Measuring the Thermal Conductivity of Biomaterials


The thermal properties and conductivity of biomaterials have can have a dramatic affect on the performance of the material for implantation or for interfacing with biological tissues. With the advent of more sophisticated implants, including those that contain electrical components, thermal conductivity and diffusivity have become more of a concern. A prime example of this concern is with the 3-D Utah electrode array, a device which is used for implantation into the brain for electrical stimulation and recording. Thermal management for the electrical components on such implanted electronics can be a problem, as traditional polymeric materials tend to act more like thermal insulators rather than conductors. This can damage the electrical components, or more likely, damage the surrounding tissue. Typically, heat transfer within a polymeric material occurs through phonons, a heat wave which can propagate through the crystalline matrix of a polymeric material. The degree of crystallinity will affect the thermal conductivity of such materials, such as poly(methyl methacrylate) vs low density polyethylene.

To create more thermally conductive polymer materials, the degree of crystallinity and stacking of a polymer material must be high as to conduct heat away from the electrically active components. Typically, this means that materials which are more amorphous tend to act as good thermal insulators, and these amorphous materials also tend to be abundant in more biologically derived materials. One method to increase thermal conductivity of an amorphous material would be to create self-assembled structures that are highly organized. These materials typically are gels or self-assembled coated materials. Gel materials have several options for implantation, including acting as cell scaffolds for implantation or as artificial tissues for implantation. Coated materials for implants can consist of polymers which coat the metal circuits. Typically, upon implantation the coating material can swell and mimic the surrounding tissues mechanical properties. While mechanical properties may be important, their thermal properties are equally important, and represent an under-explored niche within the biomedical community.

Dry polymeric materials represent ideal substrates for most techniques to measure thermal conductivity, yet dry measurements of thermal conductivity do not represent what the performance of an implanted material would exhibit in vivo. Polymers swell, and possess little to no structural integrity when implanted, and represent poor samples for traditional steady-state techniques. Additionally, these wet and soft polymers can be used as coatings for embedded electronic devices, which are very thin and geometrically constrained.

C-Therm’s TRIDENT thermal analysis platform can provide unique insight into the thermal behavior of these more difficult materials in employing the modified transient plane source (MTPS) technique. Gel type materials can be explored using MTPS measurements ensuring that the thermal properties of these tissue-mimicking materials match the surrounding implantation environment. MTPS measurements of thermal conductivity can explore electrical components or coating materials, which often possess difficult geometric constraints which other techniques find difficult to measure.

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