Bio-Inspired Suture Joints Enable Tuned Load Transfer in Digitally Fabricated Timber with Hybrid Thermoplastic Connectors

Nature employs complex interlocking geometries, such as vertebrate cranial sutures, to combine load transfer with localized compliance and geometric interlocking. These biological interfaces incorporate alternating wide regions and narrow necks that can accommodate localized deformation while maintaining mechanical interlocking. Inspired by these structural characteristics, digitally fabricated timber systems—such as Skylark–WikiHouse assemblies—can employ analogous bow-tie joint geometries to introduce controlled compliance into the connection. While conventionally manufactured from plywood, the translation of this bio-inspired geometry using hybrid thermoplastic–timber materials remains insufficiently explored. To address this gap, hybrid bow-tie joints comprising 18 mm radiata pine plywood panels and compression-molded polypropylene (PP) or high-density polyethylene (HDPE) connectors were evaluated. Ten joints underwent displacement-controlled monotonic tension at 2 mm/min. The results indicate that PP and HDPE joints achieve nearly identical mean maximum loads of 6.61 and 6.65 kN, respectively. However, substituting PP with HDPE significantly alters the global load–displacement response, yielding 33% lower global stiffness, 63% greater displacement at maximum load, and 70% greater external mechanical work. Post-test examinations revealed that final visible cracking was localized to the surrounding plywood, with no complete rupture of the thermoplastic connectors. This final damage localization is consistent with the intended suture-inspired design concept; however, the present post-test observations do not establish the temporal sequence of crack initiation and propagation or directly demonstrate crack arrest. These findings identify connector material as a practical bio-inspired design variable for tuning the global stiffness–deformation response of digitally fabricated timber joints while maintaining comparable maximum tensile load capacity.