Jeong Hoan Park

Advancements in fabrication process of microelectrode array for a retinal prosthesis using Liquid Crystal Polymer (LCP)

Liquid Crystal Polymer (LCP) has been considered as an alternative biomaterial for implantable biomedical devices primarily for its low moisture absorption rate compared with conventional polymers such as polyimide, parylene and silicone elastomers. A novel retinal prosthetic device based on monolithic encapsulation of LCP is being developed in which entire neural stimulation circuitries are integrated into a thin and eye-conformable structure. Micromachining techniques for fabrication of a LCP retinal electrode array have been previously reported. In this research, however, for being used as a part of the LCP-based retinal implant, we developed advanced fabrication process of LCP retinal electrode through new approaches such as electroplating and laser-machining in order to achieve higher mechanical robustness, long-term reliability and flexibility. Thickened metal tracks could contribute to higher mechanical strength as well as higher long-term reliability when combined with laser-ablation process by allowing high-pressure lamination. Laser-thinning technique could improve the flexibility of LCP electrode.

Feasibility of LCP as an Encapsulating Material for Photodiode-Based Retinal Implants

While retinal implants based on microphotodiode array (MPDA) could achieve great number of channels without an external camera, the absence of proper packaging technology of MPDA has limited their use in chronic implantation. Liquid crystal polymer (LCP) has gained increasing attention as a biomaterial for implantable devices due to its low water absorption rate and low permeability. In this letter, feasibility of LCP film as an encapsulation material of photodiode-based retinal prosthesis was explored. Optical property of LCP was measured to find the film thickness that can guarantee the light transmittance greater than 20%, which was achieved by developing the dry etching process of commercial LCP films to be less than 10 μm. The minimum distinguishable line pitch of the thin LCP encapsulation evaluated by a custom setup with projected grating patterns on CMOS image sensor was 90 μm, which could be equivalent to ideally 1200 channels in macular area of 10 mm2.

A distributed current stimulator ASIC for high density neural stimulation

This paper presents a novel distributed neural stimulator scheme. Instead of a single stimulator ASIC in the package, multiple ASICs are embedded at each electrode site for stimulation with a high density electrode array. This distributed architecture enables the simplification of wiring between electrodes and stimulator ASIC that otherwise could become too complex as the number of electrode increases. The individual ASIC chip is designed to have a shared data bus that independently controls multiple stimulating channels. Therefore, the number of metal lines is determined by the distributed ASICs, not by the channel number. The function of current steering is also implemented within each ASIC in order to increase the effective number of channels via pseudo channel stimulation. Therefore, the chip area can be used more efficiently. The designed chip was fabricated with area of 0.3 mm2 using 0.18 μm BCDMOS process, and the bench-top test was also conducted to validate chip performance.This paper presents a novel distributed neural stimulator scheme. Instead of a single stimulator ASIC in the package, multiple ASICs are embedded at each electrode site for stimulation with a high density electrode array. This distributed architecture enables the simplification of wiring between electrodes and stimulator ASIC that otherwise could become too complex as the number of electrode increases. The individual ASIC chip is designed to have a shared data bus that independently controls multiple stimulating channels. Therefore, the number of metal lines is determined by the distributed ASICs, not by the channel number. The function of current steering is also implemented within each ASIC in order to increase the effective number of channels via pseudo channel stimulation. Therefore, the chip area can be used more efficiently. The designed chip was fabricated with area of 0.3 mm2 using 0.18 μm BCDMOS process, and the bench-top test was also conducted to validate chip performance.

Assessment of blood flow velocity profiles in heart ventricles and aorta with phase contrast magnetic resonance imaging

Accurate measurement of blood flow is usually very difficult and invasive to make in spite of its important roles in clinical diagnosis. Therefore, an alternative method capable of determining the cardiac output non-invasively will be of good use. Furthermore, measurement of blood velocity with high resolution either in the heart or in any part of circulatory system without any cardiac catheterization would play an important role in the evaluation of patients with circulatory disorders. The MRI phase contrast technique (PC-MRI) can be used for this purpose. It can provide flow velocity information in addition to anatomic imaging by applying bipolar gradient in the velocity encoding direction. We can obtain velocity encoded MR images computed by the phase difference between two images acquired with different gradient first moments but identical zeroth moments. We combined phase contrast sequence and cine method to acquire the velocity map across the entire cardiac cycle. ECG gating was used for triggering. We calculated volume flow by pixel unit velocity analysis in the region of interest. Also by visualizing the velocity profile dynamically with 3D representation, we could assess more easily how the appearances of blood flow patterns are. The proposed method was tested both with a phantom where a pulsatile waveform was generated by ventricular assistance device and in vivo at the heart of normal volunteer on a GE 1.5 T scanner. As a conclusion, the dynamically represented 3D images obtained by the cine PC method could be confirmed as a good method for assessment of cardiac function.