Jeonghoon Yoon

Novel low-cost planar probes with broadside apertures for nondestructive dielectric measurement of biological materials at microwave frequencies

Novel planar-type probes were developed to demonstrate the possibility of replacing the existing high-cost open-ended coaxial probes. The planar probes of this study define an aperture on the broadside of the probe body. In this way, the contact area can be maximized and/or customized according to specific medical needs. The probes with various aperture sizes and shapes can also be fabricated simultaneously in a single batch process. Three probes are developed in this paper: a probe combining two laminates, a microelectromechanical systems (MEMS)-based probe with a single benzocyclobutene (BCB) layer on a quartz substrate, and another MEMS probe with two BCB layers defined on a silicon substrate. The third probe was specifically designed for monolithic integration with driving circuits on a single substrate. Limitations in the high-frequency performance of the planar probes were carefully studied, and higher order modes and incomplete shielding were found to be the main causes. The measurement results of each probe showed excellent compatibility with those of the open-ended coaxial probe up to almost 40 GHz. The proposed planar-type probes have great potentials for practical medical applications in view of low cost, disposability, and monolithic integration capability with the driving circuits.

High-frequency microwave ablation method for enhanced cancer treatment with minimized collateral damage

To overcome the limits of conventional microwave ablation, a new frequency spectrum above 6 GHz has been explored for low‐power and low collateral damage ablation procedure. A planar coaxial probe‐based applicator, suitable for easy insertion into the human body, was developed for our study to cover a wideband frequency up to 30 GHz. Thermal ablations with small input power (1–3 W) at various microwave frequencies were performed on nude mice xenografted with human breast cancer. Comparative study of ablation efficiencies revealed that 18‐GHz microwave results in the largest difference in the temperature rise between cancer and normal tissues as well as the highest ablation efficiency, reaching 20 times that of 2 GHz. Thermal profile study on the composite region of cancer and fat also showed significantly reduced collateral damage using 18 GHz. Application of low‐power (1 W) 18‐GHz microwave on the nude mice xenografted with human breast cancer cells resulted in recurrence‐free treatment. The proposed microwave ablation method can be a very effective process to treat small‐sized tumor with minimized invasiveness and collateral damages.

In-vivo measurements of the dielectric properties of breast carcinoma xenografted on nude mice.

A developing method of cancer detection is to use electromagnetic waves to compare the dielectric properties of normal and cancerous tissue. Because most of the previous studies consisted of dielectric measurements taken ex‐vivo, this study investigated the advantages of in‐vivo measurements, obtained using the newly developed insertion‐type planar probe, through the measurements of cancer (MDA MB 231), which was cultivated and implanted into the mammary fat pad of nude mice. Reflection coefficients were obtained in the broadband frequency range from 0.5 to 30 GHz, from which broadband complex permittivity data was extracted. Complex permittivity, in addition to other parameters such as conductivity and characteristic frequency, were used to make comparisons between cancerous tissue, normal muscle tissue and fat tissue, as well as comparisons between in‐vivo and ex‐vivo measurements. This study investigated the suitability of in‐vivo cancer detection using microwaves with the newly developed insertion‐type planar probe. Results showed that both sensitivity and specificity of the current method was 97%. In addition, predictive values were 99% for the positive and 94% for the negative, thus greatly enhancing the practicality of this method. In conclusion, it was demonstrated that in‐vivo measurements are highly beneficial in studying the potential of microwaves as a diagnostic tool of breast cancer, especially in combination with the newly developed insertion‐type planar probe.

In vitro and in vivo measurement for biological applications using micromachined probe

We developed a small-sized micromachined probe for the measurement of biological properties using microelectromechanical systems (MEMS) technology. We also experimentally showed the suitability of the micromachined probe for biological applications through in vivo, as well as in vitro measurements of various types of tissue. We measured the permittivities of 0.9% saline and the muscle and fat of pork using the micromachined probe after liquid calibration. The measured permittivities of 0.9% saline and pork agreed well with both the expected values of the Cole-Cole equation along with the measured values obtained through the use of a 1-mm-diameter open-ended coaxial probe. We also performed in vivo measurements of breast cancer tissue implanted in an athymic nude mouse to show the suitability of the small-sized micromachined probe for practical biological applications. Through the obtained data, the capability of the micromachined probe of distinguishing different tissue types from one another was shown. The actual aperture size of the micromachined probe is only 240 /spl mu/m /spl times/ 70 /spl mu/m and, therefore, we can extract the biological information from very small biological tissues and drastically decrease the invasiveness of this method through the implementation of the small probe created through the use of MEMS technology.

