석 사 학 위 논 문석 사 학 위 논 문석 사 학 위 논 문석 사 학 위 논 문

Master's Thesis

액정액정액정액정 디스플레이를디스플레이를디스플레이를디스플레이를 이용한이용한이용한이용한 미세입자미세입자미세입자미세입자 및및및및

혈액세포의혈액세포의혈액세포의혈액세포의 광전자적광전자적광전자적광전자적 구동구동구동구동에에에에 관한관한관한관한 연구연구연구연구

Optoelectronic manipulation of microparticles and blood cells using

a liquid crystal display

황황황황 현현현현 두두두두 ( ( ( (黃黃黃黃 炫炫炫炫 斗斗斗斗 Hyundoo Hwang))))

바이오및뇌공학바이오및뇌공학바이오및뇌공학바이오및뇌공학과과과과 Department of Bio and Brain Engineering

한 국 과 학 기 술 원한 국 과 학 기 술 원한 국 과 학 기 술 원한 국 과 학 기 술 원

Korea Advanced Institute of Science and Technology

2007

액정액정액정액정 디스플레이를디스플레이를디스플레이를디스플레이를 이용한이용한이용한이용한 미세입자미세입자미세입자미세입자 및및및및

혈액세포의혈액세포의혈액세포의혈액세포의 광전자적광전자적광전자적광전자적 구동구동구동구동에에에에 관한관한관한관한 연구연구연구연구

Optoelectronic manipulation of

microparticles and blood cells using a

liquid crystal display

Optoelectronic manipulation of

microparticles and blood cells using a

liquid crystal display

Advisor: Professor Je-Kyun Park

by Hyundoo Hwang

Department of Bio and Brain Engineering

Korea Advanced Institute of Science and Technology

A thesis submitted to the faculty of the Korea Advanced Institute

of Science and Technology in partial fulfillment of the requirements

for the degree of Master of Science in the Department of Bio and

Brain Engineering

Daejeon, Korea

2007. 6. 26.

Approved by

Professor Je-Kyun Park

액정액정액정액정 디스플레이를디스플레이를디스플레이를디스플레이를 이용한이용한이용한이용한 미세입자미세입자미세입자미세입자 및및및및

혈액세포의혈액세포의혈액세포의혈액세포의 광전자적광전자적광전자적광전자적 구동에구동에구동에구동에 관한관한관한관한 연구연구연구연구

황황황황 현현현현 두두두두

위위위위 논문은논문은논문은논문은 한국과학기술원한국과학기술원한국과학기술원한국과학기술원 석사석사석사석사 학위논문으로학위논문으로학위논문으로학위논문으로 학위논문학위논문학위논문학위논문

심사위원회에서심사위원회에서심사위원회에서심사위원회에서 심사심사심사심사 통과하였음통과하였음통과하였음통과하였음....

2002002002007777 년년년년 6666 월월월월 22226666 일일일일

심사위원장심사위원장심사위원장심사위원장 박박박박 제제제제 균균균균 ((((인인인인))))

심사심사심사심사 위원위원위원위원 남남남남 윤윤윤윤 기기기기 ((((인인인인))))

심사심사심사심사 위원위원위원위원 정정정정 기기기기 훈훈훈훈 ((((인인인인))))

i

MBiS 20063643

황 현 두. Hyundoo Hwang. Optoelectronic Manipulation of Microparticles and Blood Cells using a Liquid Crystal Display. 액정 디스플레이를

이용한 미세입자 및 혈액세포의 광전자적 구동에 관한 연구. Department of Bio and Brain Engineering. 2007. 90p. Advisor Prof. Je-Kyun Park. Text in English.

Abstract

This thesis presents two types of approaches for enhancement of a typical optoelectronic

tweezers (OET). OET has been a powerful tool for cellular-scale manipulation of

microparticles in medical and biological applications. However, in a typical OET, non-specific

interactions between particles and device surfaces interfere with the effective particle

manipulation. Under applied electric fields, some attached particles induce strong dipole forces

and pull other particles around them. In order to overcome these problems, we suggest a novel

platform called three dimensional optoelectronic tweezers (3D OET) composed of double

photoconductive layers to focus microparticles vertically, providing adsorption-free

microparticle manipulation. The 3D OET was successfully applied to focus and manipulate

polystyrene microbeads in a channel-less microenvironment without adsorptions. It showed

higher particle trapping efficiency and stability than a typical OET.

There are three types of OET platforms; digital micro-mirror display (DMD) OET, image

DEP and lab-on-a-display. DMD OET and image DEP, which uses a DMD and a projector for

image generation respectively, require complex optical structures constructed with lens and

mirror for reflecting and focusing the light. In the case of a lab-on-a-display, it has the simplest

structure for portable application because it uses a direct light transfer of LCD without any

optical components. However, the lab-on-a-display has lower performances than others. To

overcome the limitations and to harmonize the advantages of each platform, we report a LCD-

based OET system integrated with a condenser lens and an interactive control system. The

LCD OET provides much higher performances than lab-on-a-display and simpler structures

than DMD OET and image DEP. The well-integrated system is successfully utilized for

interactive manipulation of red and white blood cells. This new LCD-based OET system using

a condenser lens and an LCD may be a usable integrated system for optoelectronic

manipulation of microparticles in many biological applications.

ii

Contents Abstract ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ i

Nomenclature ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ iv

List of Tables ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ vii

List of Figures ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ viii

Chapter 1. Introduction ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 1

1.1. Tools for microparticle manipulation ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 1

1.2. Dielectrophoresis ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 5

1.3. Optoelectronic tweezers ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 8

1.4. Research objectives ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 13

1.5. Thesis outline ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 14

Chapter 2. Three-Dimensional Optoelectronic Tweezers ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 15

2.1. Introduction ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 15

2.2. Design and fabrication ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 17

2.3. Simulation of electric field distribution ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 22

2.4. Experimental ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 24

2.5. Results and discussion ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 26

2.6. Conclusion ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 34

Chapter 3. Lens-integrated LCD-based Optoelectronic Tweezers ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 35

3.1. Introduction ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 35

3.2. Design and fabrication ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 37

3.3. Simulation of electric field distribution ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 40

3.4. Experimental ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 44

3.5. Results and discussion ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 47

3.5.1. Simulation of Clausius-Mossotti factors⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 47

iii

3.5.2. Condensation of LCD image ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 51

3.5.3. Polystyrene bead manipulation ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 52

3.5.4. Blood cell manipulation ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 57

3.6. Conclusion ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 62

Chapter 4. Conclusions ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 63

Summary in Korean ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 69

References

Acknowledgements

Curriculum Vitae

iv

Nomenclature

Alphabetic letters

E Local electric field

fCM Clausius-Mossotti factor

Re(fCM) Real part of Clausius-Mossotti factor

FDEP Dielectrophoretic force

r Radius of a particle

v

Greek letters

δ Thickness of the cell membrane

εm Absolute permittivity of the suspending medium

εp Absolute permittivity of the particle

ε* Complex permittivity

εr Relative permittivity

σ Electrical conductivity

ω Natural frequency of AC voltage signal

vi

Abbreviations

CFD Computational Fluid Dynamics

CMOS Complementary Metal-Oxide Semiconductor

DEP Dielectrophoresis

DMD Digital Micro-mirror Device

iDEP Image Dielectrophoresis

ITO Indium Tin Oxide

LCD Liquid Crystal Display

MACS Magnetic-Activated Cell Sorter

N.A. Numerical Aperture

OET Optoelectronic Tweezers

PECVD Plasma Enhanced Chemical Vapor Deposition

PBS Phosphate Buffered Saline

PR Photoresist

TFT Thin Film Transistor

vii

List of Tables

Table 1. Different types of typical optoelectronic tweezers. ························12

viii

List of Figures

Figure 1. Dielectrophoretic movements of microparticles exposed to non-

uniform electric fields. ························································································7

Figure 2. Energy conversion process in optoelectronic tweezers. ····················10

Figure 3. Particle adsorptions in a typical OET device. ···································16

Figure 4. Schematic diagram of (a) optoelectronic tweezers (OET) and (b)

three-dimensional optoelectronic tweezers (3D OET).····································19

Figure 5. Fabrication process of OET device (left) and 3D OET device (right).

