Miniaturized stretchable electronic devices that can be bent and strained elastically without breaking, have drawn considerable research interest in recent years for wearable computers and integrated bio-sensor applications. Portable electrical power harvesting remains a critical challenge in flexible electronics materials. One proposed solution has been to convert waste heat from the human body into electricity using thermoelectric materials. Traditionally, however, these materials are brittle ceramic semiconductors with limited fracture resistance under deformation. The primary objective of this thesis is to address this challenge by fabricating and studying the mechanical, thermal and electrical performance of stretchable composites combining natural latex polymer with either metallic (Ni) or thermoelectric (InSb) powders. Ni-based and InSb-based latex specimens were synthesized with different powder concentrations up to 36 vol.%. The effects of the powder concentration on tensile elongation, electrical conductivity, and thermal conductivity of the composites were measured at ambient temperature. Strong dependences of mechanical and electrical properties on powder concentration were found. By contrast, thermal conductivity was observed to remain low at all concentrations, suggesting that the predominant heat transport process is through the low-conductivity latex matrix rather than the conductive particles. This thesis was conducted with the support of a Vermont Space Grant Consortium graduate research assistantship.