Micromachined Hot-Wire Thermal Conductivity Probe for Biomedical Applications

One-Dimensional Systemic Modeling of Thermal Sensors Based on Miniature Bead-Type Thermistors

Accurate measurements of thermal properties is a major concern, for both scientists and the industry. The complexity and diversity of current and future demands (biomedical applications, HVAC, smart buildings, climate change adapted cities, etc.) require making the thermal characterization methods used in laboratory more accessible and portable, by miniaturizing, automating, and connecting them. Designing new materials with innovative thermal properties or studying the thermal properties of biological tissues often require the use of miniaturized and non-invasive sensors, capable of accurately measuring the thermal properties of small quantities of materials. In this context, miniature electro-thermal resistive sensors are particularly well suited, in both material science and biomedical instrumentation, both in vitro and in vivo. This paper presents a one-dimensional (1D) electro-thermal systemic modeling of miniature thermistor bead-type sensors. A Godunov-SPICE discretization scheme is introduced, which allows for very efficient modeling of the entire system (control and signal processing circuits, sensors, and materials to be characterized) in a single workspace. The present modeling is applied to the thermal characterization of different biocompatible liquids (glycerol, water, and glycerol–water mixtures) using a miniature bead-type thermistor. The numerical results are in very good agreement with the experimental ones, demonstrating the relevance of the present modeling. A new quasi-absolute thermal characterization method is then reported and discussed. The multi-physics modeling described in this paper could in the future greatly contribute to the development of new portable instrumental approaches.

Spreadsheet Tools for Quantifying Seepage Flux Across the GW-SW Interface

Identifying the spatial distribution and magnitude of seepage flux across the groundwater-surface water (GW-SW) interface is critical for assessing potential impairments and restoration alternatives for water bodies adjacent to sites with groundwater contamination. Measurement of the vertical distribution and time-varying characteristics of temperature in sediments provides an indirect way to map out spatial and temporal patterns of seepage flux into surface water. Two spreadsheet-based calculation tools are introduced that implement four one-dimensional analytical solutions to calculate the magnitude and direction of seepage flux based on measurement of steady-state vertical temperature profiles or transient diel temperature signals at two depths within sediment. Performance of these calculation tools is demonstrated for a pond receiving contaminated groundwater discharge from an adjacent landfill. Transient versus steady-state model performance is compared, and limitations of transient modelsare illustrated for a situation with unfavorable sediment characteristics and inadequate sensor spacing. The availability of a range of analytical solutions implemented within Microsoft Excel^® is intended to encourage practitioners to explore use of this seepage flux characterization method and develop greater insight into best practices for model selection and use.

Geophysical Assessment of a Proposed Landfill Site in Fredericktown, Missouri

In cooperation with the U.S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS) collected numerous types of geophysical data at a site in Fredericktown, Missouri, in June 2018. Various geophysical surveys were collectively used to help evaluate the overall suitability of the site for use as a mine waste-soil and sediment repository, and to evaluate the suite of geophysical methods for rapid collection and preliminary assessment of sites with shallow sediments. Land-based geophysical methods, which included frequency-domain electromagnetic induction (FDEM), electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio passive seismic (HVSR), and shear-wave refraction, were used to determine the depths to crystalline bedrock and characterize the overlying unconsolidated sediments (or regolith). Water-borne FDEM profiles and forward-looking infrared (FLIR) thermal image surveys were conducted along the Fredericktown City Lake shoreline to identify locations of potential interactions between groundwater and surface water. Sediment temperature profilers were installed at two locations along the shoreline to characterize shallow unconsolidated sediment thermal properties and support the interpretation of the other geophysical surveys. Geophysical reconnaissance methods including the FDEM and HVSR methods, were used to rapidly evaluate the vertical and lateral extent of overburden, or unconsolidated sediments, overlying the bedrock at the site. The results of these methods were compared to reference geophysical methods of ERT and shear-wave refraction surveys that have greater accuracy and are more labor intensive and time-consuming. A goal of the project was the evaluation of the validity and reliability of this suite of reconnaissance geophysical methods as a means by which shallow (less than 3 meters (m)) sediments can be rapidly assessed. Two orthogonal ERT survey profiles, which used 28 electrodes spaced 1 m apart in dipole-dipole and combined Wenner-Schlumberger configurations, were collected to determine the subsurface resistivity. The results were inverted to produce electrical resistivity profiles that were compared to the FDEM and HVSR survey results. The FDEM data were collected along cleared paths through the proposed disposal cell locations. The data were inverted to generate depth-dependent estimates of electrical conductivity along the transects. An analysis of the depth of investigation (DOI) indicated the FDEM imaged to depths of about 3 m below land surface. The ERT, FDEM, and HVSR indicated the depth to crystalline bedrock was approximately 1.5 m below land surface with shallower and deeper areas. Results from this investigation indicate this suite of methods will likely perform well at sites with shallow depths to bedrock and strong conductivity and acoustic impedance contrasts, where the FDEM and HVSR methods can provide estimates of the depth to bedrock, and ERT and shear-wave refraction surveys might not be worth the added time and expense.