··························································································································21

Figure 6. Fabricated photoconductive layer.·····················································21

Figure 7. Simulated electric distributions in the liquid chamber of (a) OET and

(b) 3D OET. ······································································································23

Figure 8. Experimental setup for characterization of 3D OET. ························26

Figure 9. Microscopic photographs according to the heights of the liquid layer

of (a) OET and (b) 3D OET devices. ································································27

ix

Figure 10. Percentages of moving beads by several LCD images in OET and

3D OET devices (mean±standard deviation, n≥5).···········································29

Figure 11. Microscopic photographs of polystyrene beads (45 µm diameter)

concentrated by (a) OET and (b) 3D OET devices.··········································29

Figure 12. Percentages of moving beads according to their sizes in OET and

3D OET devices (mean±standard deviation, n≥5).···········································31

Figure 13. Bead velocities according to their sizes in OET and 3D OET devices.

(a) Bead velocities of moving beads (mean±standard deviation, n≥20). (b)

Average velocities of all beads including adsorbed beads.·······························32

Figure 14. Percentages of moving beads according to the time in OET and 3D

OET devices (mean±standard deviation, n≥5).·················································33

Figure 15. Microscopic photographs of (a) an original LCD image and (b) a

blurred image on a photoconductive layer. ·······················································36

Figure 16. Configuration of (a) a typical lab-on-a-display and (b) a new LCD-

based OET system integrated with a condenser lens. ·······································38

Figure 17. Fabrication process for the OET device. ·········································39

x

Figure 18. Microscopic photograph of the edge region of the blurred virtual

electrode and the measured light intensity of the image (thin line). The applied

voltages estimated based on the measured light intensity are described (thick

line) in the condition of 10 V bias.····································································41

Figure 19. Simulated distributions of electric field induced from (a) a clear

virtual electrode formed by condensed light and from (b) a blurred virtual

electrode formed by diffracted light in the condition of 10 V at 100 kHz.·······43

Figure 20. Plot of the square of electric field generated by a clear virtual

electrode (blue, solid) and a blurred virtual electrode (red, dotted) in the middle

of the liquid chamber. ·······················································································43

Figure 21. Schematic diagram of experimental setup for new LCD-based OET

system integrated with a condenser lens.··························································45

Figure 22. The simulated real part of Clausius-Mossotti factor of a 15 µm

diameter size polystyrene bead in mediums with conductivities of 1 S/m, 100

mS/m, 10 mS/m, 1 mS/m, and 0.1 mS/m. ························································48

Figure 23. The simulated real part of Clausius-Mossotti factor of a red blood

cell in mediums with conductivities of 1 S/m, 100 mS/m, 10 mS/m, 1 mS/m,

xi

and 0.1 mS/m. ···································································································50

Figure 24. Microscopic photographs of (a) an original LCD image and (b) a

condensed image on a photoconductive layer.··················································52

Figure 25. Bead velocities according to their sizes in a new LCD-based OET

system and a lab-on-a-display (mean±standard deviation, n≥20). ···················53

Figure 26. The captured photographs after parallel manipulation of 15 µm

polystyrene beads.·····························································································53

Figure 27. Bead velocities according to their sizes in 20 µm and 120 µm height

liquid chambers of OET device (mean±standard deviation, n≥20). ·················55

Figure 28. Interactive manipulation of a polystyrene bead for threading a maze

constructed by optically-induced virtual electrodes. ········································56

Figure 29. Concentration of red blood cells using a LCD image forming a word

“KAIST” during 60 seconds. (a) Voltage is not applied. (b) Voltage is applied.

··························································································································58

Figure 30. Parallel manipulation of red blood cells trapped in a virtual

electrodes array. ································································································58

xii

Figure 31. Single red blood cell manipulation.·················································59

Figure 32. Selection and manipulation of a single white blood cell among many

red blood cells in the condition of 7 V bias at 200 kHz.···································61

1

Chapter 1 Introduction

1.1. Tools for microparticle manipulation

The cellular-scale manipulation of microparticles plays important roles

in many fields of biological and medical applications[1, 2]. Functions such as

transporting, trapping and sorting of microscopic particles and cells are

essential to perform several biological and chemical applications including

bead-based chemical analysis, cell analysis, cell culture, and diagnostics in

clinical fields. For these purposes, several mechanisms such as optical[3],

magnetic[4], electrokinetic[5], acoustic[6] and hydrodynamic[7] forces have

been applied.

Optical tweezers which uses the optical force induced by the photon

momentum transferred from the incident light beam to the target particle was

first reported by Ashkin and his coworkers at AT&T Bell Laboratories in

1970[8, 9]. It has become a powerful tool for trapping cells and probing the

physical properties of biological polymers. A focused laser beam can generate

two forces; the scattering force which is proportional to the light intensity and

points in the direction of the incident light, and the gradient force which is

proportional to the gradient of intensity and points in the direction of the

2

intensity gradient. In practice, the gradient force needs to be more dominant

than the scattering force for stable particle trapping. For that condition, the

beam should diverge rapidly enough away from the focal point via a

microscope objective lens, whose high numerical aperture (N.A.) and well

corrected aberration focus light as tightly as possible[3]. Single cells, viruses

and bacteria were also trapped and manipulated by a tightly focused laser beam

with wavelength at the near infrared window without optical damages due to

the heat generated from adsorption of the strong incident laser beam[10, 11].

However, the small field of view of the high N.A. objective lens limits

the effective manipulation area less than 100 × 100 µm2. Because of the small

effective area for particle manipulation, the capability of parallel manipulation

is limited to a small number of cells. The manipulation area can be increased

by using a low N.A. objective lens, but the strength of the gradient force

decreases, resulting in a less stable particle trapping. A requirement of the high

light intensity for stable and multiple optical traps is also limitation of the

optical tweezers. The optical power required for multiple traps is proportional

to the number of traps[12, 13]. The maximum power of laser beam is limited

by the photo-damage threshold of light-sensitive components, limiting the

number of traps.

3

Magnetic forces are frequently used for the cell sorting, because

magnetic fields affect the target cells less than the other fields such as optical

and electrical field. Functionalized magnetic beads containing a large number

of superparamagnetic nanoparticles are used to attach to the target cells and

sort them out by applying a magnetic field in the magnetic-activated cell sorter

(MACS)[14-17]. For the single cell manipulation using a high resolution

magnetic field, microelectromagnetic matrix has been developed[18]. Recently,

a magnetic force microscope constructed with magnetic tip and trap platform

has been proposed for manipulation and sorting of single cells[4].

However, magnetic tweezers also has some limitations. We should

attach magnetic beads to the target cells for magnetic manipulation. The

magnetic bead attachment may affect the properties of the cells and interfere

with the observation. Although the magnetic force microscope and the

microelectromagnetic matrix may be used to manipulate single cells, the

requirement of a complex and expensive system and low density condition of

the cells makes them less attractive for the biological applications. In addition,

parallel manipulation of multiple cells using a magnetic field still remains

challenging.

