Strain-stiffening Modular Gels with Dynamic, Secondary Cross-Linking

Introduction:: Biological tissues prevent damage from mechanical deformations during development, wound healing, disease progression, and external stress by employing unique mechanoresponsive behavior known as strain-stiffening. In contrast, the stiffness of synthetic gels is typically fixed at the synthesis stage, and mechanical stress often results in degradation. Capturing strain-stiffening properties in synthetic soft materials is challenging yet crucial in producing tissue-mimicking biomaterials for biomedical applications. We recently demonstrated a generic approach for creating force-responsive gels wherein networks can undergo further, permanent changes in modulus upon application of repeated compressive force (1). Building on this concept, we present a modular strain-stiffening gel system capable of post-synthetic modifications. To demonstrate this modular approach, a gel network consisting of free primary amine groups is used (Figure 1A). The aldehyde bifunctional molecule (terephthaldehyde, TERE, or glutaraldehyde, GLUT) is employed as the secondary crosslinker. The mesh size of the gel network is kept so that the interchain cross-linking is prevented under static conditions. In contrast, when cyclic compression is applied to the network, new interchain cross-links are formed, stiffening the gel network.


Materials and Methods:: A copolymer of N-(3-Aminopropyl)methacrylamide hydrochloride (APMA) and 2-Methoxyethyl methacrylate (MEMA) was synthesized through RAFT polymerization. The gels were fabricated by cross-linking the AMPA/MEMA copolymer with a polyethylene glycol diacrylate (PEGDA) crosslinker of 700 g/mol molecular weight. The mechanical characterization and strain-stiffening studies were conducted on a Kinexus Pro+ rheometer (Malvern Instruments, USA) at a compression rate of 20 μm/s (up to 20 % strain). The elastic modulus was determined from the initial linear region of the stress-strain curve. The gel was placed in a petri dish containing a 50 mM solution of TERE or GLUT in DMSO to perform strain-stiffening studies. The petri dish with the gel was placed between the parallel plates of the rheometer, and the top plate was lowered such that hydrogel was confined between parallel plates. Compression and tension cycles were performed at 20 μm/s with 0−20% strain.


Results, Conclusions, and Discussions:: Results and Discussion: Gels were synthesized by varying PEGDA crosslinker (PEGDA, Mw 700 g/mol, length: ~ 3 nm) concentrations. The measured elastic moduli were between 100-300 kPa (Figure 1B). To demonstrate strain-stiffening, a secondary crosslinker was used (TERE in this example). The aldehyde functional groups in TERE can react with the primary amines in the network to form secondary cross-linking and thereby increase the modulus. However, under static conditions, the mesh size of the network doesn’t permit this process, leading to a minimum increase in the modulus (Figure 1C). When cyclic compression is applied in the presence of TERE solution, A significant modulus increase was observed (change in the elastic modulus around 75 kPa within 1 hour), demonstrating the strain-stiffening of the gel network. The increased modulus of the network is reversible owing to the reversible nature of the imine bond formed from TERE. However, if TERE is replaced with GLUT, the resultant network becomes irreversible, illustrating the post-synthetic modularity.

Conclusions: We illustrated a simple approach to create a force-responsive, modular, strain-stiffening gel system. This strategy can be adapted to develop dynamic hydrogels to study how dynamic extracellular matrices are associated with disease and injury and change cell behavior.





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References (Optional): : (1) Langmuir 2023, 39,2659−2666