학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 김성준.The retinal prosthesis is an implantable electronic device that delivers electrical stimuli containing visual information to the retina for the visual restoration of the blinds. The currently available retinal prostheses have several problems in the number of pixels. They are limited in the number of pixels, which restricts the amount of visual information they can deliver. Many research groups are trying to improve their device in this aspect. In order to achieve a significant number of pixels, retinal prosthesis needs large stimulus power dissipation. A typical device consumes more than 20 mW of power to drive 1000 channels. Some of this power can lead to temperature rise which is a safety issue. As the power dissipation scales up with the increase in the number of channels, it is desired to minimize the power per channel as much as possible. Another problem is the absence of a suitable packaging material for the long-term reliable optical window. Due to the curved and narrow implant space available for this kind of device, as well as the transparency required for the incoming wavelengths of lights, it is quite difficult to choose a material that satisfies all requirements of long-term hermetic packaging with optically transparent window. Sapphire glass with titanium metal package are too bulky and rigid, and flexible transparent polymers such as polyimide and parylene-C have high moisture absorption for the implant.
This dissertation proposes strategies and methods to solve the problems mentioned above. Two stimulation strategies are proposed. One strategy is to confine the stimulus level with a threshold that cell is activated. Thus we coin it as thresholding strategy.' The other strategy is to reduce the number of stimulation channels by using only outlines of images (outline extraction strategy). Prototype ICs were designed and fabricated for the verification of the effects of these strategies. The simulation and the measurement agree to show that retinal implant with the thresholding and outline extraction strategies consumes below one-third of the stimulus power of the conventional photodiode-based devices.
Area-efficient designs of the voltage-controlled current source are also adopted to increase the number of channels. The unit pixel area of the fabricated prototype IC was 0.0072 mm2, expanding up to 1200-channels in the macular area.
Liquid crystal polymer (LCP) is proposed as the long-term implantable packaging material with an optical window. It is an inert, biocompatible, and flexible polymer material that has a moisture absorption rate similar to Pyrex glass. We showed that an LCP film with a thickness less than 10 μm allows transmission of the lights in the visible wavelengths by more than 10 %, as the rate increases with thinner films. Thus a thinning process was developed. O2 DRIE was shown effective in reducing the roughness of the film, and the corresponding light scattering. The spatial resolution of LCP with 8.28 μm thickness showed a minimum distinguishable pitch of 90 μm, allowing a 1200 channel integration within a macular area.Chapter 1: Introduction 1
1.1. Retinal Prosthesis – State of the Arts 2
1.1.1. Retinal Prosthesis with External Camera 3
1.1.2. Retinal Prosthesis with Internal Photodiode Array 5
1.2. Photodiode-based Retinal Prosthesis 8
1.2.1. Problems 8
1.2.2. Possible Solutions 12
Chapter 2: Methods 17
2.1. Thresholding 17
2.1.1. Concept 17
2.1.2. Circuit Descriptions 19
2.2. Outline Extraction 28
2.2.1. Concept 28
2.2.2. Circuit Descriptions 30
2.3. Average Stimulus Power Estimation 40
2.3.1. Stimulus Patterns Generation of Conventional and Proposed Strategies 40
2.3.2. Minimum Distinguishable Channels to Recognize 41
2.4. Virtual Channel 43
2.4.1. Concept 43
2.4.2. Circuit Descriptions 44
2.5. Polymer Packaging 51
2.5.1. LCP as a Long-term Reliable Packaging Material 51
2.5.2. Test Methods 53
Chapter 3: Results 58
3.1. Thresholding 58
3.1.1. Fabricated IC 58
3.1.2. Test Setup 60
3.1.3. Test Results 61
3.2. Outline Extraction 65
3.2.1. Simulation Results 65
3.2.2. Fabricated IC 67
3.2.3. Test Setup 68
3.2.4. Test Results 72
3.3. Average Stimulus Power Estimation 76
3.4. Virtual Channels 79
3.4.1. Fabricated IC 79
3.4.2. Test Setup 80
3.4.3. Test Results 81
3.4.4. Two-dimensional Virtual Channel Generator– Test setup and Its Result 84
3.5. Polymer Packaging 87
3.5.1. Light Transmittance according to LCP Thickness 87
3.5.2. Thickness Control of LCP 89
3.5.3. Spatial Resolution of LCP 89
Chapter 4: Discussion 92
4.1. Average Stimulus Power 92
4.2. Visual Acuity 95
4.3. Hermeticity of the Thinned LCP Film 97
Chapter 5: Conclusion and Future Directions 99
References 103
Appendix – Generated Stimulus Patterns of Various the Number of Channels 112
국 문 초 록 139Docto