Electrokinetic mechanisms are also widely used for microparticle

4

manipulation in a microfluidic device. There are three frequently used

electrokinetic mechanisms: electrophoresis[19, 20], dielectrophoresis[5, 21],

and electro-osmosis[5, 22]. Electrophoresis refers to the migration of an

electrically charged substance such as DNA and protein under the influence of

an electric field. In the electrophoresis, the target particles or molecules are

sorted by their own electrophoretic mobility under the same electric field

strength. Dielectrophoresis (DEP) is a phenomenon in which a force is exerted

on a dielectric particle when it is subjected to a non-uniform electric field. The

dielectrophoretic force does not require the particle to be charged differently

from the electrophoresis. It is proportional to the volume of the target particles

and the square of electric field gradient. Electo-osmosis is the motion of polar

liquid under the influence of an applied electric field. Since the electro-osmotic

drag force acting on nanoparticles is significantly larger than the

dielectrophoretic forces on them, the concentration of nanoparticles using the

electro-osmotic fluids is more effective than that using the dielectrophoresis.

These electrokinetic mechanisms are the most favorable tools for the

microparticle manipulation because any modification of target particle is not

needed and the construction of patterned electrode array for the parallel

manipulation with lower power consumption and larger manipulation area is

5

possible.

However, when we use an electrokinetic mechanism for manipulating

microparticles, relative permittivity and conductivity of the target particles and

the media need to be considered. The AC frequency-dependant properties of

AC electrokinetic manipulation techniques also limit the applicable ranges of

the electrokinetic mechanisms in biological applications.

1.2. Dielectrophoresis

Dielectrophoresis(DEP) is the movement of dielectric particles under a

non-uniform electric field driven by forces arising from the interaction between

the induced electric dipole of the particle and the applied electric field[23]. The

dielectrophoretic force is[24, 25]:

FDEP = 2πr3εmRe(fCM)∇E2

(1)

where is the radius of the particles, εm is the permittivity of media, E is the

local electric field, and Re(fCM) is the real part of the Clausius-Mossotti factor

which is described as below:

6

(2)

where ε* is the complex permittivity (ε* = ε-jσ/ω); σ is the conductivity, and ω

is the electric field frequency.

When the real part of the Clausius-Mossotti factor, Re(fCM) is positive,

the particles move toward the region of the maximum electric field (positive

DEP). On the other hand, when this value is negative, the particles are repelled

from the region of the maximum electric field (negative DEP). The illustration

of positive DEP and negative DEP is shown in Figure 1.

7

Figure 1. Dielectrophoretic movements of microparticles exposed to non-

uniform electric fields. For positive dielectrophoresis, particles move

toward the region of the highest electric field (the left particle). For

negative dielectrophoresis, particles are repelled from the region of the

highest electric field (the right particle).

8

1.3. Optoelectronic Tweezers

Electrokinetics, especially DEP, has become one of the most attractive

techniques for microparticle manipulation in a microfluidic device or a lab-on-

a-chip due to its ability to handle microparticles without any modification of

them. Many research groups have revealed the dielectric properties of

polystyrene microbeads[26-28], nanoparticles[29], carbon nanotubes[30, 31],

DNA[32, 33], bacteria[34], viruses[35], leukocytes[36], erythrocytes[37], liver

cells[38], and tumor cells[39].

Recently, a programmable lab-on-a-chip system using DEP electrode

array has been a key issue for parallel manipulation of individual cells or

particles[39, 40]. Manaresi and his colleagues reported a programmable DEP

technique which uses complementary metal-oxide semiconductor (CMOS)

circuit[41]. It was demonstrated that the programmable DEP manipulator with

individually addressable two-dimensional electrode array could be used for the

parallel manipulations of biological cells and microparticles. Since the

activated electrode patterns are movable and reconfigurable, it has many

advantages such as single particle addressing and selection, parallel

manipulation, and channel-less structure. However, it has a potential drawback

for disposable applications due to high manufacturing cost. Integration of on-

9

chip circuits increases the cost of the device, making it less attractive for

disposable applications. The wiring and interconnecting of the large number of

electrode array are also remained challenging issues.

To deal with these problems, optoelectronic tweezers (OET) was

proposed by Chiou and his colleagues in 2005[42]. In their study, replacing the

patterned electrodes with a pattern-less photoconductive layer, light-induced

virtual electrodes were formed on the photoconductive layer by using the

transmitted dynamic images generated from a digital micro-mirror device

(DMD). The conversion process among optical, electrical and mechanical

energy in OET is illustrated in Figure 2. In OET, the optical energy is

converted to the electrical energy via photoconductive material, triggering

virtual electrodes on the photoconductive surface. The virtual electrodes induce

the mechanical forces for microparticle manipulation via electrokinetic

mechanisms such as electrophoresis, dielectrophoresis, and electro-osmosis.

10

Figure 2. Energy conversion process in optoelectronic tweezers. Optical

energy is indirectly converted to mechanical energy by photoconductive

materials and several electrokinetic mechanisms.

Actually, the concept of light-induced electrodes has been frequently

used in electro-photography[43]. The patterning and concentration of nano-

and microparticle using optically-induced virtual electrodes was also

reported[44, 45]. Based on the concept of light-induced electrodes, OET which

uses image-driven DEP for parallel manipulation of single particles and live

cells has been developed[42, 46]. This concept provided a solution to the

disposable and interconnecting issues in the parallel manipulation of multiple

cells using DEP forces. Moreover, the optoelectronic mechanism requires a

much lower optical power and offers a much larger manipulation area than a

typical optical mechanism such as optical tweezers.

After the first OET platform was proposed, three types of OET have

been developed: DMD OET[42], image DEP[47], and lab-on-a-display[48]

11

(Table 1). The DMD OET is the first reported OET technique proposed by

Chiou and his colleagues as we mentioned. The DMD and optical lens

constructions offer the high resolution images to induce a virtual electrode.

However, the optical constructions for DMD projection lead to difficulties in

portable applications. The reflective structure resulted in complex structure.

Additionally, the optical lens between DMD and OET requires optical

alignment to focus the projected image on the photoconductive layer.

An OET system called image DEP (iDEP) uses a TFT-LCD projector

for image projection. This platform also needs a complex optical structure. In

addition, the images from the projector are diffracted more than DMD OET,

resulting in the lower-resolution virtual electrodes for particle manipulation.

The concept of an effective radius which is the maximum distance from a light

spot to manipulating particles is presented in their approach.

A lab-on-a-display, reported by Choi and his colleagues in 2007, is

constructed with an OET device and an LCD. The direct image transfer on

LCD is utilized. There is no optical component between LCD and OET, thus

the OET part is just placed on the display device. It offers the simplest structure

and the largest manipulation area among three types of OET platforms. In

addition, due to the elimination of lens and optical alignment, this platform is

12

relatively thin and tolerant to vibrations, providing more suitable platform for

portable applications. However, the lens-less structure results in the image

blurring by light diffraction, limiting the effective particle manipulation.

Table 1. Different types of typical optoelectronic tweezers.

13

1.4. Research Objectives

In this paper, we have developed a new platform for optoelectronic

particle manipulation called three-dimensional optoelectronic tweezers (3D

OET) composed of two photoconductive layers. The 3D OET was successfully

applied to focus polystyrene beads vertically and to manipulate them without

adsorptions. This work demonstrates that vertical focusing of microbeads

prevents non-specific interactions between the particles and the surfaces of the

OET device, and provides increased trapping efficiency and adsorption-free

manipulation of the particles. The performance of 3D OET is also compared

with that of a typical OET.

Here, we also propose a new LCD-based OET platform integrated with

a condenser lens for condensing LCD images to prevent image blurring by

light diffraction. A simple experimental setup and high performances of new

LCD OET are demonstrated. Simulation studies for electric field distribution in

liquid chamber by blurred virtual electrodes in lab-on-a-display and clear

virtual electrodes in new LCD OET are also performed. The microbead

velocities according to their sizes and gap heights of liquid chamber in LCD-

condenser OET are measured and compared with those in a typical lab-on-a-

display. We also manipulated blood cells using this new LCD OET system.

