Dextran–derived Thermoset Substrate Enabling Low Transmission Loss at High Frequencies

Sustainable communication devices require insulating substrates that combine low transmission loss with renewable material origins. Here, we developed a cross-linkable dextran (α-1,6-glucan) ester bearing both unsaturated and saturated substituents, enabling the fabrication of high-performance thermoset substrates. Peroxide-assisted cross-linking at 180 °C yielded films with a glass transition temperature above 200 °C, durable for soldering, and excellent dielectric properties (dielectric constant, Dk = 2.5, dissipation factor, Df = 0.009), superior to conventional epoxy-based substrates represented by FR-4 (∼4, ∼0.015). These films could be directly laminated with copper foil without adhesives. Coplanar transmission lines were subsequently fabricated, and signals up to 67 GHz were successfully transmitted with 40% lower loss compared with FR-4. This work represents the first demonstration that a biomass-derived thermoset polymer can support low-loss signal propagation in the 5G and 6G frequency bands, offering a sustainable pathway toward next-generation electronic substrates that unite advanced performance with environmental responsibility.