Improving ionic thermoelectrics Using ions as charge carriers in thermoelectric devices usually requires using either thermal diffusion or redox reactions at two electrodes with different temperatures. Han et al. leveraged both of these strategies to develop a gelatin-based ionic thermoelectric device that uses alkali salts and an iron-based redox couple to generate a large thermopower. This device is capable of generating useful amounts of energy from body heat. Science , this issue p. 1091