Chulwoo Son

A multiaxial stretchable interconnect using liquid-alloy-filled elastomeric microchannels

We report on the fabrication and characterizations of a multiaxial stretchable interconnect using room-temperature liquid-alloy-filled elastomeric microchannels. Polydimethylsiloxane (PDMS) microchannels coated at the bottom with a gold wetting layer were used as the reservoirs which were subsequently filled by room-temperature liquid alloy using microfluidic injection technique. Using a diamond-shaped geometry to provide biaxial performance, a maximum stretchability of 100% was achieved (ΔR=0.24Ω). Less than 0.02Ω resistance variation was measured for 180° bending. Active electronics, light emitting diode, was also integrated onto the PDMS substrate with stretchable interconnects to demonstrate stable electrical connection during stretching, bending, and twisting.

A Wireless Implantable Passive Microdosimeter for Radiation Oncology

Wireless measurement of ionizing radiation in close proximity or/and within an irradiated solid tumor is extremely valuable for dose verification and quality control in radiation oncology. For such applications, it is preferable to manufacture such sensors using passive components since high levels of ionizing radiation can damage active electronics. In addition, passive implementation can reduce the cost associated with fabrication and assembly. This paper reports on the development of an implantable micro- machined passive LC transponder for in situ radiation measurement. Dose measurement is performed by monitoring the resonance frequency change associated with the decay of surface change of an electret upon exposure to radiation. This is achieved through a micromachned capacitor with a movable plate that is partially filled with a Teflon electret and connected in parallel with an inductor, thus forming a passive LC tank circuit. For an implantable prototype encapsulated in a glass capsule (2.5 mm in diameter, 2.8 cm in length), test results show that a dose of 30 Gy (from a Co60 source) can produce 1.46 MHz frequency shift resulting in a sensitivity of 49 kHz/Gy.

Selective nanofiber deposition via electrodynamic focusing

In this paper, we demonstrate the effect of electrodynamic focusing through a gold-coated PDMS shadow mask on the selective deposition of electrospun nanofibers. Under a suitable applied voltage, the PDMS mask repels the fibers from its surface while simultaneously forcing them into micron-sized holes and onto a collecting substrate. The presented technique is simple and can be used to produce lithographic-scale nanofiber deposition using a wide range of materials.

A Micromachined Electret-Based Transponder for In Situ Radiation Measurement

This letter reports on the development of a micromachined passive transponder for in situ measurement of ionizing radiation. gamma-ray exposure is remotely measured by monitoring the resonance frequency change correlated with a change in the surface charge of an electret. This is achieved through a micromachined capacitor with a movable plate that is partially filled with a Teflon electret and connected in parallel with an inductor, forming a passive LC tank (a microionization chamber, 1.5 cm in diameter and 3 mm in thickness). Test results show that an exposure of 60 000 R from Cs137 produces a 687-kHz resonance frequency shift (sensitivity of 11.45 kHz/kR)

Aqueous microdrop manipulation and mixing using ferrofluid dynamics

In this letter, the authors present a simple method to manipulate free microdroplets using ferrofluid dynamics. For droplet transport, a set of periodic lines of ferrofluid on top of a silicon wafer is created by a single strip magnet and dynamically changed by the rotation of a magnetic stirrer underneath it. It is demonstrated that the speed of droplet movement depends on the rotation speed of the magnetic stirrer as well as the size of the droplet. For better droplet mixing efficiency, a discontinuous pattern at the mixing spots is created by adding a smaller strip magnet to the above setup.

An implantable wireless microdosimeter for radiation oncology

Wireless measurement of ionizing radiation in close proximity or within an irradiated solid tumor is extremely valuable for dose verification and quality control in radiation oncology. For such applications, it is preferable to manufacture such sensors using passive components since high levels of ionizing radiation can damage active electronics. This paper reports on the development of an implantable micromachined passive LC transponder (2.5 mm in diameter and 2.8 cm in length.) Dose measurement is performed by monitoring the resonance frequency change associated with the decay of surface change of an electret upon exposure to radiation. The test results show that a dose of 30 Gy can produce 1.46 MHz frequency shift resulting in a sensitivity of 49 kHz/Gy.

Electret Based Wireless Micro Ionizing Radiation Dosimeter

Wireless ionizing radiation sensors have a wide range of applications in areas such as radiation therapy, environmental monitoring, and homeland security. It is preferable to manufacture such sensors using passive elements since ionizing radiation can damage active electronic components. This paper reports the development of a first micromachined totally passive electret based wireless ionizing radiation dosimeter for high dose radiation therapy applications. Wireless Ionizing radiation measurement is performed by monitoring the resonance frequency change with a phase-dip technique. Device test results show that 60,000R gamma-ray ionizing radiation exposure produces a 687kHz resonance frequency shift resulting in a sensitivity of 11.45kHz/kR.

Pull-in instability of parallel-plate electrostatic microactuators under a combined variable charge and voltage configuration

In this letter, we present a theoretical analysis of pull-in instability of parallel-plate electrostatic microactuators under a combined variable charge and voltage configuration. This occurs in parallel plate capacitors partially filled with a charged dielectric (electret) where the plates are connected together through an impedance. Our results demonstrate the possibility of achieving full range motion under certain conditions. For Teflon® electrets much thinner than the air gap; the maximum traveling distance is similar to the voltage controlled electrostatic actuators (gap/3), whereas for electrets four times thicker than the initial gap the maximum traveling distance can approach 100% of the gap.

multi-axial super-stretchable interconnects with active electronics

We report on the fabrication and characterization of stretchable interconnects using room temperature liquid alloy filled PDMS microchannels. Several novel fabrication techniques are presented to increase the functionality and manufacturing yield. These include selective deposition of a gold wetting layer at the bottom of the channels, microfluidic injection of room temperature liquid alloy, and reservoirs for accommodation of surface mount components. Achieved maximum stretchability of a bi-axial diamond-shaped structure is 100% with a 0.24 Omega resistance variation. Active electronics is also integrated onto the PDMS substrate with stretchable interconnects and tested for functionality.

Fabrication of Polymeric 3-D Micro-Structures Using Ferrofluid Molds

In this paper, we report a new and simple fabrication method for 3-D polymeric microstructures using low viscosity mineral oil-based ferrofluids in combination with polydimethylsiloxane (PDMS) and UV curable epoxy. The diameter and height of ferrofluid spikes/domes which usually form upon the application of a magnetic field was controlled by a micromachined membrane having holes of various dimensions. Holes or various dimensions (60, 100, and 200 µ m) were fabricated in a thin silicon membrane covering a ferrofluid reservoir. The heights of fabricated polymeric microstructures corresponding to the above mentioned hole sizes were equal to the hole diameters, while their base diameters were reduced by 10-30%. This approach provides a new technique to easily fabricate various size 3-D micro-structures on the same plane by controlling the magnetic field strength, ferrofluid viscosity, and hole dimensions.