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Compliant sutures surrounded by stiff matrices are present in biological armors and carapaces, providing enhanced mechanical performance. Understanding the mechanisms through which these sutured composites achieve outstanding properties is key to developing engineering materials with improved strength and toughness. This article studies the impact of suture geometry and load direction on the performance of suture joints using a two-stage reactive polymer resin that enables facile photopatterning of mechanical heterogeneity within a single polymer network. Compliant sinusoidal sutures with varying geometries are photopatterned into stiff matrices, generating a modulus contrast of two orders of magnitude. Empirical relationships are developed connecting suture wavelength and amplitude to composite performance under parallel and perpendicular loading conditions. Results indicate that a greater suture interdigitation broadly improves composite performance when loading is applied perpendicular to suture joints, but has deleterious effects when loading is applied parallel to the joint. Investigations into the failure mechanisms under perpendicular loading highlight the interplay between suture geometry and crack growth stability after damage initiation occurs. Our findings could enable a framework for engineering composites and bio-inspired structures in the future. 

Reversibly programmable liquid crystal elastomer microparticles (LCEMPs), formed as a covalent adaptable network (CAN), with an average diameter of 7 μm ± 2 μm, were synthesized via a thiol-Michael dispersion polymerization. The particles were programmed to a prolate shape via a photoinitiated addition–fragmentation chain-transfer (AFT) exchange reaction by activating the AFT after undergoing compression. Due to the thermotropic nature of the AFT-LCEMPs, shape switching was driven by heating the particles above their nematic–isotropic phase transition temperature ( T NI ). The programmed particles subsequently displayed cyclable two-way shape switching from prolate to spherical when at low or high temperatures, respectively. Furthermore, the shape programming is reversible, and a second programming step was done to erase the prolate shape by initiating AFT at high temperature while the particles were in their spherical shape. Upon cooling, the particles remained spherical until additional programming steps were taken. Particles were also programmed to maintain a permanent oblate shape. Additionally, the particle surface was programmed with a diffraction grating, demonstrating programmable complex surface topography via AFT activation. 

Due to a mismatch in mechanical moduli, the interface between constituent materials in a composite is the primary locus for crack nucleation due to stress concentration. Relaxation of interfacial stresses, without modifying the properties of constituent materials, is a potent means of improving composite performance with broad appeal. Herein, we develop a new type of adaptive interface that utilizes thiol–thioester exchange (TTE) at the filler–polymer interface. Specifically, dynamic covalent bonds sequestered at material interfaces are reversibly exchanged in the presence of thioester moieties, excess thiol and a base/nucleophile catalyst. Employing this active interface effectively mitigates deleterious growth of interfacial stresses, thereby enhancing the composite's mechanical performance in terms of reductions in polymerization shrinkage stress and improvement in toughness. Activating interfacial TTE in an otherwise static matrix resulted in 45% reduction in the polymerization stress, more significant post-polymerization stress relaxation and drastically increased toughness relative to control composites incapable of TTE bond exchange but otherwise identical. In particular, the higher fracture toughness in TTE-activated composites is attributed to the alleviation of crack tip strain concentration, as revealed by digital image correlation.