Microfabricated thermal conductivity sensor: a high resolution tool for quantitative thermal property measurement of biomaterials and solutions

Abstract

Obtaining accurate thermal properties of biomaterials plays an important role in the field of cryobiology. Currently, thermal needle, which is constructed by enclosing a manually winded thin metal wire with an insulation coating in a metallic sheath, is the only available device that is capable of measuring thermal conductivity of biomaterials. Major drawbacks, such as macroscale sensor size, lack of versatile format to accommodate samples with various shapes and sizes, neglected effects of heat transfer inside the probe and thermal contact resistance between the sensing element and the probe body, difficult to mass produce, poor data repeatability and reliability and labor-intense sensor calibration, have significantly reduced their potential to be an essential measurement tool to provide key thermal property information of biological specimens. In this study, we describe the development of an approach to measure thermal conductivity of liquids and soft bio-tissues using a proof-of-concept MEMS based thermal probe. By employing a microfabricated closely-packed gold wire to function as the heater and the thermistor, the presented thermal sensor can be used to measure thermal conductivities of fluids and natural soft biomaterials (particularly, the sensor may be directly inserted into soft tissues in living animal/plant bodies or into tissues isolated from the animal/plant bodies), where other more standard approaches cannot be used. Thermal standard materials have been used to calibrate two randomly selected thermal probes at room temperature. Variation between the obtained system calibration constants is less than 10%. By incorporating the previously obtained system calibration constant, three randomly selected thermal probes have been successfully utilized to measure the thermal conductivities of various solutions and tissue samples under different temperatures. Overall, the measurements are in agreement with the recommended values (percentage error less than 5%). The microfabricated thermal conductivity sensor offers superior characteristics compared to those traditional macroscopic thermal sensors, such as, (a) reduced thermal mass and thermal resistivity, (b) improved thermal contact between sensor and sample, (c) easy to manufacture with mass production capability, (d) flexibility to reconfigure sensor geometries for measuring samples with various sizes and shapes, and (e) reduced calibration workload for all sensors microfabricated from the same batch. The MEMS based thermal conductivity sensor is a promising approach to overcome the inherent limitations of existing macroscopic devices and capable of delivering accurate thermal conductivity measurement of biomaterials with various shapes and sizes.