Characterization of Latent Heat-Releasing Phase Change Materials for Dermal Therapies

Phase change materials (PCMs) have been developed as heat storage solutions, particularly for the supply of green energy. One interesting use of PCMs is long-term energy storage, in which a phase transition is triggered externally. In addition, PCMs are a potential method of administering heat for therapeutic purposes (therapeutic hyperthermia). This exciting therapeutic intervention provides an adjunct to chemotherapy and the potential to improve drug absorption. The purpose of this study was to characterize and control the heat generation process from supercooled salt PCMs. Salts were selected on the basis of their lack of toxicity and previous use in medicine. The crystallization of sodium formate, acetate (SA), and thiosulfate (ST) was characterized and monitored for heat generation, Only SA and ST had the wide metastable limits and exothermic crystallization appropriate for safe dermal (32-50 degrees C). The crystallization rate, heating profile, and heat capacities (C-p) of the PCM and dry crystals at different supersaturation levels (high (H) and low (L)) were determined. A low degree of supersaturation produced lower maximum (e.g., SA(H) T-max = 55.7 +/- 0.3 degrees C, SAL T-max = 39.1 +/- 0.3 degrees C), whereas low crystallization rates provide prolonged hyperthermia (e.g., STH DUR 1125 s). The heat release profiles can be optimized for therapeutic applications by controlling the following variables: (1) mass of salt which crystallizes (enthalpy released), (2) rate of crystallization, and (3) relative heat capacities of the supercooled solution and crystalline material. As an example, therapeutically relevant hyperthermia increased lidocaine flux across a model skin membrane from 7.18 +/- 1.8 to 33.7 +/- 2.6 mu g cm(-2). The current study has characterized the thermogenesis from pharmaceutically acceptable materials and provides for the development of PCMs as reliable, effective, and therapy-specific heat administration systems.