14

1.5. Thesis outline

The thesis consists of four chapters. In Chapter 1, backgrounds and

research objectives are mentioned. Chapter 2 describes design, fabrication,

simulation, and experimental demonstration of 3D OET comparing with

conventional OET. Chapter 3 describes a new LCD-based OET system using a

condenser lens. The last chapter is the conclusions of this thesis.

15

Chapter 2 Three-Dimensional Optoelectronic Tweezers

2.1. Introduction

In a typical OET device, not only lateral forces but also vertical forces

act on the particles. The vertical forces make the particles move upward when a

negative DEP is induced. Consequently, the particles positioned at the upper

region of the liquid layer are frequently attached to the top ground layer by

surface-particle electrostatic interactions, interfering with the effective and

non-contact particle manipulation using OET. The attachment of microparticle

is shown in Figure 3(a). Some attached particles pull other nearby particles

around them by dipole forces and interfere with their smooth movements

(Figure 3(b)). Additionally, some relatively small particles are sometimes put

out of focus, limiting the particle trapping efficiency to be low. These problems

become intensified when we need to freely handle a microparticle in the

channel-less environments. In the case of the biological samples which can be

contaminated or damaged by an external stimulation easily, the interactions

with surfaces may exert fatal influences on them. Nevertheless, the particle-

surface interactions are unavoidable if we use a conventional OET which uses

DEP forces acted on only one direction.

16

(a) (b)

Figure 3. Particle adsorptions in a typical OET device. (a) Not only lateral

forces but also vertical forces which push the particle in the upper

direction are induced by negative DEP. In consequence, the particles

become adsorbed to the surface of ground layer by electrostatic surface-

particle interactions. (b) Some adsorbed particles pull other particles

around them by dipole forces and interfere with their movements.

To prevent the adsorption and out-focusing of microparticles, we need

to push the particles from the top layer and vertically focus them via another

force acted in the opposite direction. That is, three dimensional trapping of

particles, like an optical tweezers, is needed. Therefore, here we propose a new

OET platform called three-dimensional optoelectronic tweezers (3D OET) for

3D particle trapping and manipulation. The 3D particle trapping via 3D OET

17

allows an adsorption-free particle manipulation to be possible. In addition, the

3D focused particles or live cells become free from a shear stress and other

external stimuli.

2.2. Design and fabrication

In the 3D OET device, the liquid layer containing microparticles is

sandwiched between two photoconductive layers (Figure 4(b)) differently from

the OET device composed of a photoconductive and a ground layer (Figure

4(a)). When an AC bias voltage is applied between two photoconductive layers,

the transmitted light beam forms virtual electrodes on the surfaces of the top

and the bottom photoconductive layer, resulting in an electric field gradient in

the liquid. This electric field gradient generates dipole moments of neutral

particles, which causes a DEP force for microparticle manipulation. The virtual

electrodes in 3D OET generate a 3-dimensional DEP cage to trap the particles

and focus them vertically using DEP forces. Consequently, we can manipulate

the vertically focused microparticles under the perfectly non-contact and

adsorption-free environments.

We used an indium tin oxide (ITO) layer as the transparent and

conductive layer for the ground. The photoconductive layer was comprised of

18

four layers: a 180 nm thick ITO layer, a 50 nm thick n+ doped hydrogenated

amorphous silicon (n+ a-Si:H) layer, a 800 nm thick intrinsic hydrogenated

amorphous silicon (intrinsic a-Si:H) layer, and a 20 nm thick silicon nitride

(SiNx) layer. Since the intrinsic a-Si:H has shorter carrier diffusion length and

higher optical absorption coefficient than crystalline silicon, it is a good

photoconductive material to make virtual electrode patterns of higher-

resolution[49, 50]. The 50 nm thick n+ a-Si:H layer was used to reduce the

contact resistance between the ITO layer and the intrinsic a-Si:H layer. The 20

nm thick SiNx layer was used to prevent electrolysis.

19

(a)

(b)

Figure 4. Schematic diagram of (a) optoelectronic tweezers (OET) and (b)

three-dimensional optoelectronic tweezers (3D OET). The 3D OET device

is composed of two photoconductive layers, while the OET device is

composed of one photoconductive layer and a ground layer.

20

To fabricate our OET device, we purchased glass substrates coated

with a 180 nm thick ITO layer from Samsung-Corning Precision Glass (Asan,

Korea). Triple layers of n+ a-Si:H, intrinsic a-Si:H, and SiNx were

consecutively deposited by plasma enhanced chemical vapor deposition

(PECVD) method on the ITO-coated glass substrate in a single chamber reactor.

The n+ a-Si:H was deposited from a gas ratio of 1.5% PH3 in SiH4 and then

intrinsic a-Si:H was deposited from a gas mixture of 20% SiH4/He = 300 sccm

and H2 = 100 sccm at 280 ºC. The SiNx layer was deposited by a SiH4, NH3

and N2 mixture. Then, some regions were etched by reactive ion etch (RIE) to

expose the ITO layer for bias connections. After dicing both the

photoconductive layer into 37.5 mm × 25.0 mm, a wrapping wire was

connected for biasing and we turned one layer upside down and put it on top of

the other layer at regular gap space using double-stick tapes as spacers (Figure

5). Fabricated photoconductive layer is shown in Figure 6.

21

Figure 5. Fabrication process of OET device (left) and 3D OET device

(right). Both fabrication processes for photoconductive layers of each

device are identical. In 3D OET, the liquid layer containing microparticles

is sandwiched between two photoconductive layers.

Figure 6. Fabricated photoconductive layer. Some regions of ITO layer are

exposed for bias connections.

22

2.3. Simulation of electric field distribution

The electric field distribution in the liquid chamber of 3D OET was

simulated and described as compared with that of OET in Figure 7. The electric

field was calculated by using a commercial CFD solver (CFD-ACE+; ESI US

R&D Inc., Huntsville, AL, USA). We assumed that 10 V bias at 100 kHz was

applied to some area of the bottom photoconductive layer of both devices. In

OET device, all area of the top ground layer was assumed to be a ground. In

3D OET device, only the exposed area of the top photoconductive layer, which

is the same with that of the bottom photoconductive layer, was assumed to be a

ground. We can find the electric field differences between OET and 3D OET

through the simulation.

According to the simulation results (Figure 7), the microparticles

which follow negative DEP would move in the direction from the dark region

to the light region. That is, microparticles in 3D OET device would be focused

in the middle of the liquid layer and kept apart from the surfaces of device.

Consequently, we can manipulate the particles free from the particle-surface

interactions using 3D OET devices. In addition, since DEP forces acted on a

particle are proportional to the gradient of the square of electric field, the

focused particles in 3D OET can be move faster than those in OET.

23

(a)

(b)

Figure 7. Simulated electric distributions in the liquid chamber of (a) OET

and (b) 3D OET. The estimated particle movements were also presented.

In OET device, microparticles which follow negative DEP would be moved

upward and adsorbed to the surface of ground layer. In 3D OET, the

microparticles would be focused into the middle of the liquid chamber and

kept apart from the surfaces of the top layer.

24

2.4. Experimental

Experimental setup for 3D OET is shown in Figure 8. A 1.3 in

monochromatic LCD module (26.4 mm × 19.6 mm image area) from the

conventional projector (EMP-5300; Epson, Japan) was used in this research. It

was consisted of 800 × 600 pixel array with 33 µm pixel pitch. Its dimensions

were 42 mm in length, 40 mm in width, and 5 mm in thickness. The LCD

module was operated by the LCD driver circuit of the projector. The LCD

images were drawn by a standard presentation software program (Microsoft

PowerPointTM) or an interactive program we developed on a computer.

Plain polystyrene beads (PolySciences, PA, USA) were used for

particle manipulation in the condition of 20 V bias at 100 kHz. The sample was

prepared by dilution with deionized water (σ = 0.23 mS/m) to the final

concentration of about 2.5 × 105 particles/mL. A sample droplet of 5-7 µL was

sandwiched between two photoconductive layers using double-stick tape as a

spacer, ensuring the liquid chamber of 120 µm thick. The bias voltage

produced from a function generator (MXG-9802A; Seowon Family Co., Korea

or AGF3022; Tektronix, USA) was applied.

