel may clog the needle because the permanent shapes of cryogels are complicated; (2) a shape memory cryogel with two permanent shapes should have a smaller, rod-like shape for smooth injection and a larger, more complex shape after exposure to 37°C water (body temperature); (3) the cryogel is required to be biodegradable. Such kind of shape memory cryogel is rarely developed, but Hsu did it. Hsu selected chitosan, which is derived from crab and oyster shells and is biodegradable, as the polymer matrix. Hsu used the shape memory elastomer from the previous research 2 and modified the end group to ensure it could crosslink with the chitosan in an aqueous solution at freezing temperatures to form a network. After the ice was melted, a cryogel that exhibited shape memory was formed because of the shape memory crosslinker. Although SAXS revealed that the crystalline orientation of the polymer network accounted for the shape memory mechanism, the interaction between the chitosan and the shape memory crosslinker deflected the SAXS signals. This first-generation shape memory cryogel was injectable and expanded in 37°C water. Moreover, the aqueous solution was able to be three-dimensionally printed and frozen to form custom- shaped memory cryogel ( Fig. 2 ). 4 Hsu developed the second generation of shape memory cryogel with an increased degree of shape memory. Instead of shape memory elastomer, Hsu used a simpler crosslinker, cellulose nanofiber, to form a chitosan network at low temperature. Cellulose nanofiber is from a green resource (obtained from trees) and can be modified to contain