A soft biomaterial that stiffens near body temperature for tendon reinforcement

Soft polymer biomaterials are attractive for tendon and ligament interfaces because they can bend, deform, and adapt to irregular tissue surfaces, yet their softness often limits the strength, toughness, and load-bearing capacity required after placement. Here, we address this conflict using a crystallization-programmed poly(ethylene glycol) diacrylate (PEGDA)-based network that remains soft before application but stiffens near body temperature. This design uses crystallization of poly(ethylene glycol) (PEG) segments as a latent reinforcing mechanism, allowing a soft, shape-adaptable material to become mechanically supportive at tendon and ligament interfaces.

Networks were synthesized from PEGDA, a dithiol chain extender, and a four-arm thiol crosslinker by varying the molar ratios of these components, thereby controlling the molecular weight between crosslinks (Mc). Differential scanning calorimetry was used to determine crystallization temperature (Tc), melting temperature (Tm), and transition enthalpy (ΔH), while temperature-sweep and isothermal rheology quantified changes in storage modulus (G′) and loss modulus (G″) during crystallization. The networks showed a composition-dependent increase in G′, reaching up to approximately three orders of magnitude in selected formulations, consistent with crystal-derived physical crosslinks. Higher Mc and more mobile chain segments promoted crystallization by reducing covalent constraints on PEG chain ordering.

These thermomechanical characteristics suggest a body-temperature-activated reinforcement strategy for tendon and ligament interfaces. Overall, this work frames PEGDA crystallization as a functional biomaterial design strategy, demonstrating how a soft network can be programmed to become mechanically supportive after placement and offering a design principle for conformable, adaptive biomaterials.