We used two illuminations of the microscope: one for actuation and the

other for observation. The downside illumination with high intensity was used

25

for actuation, i.e., to create the image for virtual electrodes. The upside

illumination with low intensity was used for observation, because it was

difficult to see particles in a dark region without this upside illumination. The

movements of beads were observed and recorded using upright microscope

(Zeiss Axioskop 40; Carl Zeiss, Germany) with a digital camera (Coolpix5400;

Nikon, Japan). To investigate the effects of bias voltage and bead size on the

bad velocity, we recorded the bead movements and analyzed the video images

frame by frame using a analysis program that we made using MATLAB.

The microscopic pictures were taken without any image patterns of

LCD, because the optical observation of the beads in the dark region was

limited in the 3D OET device. To observe and capture the moved particles in

the dark region, we brightened whole area of LCD with a white full screen

after all manipulation experiments were finished.

26

Figure 8. Experimental setup for characterization of 3D OET. This

experimental setup was also utilized to OET device for comparing two

platforms (OET and 3D OET) with each other.

2.5. Results and discussion

Microscopic pictures on three different heights – bottom, middle, and

top – of the liquid chamber are shown in Figure 9. In the 3D OET device, the

27

polystyrene beads were successfully focused into the middle of the liquid layer

by a 3D DEP cage (Figure 9(b)), while they were adsorbed on the surface of

the ground layer by electrostatic surface-particle interactions in the OET device

(Figure 9(a)).

(a)

(b)

Figure 9. Microscopic photographs according to the heights of the liquid

layer of (a) OET and (b) 3D OET devices. Schematic diagrams of cross-

sections are also represented. In the 3D OET device, the polystyrene beads

(45 µm diameter) were focused into the middle of the liquid layer, while

they were adsorbed on the surface of the ground layer by negative DEP in

the OET device.

28

Since the number of moving particles speaks well for higher trapping

efficiency and adsorption-free particle manipulation of 3D OET, we have

compared the number of moving particles in 3D OET device with that in OET

device according to LCD images, bead sizes, and manipulation times. The

manipulation of 45 µm diameter size polystyrene beads was performed in the

condition of 20 V bias at 100 kHz. DI water whose measured electrical

conductivity is 0.23 mS/m was used as a buffer solution.

Figure 10 shows the number of moving particles by several LCD

images in OET and 3D OET devices. The image appears gradually during 60

seconds, forming letter “I”, “X” and “O”. We assumed that the microparticles

were uniformly spread out at the initial state, and calculated the percentages

using the number of particles moved by the images and the total number of

particles. The number of particles successfully moved by 3D OET was about

two times more than that of particles moved by OET. The microscopic

photographs of moved particles by LCD images forming letter “O” (the inside

of dotted lines is dark region) in OET and 3D OET devices are shown in Figure

11(a) and 11(b), respectively. The increased particle trapping efficiency of 3D

OET is shown.

29

I Type O Type X Type0

20

40

60

80

100 3D OET OET

Per

cent

age

of m

ovin

g be

ads

(%)

Pattern type

Figure 10. Percentages of moving beads by several LCD images in OET

and 3D OET devices (mean±standard deviation, n≥5). The images appears

gradually during 60 seconds, forming letter “I”, “X ” and “O”.

(a) (b)

Figure 11. Microscopic photographs of polystyrene beads (45 µm

diameter) concentrated by (a) OET and (b) 3D OET devices. A LCD image

appeared gradually during 60 seconds, and formed letter “O”. The

microbeads should have been moved to the inside of the dotted line.

30

The number of moving particles according to their sizes in OET and

3D OET devices is shown in Figure 12. Few particles were adsorbed in 3D

OET device, while less than 50% of particles could be moved in OET device.

We could find the difference of performances between 3D OET and OET

through comparing the number of moved 90 µm particles in 120 µm gap height

of liquid chamber. The adsorptions of microparticles by surface-particle

interactions become more serious when we manipulate relatively larger beads

with lower gap height of liquid chamber. In 3D OET, we could manipulate

most of the 90 µm diameter size beads although they are relatively large size

beads in 120 µm liquid chamber. On the other side, more than 90% of the 90

µm diameter beads were adsorbed to the surface of the ground layer and failed

the successful manipulation in OET device.

31

15um 45um 75um 90um0

20

40

60

80

100 3D OET OET

Per

cent

age

of m

ovin

g be

ads

(%)

Bead diameter (µm)

Figure 12. Percentages of moving beads according to their sizes in OET

and 3D OET devices (mean±standard deviation, n≥5). The gap height of

liquid chamber was 120 µm and 20 V bias at 100 kHz was applied.

The velocities of beads according to their sizes were also measured in

OET and 3D OET. The velocities only around the image edge were measured

because the beads moved fastest there. Firstly, we selected only moving beads

and measured their velocities. After that, we calculated the average velocities

of all particles based on the percentages of moving particles shown in Figure

12. The velocity of moving particles increases as their diameter increases in

both OET and 3D OET devices (Figure 13(a)). This result is in a good

agreement with the equation (1). However, the average velocity of all bead

including adsorbed beads shows large differences between OET and 3D OET

(Figure 13(b)). As the diameter of bead increases, the difference also increases.

32

(a)

15 30 45 60 75 90

0

10

20

30

40

50 3D OET OET

Bea

d ve

loci

ty (

µm/s

)

Bead diameter (µm)

(b)

15 30 45 60 75 90

0

10

20

30

40

50 3D OET OET

Bea

d ve

loci

ty (

µm/s

)

Bead diameter (µm)

Figure 13. Bead velocities according to their sizes in OET and 3D OET

devices. (a) Bead velocities of moving beads (mean±standard deviation,

n≥20). (b) Average velocities of all beads including adsorbed beads. The

average velocities of all beads were calculated based on the percentages of

moving particles.

33

The result of time domain analysis is also described in Figure 14.

While the number of moving beads in OET decreases as time goes by, most of

the particles in 3D OET device were moved continuously without attachments.

The attached particles in OET induced strong electrostatic particle-particle

interactions and pulled other particles around them. This phenomenon

interferes with a continuous particle manipulation for a long time. It becomes a

very serious problem when a long-term particle manipulation is required.

0 90 180 270 360 4500

20

40

60

80

100

Per

cent

age

of m

ovin

g be

ads

(%)

Time (s)

3D OET OET

Figure 14. Percentages of moving beads according to the time in OET and

3D OET devices (mean±standard deviation, n≥5). While the number of

moving beads in OET decreases as time goes by, most of the particles in 3D

OET device were moved continuously without adsorptions.

34

Our 3D OET device is adapt to the lab-on-a-display platform which

uses no optical components. If a light is transmitted through a high N.A. lens

like a DMD OET and an iDEP platform, the light cannot be transmitted to both

two photoconductive layers since the lens can focus the light into only one

photoconductive layer. Consequently, the optical components such as objective

lens interfere with the normal operation of 3D OET. So, we used only lab-on-a-

display platform which uses an OET device on a LCD, resulting in direct light

transfer, for operating 3D OET.

2.6. Conclusion

In this work, 3D OET, which contains two photoconductive layers as a

pair, for 3D trapping and manipulation of microparticles has been developed.

We have successfully manipulated the polystyrene beads without attachments

using 3D OET on a LCD. The performances of 3D OET were compared with

those of OET according to several parameters; LCD images, bead sizes, and

manipulation times. The 3D OET shows higher particle trapping efficiency and

less particle adsorptions than a typical OET. However, the optical observation

of the beads in a dark region was limited when a thick photoconductive layer is

used as the upper photoconductive layer.