The Feasibility of a Smart Surgical Probe for Verification of IRE Treatments Using Electrical Impedance Spectroscopy

SIGNIFICANCE

Irreversible electroporation (IRE) is gaining popularity as a focal ablation modality for the treatment of unresectable tumors. One clinical limitation of IRE is the absence of methods for real-time treatment evaluation, namely actively monitoring the dimensions of the induced lesion. This information is critical to ensure a complete treatment and minimize collateral damage to the surrounding healthy tissue.

GOAL

In this study, we are taking advantage of the biophysical properties of living tissues to address this critical demand.

METHODS

Using advanced microfabrication techniques, we have developed an electrical impedance microsensor to collect impedance data along the length of a bipolar IRE probe for treatment verification. For probe characterization and interpretation of the readings, we used potato tuber, which is a suitable platform for IRE experiments without having the complexities of in vivo or ex vivo models. We used the impedance spectra, along with an electrical model of the tissue, to obtain critical parameters such as the conductivity of the tissue before, during, and after completion of treatment. To validate our results, we used a finite element model to simulate the electric field distribution during treatments in each potato.

RESULTS

It is shown that electrical impedance spectroscopy could be used as a technique for treatment verification, and when combined with appropriate FEM modeling can determine the lesion dimensions.

CONCLUSIONS

This technique has the potential to be readily translated for use with other ablation modalities already being used in clinical settings for the treatment of malignancies.

High accuracy thermal conductivity measurement of aqueous cryoprotective agents and semi-rigid biological tissues using a microfabricated thermal sensor

An improved thermal-needle approach for accurate and fast measurement of thermal conductivity of aqueous and soft biomaterials was developed using microfabricated thermal conductivity sensors. This microscopic measuring device was comprehensively characterized at temperatures from 0 °C to 40 °C. Despite the previous belief, system calibration constant was observed to be highly temperature-dependent. Dynamic thermal conductivity response during cooling (40 °C to -40 °C) was observed using the miniaturized single tip sensor for various concentrations of CPAs, i.e., glycerol, ethylene glycol and dimethyl sulfoxide. Chicken breast, chicken skin, porcine limb, and bovine liver were assayed to investigate the effect of anatomical heterogeneity on thermal conductivity using the arrayed multi-tip sensor at 20 °C. Experimental results revealed distinctive differences in localized thermal conductivity, which suggests the use of approximated or constant property values is expected to bring about results with largely inflated uncertainties when investigating bio-heat transfer mechanisms and/or performing sophisticated thermal modeling with complex biological tissues. Overall, the presented micro thermal sensor with automated data analysis algorithm is a promising approach for direct thermal conductivity measurement of aqueous solutions and soft biomaterials and is of great value to cryopreservation of tissues, hyperthermia or cryogenic, and other thermal-based clinical diagnostics and treatments.

Measurement of the thermal conductivity of carbon nanotube--tissue phantom composites with the hot wire probe method

Developing combinatorial treatments involving laser irradiation and nanoparticles require an understanding of the effect of nanoparticle inclusion on tissue thermal properties, such as thermal conductivity. This information will permit a more accurate prediction of temperature distribution and tumor response following therapy, as well as provide additional information to aid in the selection of the appropriate type and concentration of nanoparticles. This study measured the thermal conductivity of tissue representative phantoms containing varying types and concentrations of carbon nanotubes (CNTs). Multi-walled carbon nanotubes (MWNTs, length of 900-1200 nm and diameter of 40-60 nm), single-walled carbon nanotubes (SWNTs, length of 900-1200 nm and diameter <2 nm), and a novel embodiment of SWNTs referred to as single-walled carbon nanohorns (SWNHs, length of 25-50 nm and diameter of 3-5 nm) of varying concentrations (0.1, 0.5, and 1.0 mg/mL) were uniformly dispersed in sodium alginate tissue representative phantoms. The thermal conductivity of phantoms containing CNTs was measured using a hot wire probe method. Increasing CNT concentration from 0 to 1.0 mg/mL caused the thermal conductivity of phantoms containing SWNTs, SWNHs, and MWNTs to increase by 24, 30, and 66%, respectively. For identical CNT concentrations, phantoms containing MWNTs possessed the highest thermal conductivity.