An Optimum Design Methodology for Planar-Type Coaxial Probes Applicable to Broad Temperature Permittivity Measurements

A planar-type coaxial probe applicable to wide temperature and frequency range has been developed. The probe employs a dielectric with a low coefficient of thermal expansion to minimize the effect of thermal deformation for broad temperature measurements. Additionally, a detailed design methodology has been developed to optimize the probe apertures in an effort to minimize the measurement uncertainty while maximizing the operating bandwidth. For this purpose, thorough sensitivity analysis has been employed to correlate the probe structures and dimensions to the individual sensitivity parameters. The analysis has been validated by parametric experiments. By using the dielectric with a low coefficient of thermal expansion and optimizing the probe dimensions, accurate permittivity measurements have been demonstrated from 30 C to 75 C with 40-GHz bandwidth. With the application of the error-correction method, the measurement temperature range has been further extended all the way up to the boiling temperature of water C). Furthermore, the complex permittivities of methanol have been measured from 30 C to 50 C and the dispersion parameters and full interpolation formulas have been extracted.

Planar type probe with multiple-polarization response for in-vivo permittivity measurements of heterogeneous biological tissues

A three-way planar-type probe optimized for in-vivo permittivity measurements of biological materials has been developed. The probe in this letter consists of three orthogonally-faced probing apertures, which allows one to make accurate and uninterrupted measurements in three different directions per each insertion. As a result, this probe is significantly less invasive for the collection of the desired information of the biological materials, thus providing a vastly improved solution for in-vivo measurements. This probe can also be used for microwave ablation, in which case the treating region can be significantly expanded

A High-Temperature Capable Planar-type Coaxial Probe for Complex Permittivity Measurements Up to 40 GHz

A planar-type probe applicable to a wide temperature and frequency range has been developed. The probe employs a low CTE (coefficient of thermal expansion) dielectric material to minimize the effect of thermal expansion for high temperature measurement. Additionally, the coaxial aperture dimensions have been optimized through sensitivity analysis to minimize the uncertainty of the extracted complex permittivity due to variations of the measured S-parameters. Also, the error correction method was used at extreme temperatures to compensate for the effects of the excessive thermal expansion. For validation, distilled water was measured from 0degC to 100degC in the frequency range of 0.5~40 GHz. The measured complex permittivity shows good agreement with the reference data up to 75degC without the error correction and up to the boiling point (92degC) with the error correction. To our knowledge, this is the first demonstration of low-cost planar-type probes capable of high temperature operation.

Silicon MEMS probe using a simple adhesive bonding process for permittivity measurement

We developed a silicon MEMS probe for permittivity measurements using an adhesive bonding process. Only two photolithographic masks are required to fabricate the probe, which can be implemented through simple bonding processes using silicon substrates and a benzo cyclo butene (BCB) adhesive layer. Undoped silicon substrates with thicknesses of 300 ?m are used as the dielectric layers of the proposed probe. BCB layers, which have good electrical properties at high frequencies as well as adhesive properties for the bonding process, play the role of bonding materials between the two silicon substrates. The length of the probe is 30 mm, and the aperture located at the tip of the probe is 1.1 mm ? 0.62 mm. The permittivity of 0.5% saline was measured, and the results agreed with the values obtained through the Cole?Cole equation. To validate the feasibility of this probe for practical biological applications, we also performed in vivo measurements of the muscle, skin and blood of mice. Due to the simple fabrication process, the cost of the probe can be reduced in comparison with the previous micromachined probe (Kim et al 2005 J. Micromech. Microeng. 15 543?50) as well as the conventional laser machined probe. Low cost leads to disposability, which is an important factor for practical biomedical applications; and thus, coupled with the probes capabilities of MMIC integration and CMOS compatibility, this probe has excellent potential in the field of microwave permittivity measurements.

High-power reflection coefficient measurement of biological material applicable to microwave hyperthermia

A measurement method for the high-power reflection coefficients of biological material applicable to microwave hyperthermia is presented. To extract the power-dependent complex permittivities, scalar S-parameter measurement setup consisting of a planar coaxial probe, impedance tuner, directional couplers, and power meters have been used. The tuner impedance was tuned until the reflected power reading was zeroed out, and subsequent de-embedding steps accounting for the non-idealities were followed to extract the reflection coefficient of the material under test at the coaxial aperture. To validate the proposed measurement method, the reflection coefficients of the distilled water were measured with various microwave powers at heated temperatures. The extracted complex permittivities showed good agreement with the reference data over broad temperature and frequency range. The method provides an effective way of in-situ monitoring during the hyperthermia and experimental data for thermal-electromagnetic analysis.