35

Chapter 3 Lens-integrated LCD-based Optoelectronic Tweezers

3.1. Introduction

Several types of OET platforms have been proposed since the

optoelectronic principles were reported in 2005[42, 47, 48]. The OET can be

classified in three categories based on the device for image generation: a DMD,

a projector and a LCD. Another classificatory criterion is whether optical

components such as objective lens and mirror structures need to be applied or

not (Table 1).

Since the DMD should reflect a light using micro-mirror array to make

an image and the projector, in general, flashes a largely diffracted image to

project on a large screen, the DMD OET and the image DEP applied a

projector ought to use the optical components for condensing, reflecting and

focusing an image. These optical components provide higher optical power and

smaller minimum pixel sizes by condensing original images, resulting in higher

manipulation performances. However, the optical components make their

practical usage difficult, and limit the portability because of their complex and

large structures. In lab-on-a-display which utilized a LCD for image generation,

no optical structures were constructed and the direct image transfer from LCD

36

to an OET device was used to obtain a large manipulation area, a simple

structure, and portability. However, the LCD images directly transmitted into

an OET device were diffracted and induced the blurred virtual electrodes on a

photoconductive layer, limiting its performances of microparticle manipulation

(Figure 15).

To overcome the limitations of each platform and to harmonize their

own advantages, here we propose a new OET platform using a LCD and a

condenser lens. In this study, we could construct a new LCD-based OET

system integrated with a condenser lens which offers a well-packaged structure.

This new platform is much simpler and easier to use than DMD OET and iDEP.

Additionally, it has much higher manipulation performances than lab-on-a-

display platform, comparable to DMD OET.

(a) (b)

Figure 15. Microscopic photographs of (a) an original LCD image and (b)

a blurred image on a photoconductive layer. The image blurring is

occurred by diffraction of the light.

37

3.2. Design and fabrication

To prevent the diffraction of light in the lab-on-a-display, we utilized a

condenser lens positioned between an LCD and an OET device (Figure 16).

Although the optical component makes the OET system have higher

complexity in a general way, an integrated system which is thinner and simpler

than a typical OET system was constructed by using the direct light penetration

through LCD and the condenser lens integrated in a microscope. Through this

novel approach, we could obtain a high manipulation performance comparable

to a typical DMD OET platform as well as a well-integrated system as simple

as a typical lab-on-a-display platform.

The fabrication process of a photoconductive layer is the same with

Figure 5. In this study, we additionally fabricated a spacer using a

photolithography process for lower gap height (20 µm) of liquid chamber. The

device which has lower gap height was used for manipulation of smaller beads

or blood cells. Fabrication process of whole device including a

photoconductive layer and a spacer-patterned ground layer are shown in Figure

17.

38

(a)

(b)

Figure 16. Configuration of (a) a typical lab-on-a-display and (b) a new

LCD-based OET system integrated with a condenser lens. By using a

condenser lens integrated in a microscope, we could construct a simple

and well-packaged OET system as well as obtain higher performances for

microparticle manipulation.

39

Figure 17. Fabrication process for the OET device. Photoresist spacers are

patterned on a ground layer by using a photolithography process. The

spacer-patterned ground layer is lain upon a photoconductive layer and

then clamped.

40

A photolithography process was used to fabricate 20 µm thick spacers

on a ground layer. Firstly, we purchased an ITO-coated glass substrate. THB-

151N negative photoresist (JSR Corp., Japan) was spin-coated (20 sec at 300

rpm, 30 sec at 2500 rpm) and exposed to UV light for pattern-transfer. The

latent image was developed and hardened in bake-oven. After dicing the

spacer-patterned ground layer into 37.5 mm × 25.0 mm, a wrapping wire was

connected for biasing. Finally, we clamped the photoresist-patterned ground

layer and a photoconductive layer after sampler injection. We also used a

double-stick tape as a 120 µm thick spacer.

3.3. Simulation of electric field distribution

In a lab-on-a-display and iDEP systems, the image blurring is occurred

by light diffractions. To investigate the relation between the image blurring and

the electric field distribution, we simulated the square of electric field in liquid

chamber using a commercial CFD solver (CFD-ACE+; ESI US R&D Inc.,

Huntsville, AL, USA). Firstly, we estimated the voltage distribution of virtual

electrodes formed on the photoconductive layer by blurred images. We

assumed that the photoconductivity is linearly proportional to the light intensity

at the light intensities under 2000 lx[48]. Based on the light intensities of

41

blurred images measured by a commercial measurement program (i-Solution;

Image and Microscope Technology, Korea), the applied voltages on the region

of blurred images were calculated and described in Figure 18. The voltage

signal applied across a photoconductive layer and a ground layer was assumed

to have a constant value of 10 V at 100 kHz.

Figure 18. Microscopic photograph of the edge region of the blurred

virtual electrode and the measured light intensity of the image (thin line).

The applied voltages estimated based on the measured light intensity are

described (thick line) in the condition of 10 V bias.

42

The simulated electric field distribution in the liquid chamber is shown

in Figure 19. If a diffraction of transmitted images occurs in the lab-on-a-

display platform, the edge region of virtual electrode will be blurred and the

electric field would be distributed as shown in Figure 19(b). However, if the

image blurring is prevented by a condenser lens, a clear virtual electrode will

be formed (Figure 19(a)). The square of electric field distribution on the edge

region of the blurred electrodes and the clear electrodes were also plotted in

Figure 20. The gradient of the square of electric field on the edge region of

clear electrodes was one and half times higher than that of blurred electrodes.

That is, when we used a condenser lens for condensing and inducing a clear

virtual electrode on a photoconductive layer, the DEP forces acted on a

microparticle at the edge region of electrode would be stronger.

43

Figure 19. Simulated distributions of electric field induced from (a) a clear

virtual electrode formed by condensed light and from (b) a blurred virtual

electrode formed by diffracted light in the condition of 10 V at 100 kHz.

Figure 20. Plot of the square of electric field generated by a clear virtual

electrode (blue, solid) and a blurred virtual electrode (red, dotted) in the

middle of the liquid chamber. The gradient of the square of electric field

generated by a clear image is 1.5 times higher than that by a blurred

image.

44

3.4. Experimental

A 0.5 in monochromatic LCD module (11.3 mm × 8.5 mm) from the

conventional projector (EMP-S4; Epson, Japan) was used in this research. It

was consisted of 800 × 600 pixel array with 14 µm pixel pitch. Its dimensions

were 19.2 mm in length, 21.8 mm in width, and 5 mm in thickness. The LCD

module was operated by the LCD driver circuit of the projector. The LCD

images were drawn by using an interactive control program we developed on a

computer.

We used two illuminations of the microscope: one for actuation and the

other for observation. The downside illumination with high intensity was used

for actuation, i.e., to create the image for virtual electrodes. The upside

illumination with low intensity was used for observation, because it was

difficult to see particles in a dark region without this upside illumination. The

maximum light intensity from the LCD was measured using an illuminance

meter (Illuminance Meter T-10M; Konica Minola Inc., Japan).

The LCD module was installed on the downside illumination of

upright microscope (Zeiss Axioskop 40; Carl Zeiss, Germany). The light

passed through LCD module was transmitted to a condenser lens (N.A. = 0.9)

of the microscope, condensing and focusing into a photoconductive layer of

45

OET device. This simple experimental setup offers well compatible OET

system in a practical manner (Figure 21).

Figure 21. Schematic diagram of experimental setup for new LCD-based

OET system integrated with a condenser lens.

46

Plain polystyrene beads (PolySciences, PA, USA) were used for

particle manipulation in the condition of 20 V bias at 100 kHz. The sample was

prepared by diluting with deionized water (σ = 0.23 mS/m) to the final

concentration of about 2.5 × 105 particles/mL. A sample droplet of 5-7 µL was

sandwiched between a photoconductive layer and a ground layer. When a 120

µm thick liquid chamber was required, we used a double-stick tape as a spacer.

