(573i) Geometry and Composition of Soft Polymer Films Embedded with Nanoparticles Enhance Rates for Optothermal Heat Dissipation

Embedding soft matter with nanoparticles (NPs) can provide electromagnetic tunability at sub-micron scales for growing number of uses in biomedicine including image contrast agents, biosensors, tissue culture substrates, photothermal therapy, drug delivery, and gene transfection. But use of NP-embedded soft material in temperature-sensitive applications has been constrained by difficulty in validating prediction of rates for energy dissipation across thermally insulating to conducting behavior. For example, it has been shown that increasing NP concentration in photothermal therapy paradoxically resulted in tumor regrowth despite increased temperatures, because of reduced depth of laser penetration.

To date, rigorous evaluation of NP-induced photothermal effects in biological systems has been largely limited to characterizing steady state heat flux or temperature distributions. However, the (dynamic) rate of temperature increase, i.e., heating rate, in such systems is largely unexplored. The rate of heat dissipation is a critical factor influencing a variety of biomedical outcomes. For example, a heat rate not less than one degree celsius per minute is the European Society for Hyperthermic Oncology guideline for superficial hyperthermia clinical trials. In NP-containing materials, heating rate was reported to vary nearly 3-fold depending on the configuration of the NP. And heating transients have been shown to influence rate of drug release from polydimethylsiloxane (PDMS) films and phase change materials.

The present work examined changes in the transient rate for heat dissipation as inter-NP separation was reduced from greater than 5 microns to sub-micron intervals while varying the 3D geometry of a polymer and its NP density 2- to 15-fold.To our knowledge this is the first quantitative comparison of simulated and experimentally measured plasmon-thermal heating transients that spans thermally conductive to thermally insulative regimes. In order to achieve this result, embedment of monodisperse NP was improved using a refined fabrication process to stably decrease inter-NP spacings in biocompatible polymer to nano-scales. Lumped-parameter and finite element analyses were refined to apportion effects of structure and composition of NP-embedded soft polymer to rates for conductive, convective, and radiative heat dissipation. These advances allowed rational selection of PDMS size and NP composition to optimize measured rates of internal (conductive) and external (convective and radiative) heat dissipation.

Stably reducing distance between monodisperse NP to nano-scale intervals increased overall heat dissipation rate by up to 29%. Refined fabrication of NP-embedded polymer enabled tunability of dynamic thermal response, the ratio of internal to external dissipation rate, by a factor of 3.1 to achieve a value of 0.091, the largest reported to date. Heat dissipation rates simulated a priori were consistent with 130 µm resolution thermal images across 2- to 15-fold changes in NP-PDMS geometry and composition. Nusselt number was observed to increase with the fourth root of Rayleigh number across thermally insulative and conductive regimes, further validating the approach. These developments support model-informed design of soft media embedded with nano-scale spaced NP to optimize heat dissipation rates for evolving temperature-sensitive diagnostic and therapeutic modalities as well as emerging uses in flexible bioelectronics, cell and tissue culture, and solar-thermal heating.