If a 20 µm thick liquid chamber is needed to manipulate smaller beads (~15

µm), we used a ground layer that a photoresist spacer is fabricated on it.

We used fresh whole blood of a Sprague-Dawley rat as a blood sample.

Firstly, a 0.5 mL whole blood sample was taken from a healthy rat and kept in a

heparin-coated tube. To avoid coagulation of blood, we completed the

following procedure in a short period. After an isotonic glucose solution (8.5%

dextrose, 0.3% sucrose) was added to the blood sample, it was centrifuged and

the supernatant fluid was removed. The process was repeated two times for

washing and changing buffer solution. Finally, we obtained the blood cells 10:1

diluted with isotonic glucose solution (σ = 0.23 mS/m). In the condition of 7 V

bias at 200 kHz, we used the prepared blood sample for blood cell

manipulation.

47

3.5. Results and discussion

3.5.1. Simulation of Clausius-Mossotti factors

The simulation of Clausius-Mossotti factor should be performed before

the DEP experiment to acquire a proper AC voltage condition. The simulated

Clausius-Mossotti factors of polystyrene beads according to the AC frequency

are shown in Figure 22. The real part of Clausius-Mossotti factor of

polystyrene beads was simulated by using equation (2). The electrical relative

permittivity and conductivity of 15 µm polystyrene beads is 2.56 and 1.15

mS/m, respectively. When the AC frequency is higher than 200 kHz, the

polystyrene beads move by negative dielectrophoretic forces. On the other

hands, when the AC frequency is lower than 200 kHz, the polystyrene beads

show positive dielectrophoretic motions with the media conductivity larger

than 100 mS/m.

48

Figure 22. The simulated real part of Clausius-Mossotti factor of a 15 µm

diameter size polystyrene bead in mediums with conductivities of 1 S/m,

100 mS/m, 10 mS/m, 1 mS/m, and 0.1 mS/m. The relative permittivity and

conductivity of the polystyrene bead is assumed to be 2.56 and 1.15 mS/m,

respectively.

49

The simulation of frequency-dependent Clusius-Mossotti factors of

erythrocytes was also performed (Figure 23). In the case of the erythrocytes,

they were represented by a single-shell model[51]. In practice, the biconcave

erythrocyte model has been used, but the spherical model with an effective

radius of erythrocytes used in this work for simplification. The electrical values

of erythrocytes which have been extensively used in the literature were

utilized[52-55]. The cytoplasm is a physiological saline solution with a protein

volume fraction of 0.26, relative electrical permittivity εr = 50 and conductivity

σ = 0.52 S/m. The membrane is represented by a shell of constant thickness δ =

8 nm that has a very low conductivity σ = 1 µS/m and a relative permittivity εr

= 5. The cell is immersed in an external media whose relative permittivity εr =

78.

The real part of Clausius-Mossotti factor of erythrocytes is switched

from negative to positive at the AC frequency less than 10 MHz. The switching

frequency increases as the AC frequency increases. However, when the

conductivity of media is larger than 1 S/m, the negative dielectrophoretic

forces are acted on the erythrocytes at the all frequency ranges. On the other

hands, the conductivity of media is smaller than 1 mS/m, the positive DEP is

always shown.

50

Figure 23. The simulated real part of Clausius-Mossotti factor of a red

blood cell in mediums with conductivities of 1 S/m, 100 mS/m, 10 mS/m, 1

mS/m, and 0.1 mS/m. The electrical conductivity of cytoplasm is assumed

to have a value of σ = 0.52 S/m. The membrane is assumed to have a

constant thickness δ = 8 nm that has a very low conductivity, σ = 1 µS/m.

51

3.5.2. Condensation of LCD image

Figure 24 shows two LCD images; one is an original LCD image and

the other is a condensed image focused on a photoconductive layer. We could

prevent the image blurring by light diffraction using a condenser lens. The

magnification and N.A. of the condenser lens are ×5 and 0.9, respectively.

Consequently, we could get five times smaller minimum pixel size (2.8 µm)

than original LCD pixel size (14 µm). The minimum pixel size can be smaller

if we use another LCD module which has smaller pixels or another lens

structure which has higher magnification. However, the manipulation area of

this lens-integrated LCD OET system (3.8 mm2) became much smaller than

that of a lab-on-a-display which directly uses transferred images from LCD

without any optical components. We cannot get a wide manipulation area with

smaller minimum pixel size because the manipulation area decreases as the

minimum pixel size decreases.

52

Figure 24. Microscopic photographs of (a) an original LCD image and (b)

a condensed image on a photoconductive layer. The image blurring is

prevented by a condenser lens.

3.5.3. Polystyrene bead manipulation

The bead velocities according to their diameter in our new LCD OET

system and in a typical lab-on-a-display system are shown in Figure 25. The

measured bead velocity in new LCD OET platform integrated with a condenser

lens was much higher than that in a typical lab-on-a-display platform. In a

typical LCD OET platform, lab-on-a-display, manipulation of microbeads

smaller than 18 µm diameter size was almost impossible in the conditions of 20

V bias at 100 kHz. On the other hand, we could successfully trap and

manipulate 15 µm diameter size beads using a new LCD-based OET platform

integrated with a condenser lens to prevent a blurring effect by light

diffractions (Figure 26). The maximum velocity of 15 µm diameter size beads

was about 8 µm/s in 120 µm height liquid chamber.

53

15 30 45 60 75 90

0

30

60

90

120 Gap height : 120 µm Condenser-LCD Only LCD

Bea

d ve

loci

ty (

µm/s

)

Bead diameter (µm)

Figure 25. Bead velocities according to their sizes in a new LCD-based

OET system and a lab-on-a-display (mean±standard deviation, n≥20). The

LCD-Condenser OET system shows higher manipulation performances

than the lab-on-a-display platform which uses only LCD without any lens

structures.

Figure 26. The captured photographs after parallel manipulation of 15 µm

polystyrene beads. The maximum velocity of the microbeads is 8 µm/s in

the condition of 20 V at 100 kHz.

54

According to those experimental results, we could find that the blurred

virtual electrodes induced from the diffracted LCD image in lab-on-a-display

limit the effective microparticle manipulation. We already confirmed that the

square of electric field gradient induced from a clear virtual electrode is one

and a half times larger than that induced from a blurred virtual electrode

through the simulation study (Figure 22). We also measured the maximum light

intensity of images transmitted to a photoconductive layer in each case. When

we used a lens to condense the light from LCD, the optical power was

measured ten times stronger than when we did not use any optical components

for condensing LCD images. These factors would affect the improvement of

performances of the new LCD OET system.

However, the polystyrene beads smaller than 10 µm (diameter) did not

move even when we used the new LCD OET for particle manipulation. To

solve this problem, we lowered the gap height of liquid chamber of OET

device from 120 µm to 20 µm. To investigate the relation between the height of

liquid chamber and the induced dielectrophoretic forces, other experimental

conditions were fixed and only the ground layer was changed to the 20 µm

thickness spacer-patterned ground layer. The measured bead velocities are

shown in Figure 27. We could successfully manipulate 8 µm diameter size

55

polystyrene beads with 11 µm/s average velocity in 20 µm height liquid

chamber. Larger dielectrophoretic force would be generated since the electric

field gradient increases as the gap height of liquid chamber decreases.

Accordingly, smaller beads may also be trapped and manipulated if an OET

device which has lower liquid chamber is used.

8 10 12 14 16

0

10

20

30

Gap height 20 µm 120 µm

Bead diameter (µm)

Bea

d ve

loci

ty (

µm/s

)

Figure 27. Bead velocities according to their sizes in 20 µm and 120 µm

height liquid chambers of OET device (mean±standard deviation, n≥20).

As the height of liquid chamber decreases, the optically-induced

dielectrophoretic force increases.

56

We trapped a 10 µm diameter size bead using a virtual DEP cage and

go through a maze constructed by virtual electrodes (Figure 28). The gap

height of liquid chamber was 20 µm and the applied voltage was 20 V bias at

100 kHz. The measured maximum velocity of the bead was 16 µm/s. After 3

minutes, we could find the way to thread the maze. This interactive

manipulation of polystyrene beads was performed by using an interactive

control program we developed. We can interactively draw and move an image,

namely a virtual electrode, as seeing a real-time microscopic movie through the

interactive control program.

Figure 28. Interactive manipulation of a polystyrene bead for threading a

maze constructed by optically-induced virtual electrodes.

57

3.5.4. Blood cell manipulation

We have successfully concentrate blood cells using optically-induced

positive DEP (Figure 29). A number of blood cells were transported to the

bright (red) area of the LCD image forming a word “KAIST” during 60

seconds. When operating voltage was applied, red blood cells were arranged in

the direction of parallel to the electric field direction. Some red blood cells

were aggregated by dipole interactions.

Figure 30 demonstrates the trapping and transporting of individual red

blood cells using optically-induced virtual electrode array. One to three red

blood cells were trapped in each light spot by positive DEP under the voltage

condition of 7 V at 200 kHz. By programming the LCD image, a column of the

array could be selected and moved. The moving column successfully

transported the trapped red blood cells individually in the upper direction.

58

(a) (b)

Figure 29. Concentration of red blood cells using a LCD image forming a

word “KAIST” during 60 seconds. (a) Voltage is not applied. (b) Voltage is

applied.

Figure 30. Parallel manipulation of red blood cells trapped in a virtual

electrodes array. One to three cells are trapped in each light spot by

positive DEP. A selected column of the array moves in the upper direction,

transporting red blood cells.

59

The single red blood cell trapping and manipulation were also

performed (Figure 31). By using the interactive control program, we selected a

target red blood cell and move the virtual electrode for transporting the selected

red blood cell. The maximum velocity of red blood cell was about 5 µm/s in

the condition of 7 V bias at 200 kHz.

Figure 31. Single red blood cell manipulation. A red blood cell is trapped

and transported by positive DEP. The measured velocity of red blood cell

is about 5 µm/s in the condition of 7 V bias at 200 kHz.

60

Figure 32 demonstrates selection and manipulation of single white

blood cell. We could find a target white blood cell among many red blood cells

through real-time movie on a monitor. After finding and selecting the target

white blood cell, a virtual dielectrophoretic tweezers was generated and we

dragged the selected white blood cell out from many unwanted red blood cells.

These processes were performed by using the interactive control program in

the same voltage condition and a light spot whose diameter is about 6 µm was

used for trapping a white blood cell. If the size of light spot increased, the red

blood cells moved to the virtual electrode by positive DEP interfere with the

selection and transport of a white blood cell.

61

Figure 32. Selection and manipulation of a single white blood cell among

many red blood cells in the condition of 7 V bias at 200 kHz. The minimum

size of light spot for trapping a white blood cell is about 6 µm.

62

3.6. Conclusion

In this paper, we have developed a new LCD-based OET system which

uses a LCD and a condenser lens for projecting an image into an OET device.

By using the condenser lens integrated in a microscope, we could successfully

construct a simple integrated OET system. Bead velocities according to their

size were measured using this new platform, comparing with a typical lab-on-

a-display platform. The bead velocities under different gap height of liquid

chamber in OET device were measured. We could also manipulate blood cells

using this new platform.

Our new LCD OET provides much higher performances than a typical

lab-on-a-display, comparable to DMD OET, by preventing the image blurring

induced from light diffraction using condenser lens. Additionally, this new

platform is much simpler and easier to use than typical OET platforms which

use some optical components in a practical manner. This new LCD-based OET

system using a condenser lens and a LCD may be a usable integrated system

for optoelectronic manipulation of microparticles including live cells because

of its simple structures and high performances.

63

Chapter 4 Conclusions

In this research, we have demonstrated two approaches for

enhancement of typical OET platforms; 3D OET and lens-integrated LCD OET.

First, the 3D OET composed of two photoconductive layers was successfully

utilized to focus microparticles into the middle of the liquid chamber using

negative DEP. Consequently, we could keep the microparticles apart from the

surface of device, preventing adsorptions by electrostatic surface-particle

interactions in OET device. We compared the performances of a 3D OET

device with those of a typical OET device according to several parameters;

LCD images, bead sizes and manipulation times. In 3D OET device, about

90 % of target particles were successfully manipulated continuously for a long

time (t ≥ 450 sec), while about 50 % of particles were adsorbed to the device

surface or focused out by vertical forces in OET device. In the case of 90 µm

diameter beads in 120 µm gap height of liquid layer, about 95 % of them were

adsorbed. If we use relatively low gap height of liquid chamber, the gradient of

the square of electric field which is proportional to the DEP forces would be

increased, but the adsorptions of manipulated particles would be also increased.

Therefore, we need to consider the mutual exclusion before we decide the gap

64

height of liquid chamber when we use the OET for particle manipulation.

However, when we used the 3D OET, all particles could be manipulated

without adsorptions regardless of their size and the gap height of liquid

chamber. The higher particle trapping efficiency and less particle adsorptions

of 3D OET make it more compatible tools for manipulation of microparticles

including live cells and polymer beads.

However, some limitations which need to be conquered are remained.

The optical observation of the beads in a dark region was limited when a thick

photoconductive layer is used as the upper photoconductive layer. Additionally,

the 3D OET device is only adapted to the lab-on-a-display platform which uses

no optical components, resulting in direct light transfer, because a high N.A.

lens makes the transmitted light to be focused into only one photoconductive

layer. However, as we mentioned previously, the image blurring is occurred in

the lab-on-a-display, resulting in smaller DEP forces for particle manipulation.

To overcome the drawbacks, we can utilize a new display device which can

project the image into both two photoconductive layers of 3D OET. Two light

sources can be also applied; one is for inducing the virtual electrodes on the

upper photoconductive layer and the other is for inducing them on the bottom

one. If the double-light source structure can be constructed, the ‘real’ three-

65

dimensional particle manipulation, not only focusing but also control of

vertical positions of each particle, may also be possible.

Second, we have also developed a new LCD-based OET system using

a condenser lens for preventing light diffractions and providing higher

manipulation performances and virtual electrode resolution. By using an LCD

for image generation and a condenser lens integrated in a microscope, we could

obtain not only higher particle manipulation performances, but also a simple

and well-packaged OET system. An image blurring was completely reduced by

using the condenser lens whose magnification and N.A. are ×5 and 0.9,

respectively. The minimum pixel size of virtual electrode was 2.8 µm when we

use an LCD module whose pixel size is 14 µm. Because of the clear and

smaller virtual electrodes, the optically-induced DEP forces were increased

about 1.5 times. Additionally, in the lens-integrated LCD OET, the

condensation of light by the condenser lens offered optical power 10 times

stronger than the conventional lab-on-a-display platform. When the gap height

of liquid chamber was lowered from 120 µm to 20 µm, the electric field

gradient was increased significantly and we could manipulate smaller

microbeads (8 µm diameter polystyrene beads) with 11 µm/s average velocity

using the optically-induced DEP. We could also observe an electro-osmosis of

66

5.4 µm diameter fluorescent microbeads with a certain voltage condition. It

shows a possibility of our LCD OET platform for AC-electro-osmotic

nanoparticle manipulation. An interactive control program was also developed

and demonstrated for manipulation of polystyrene beads and blood cells.

Selection and transport of a single red blood cell and a single white blood cell

were successfully demonstrated using this platform.

In the case of blood cells, dielectrophoretic aggregation and positive

dielectrophoretic movements of them interfered the effective single cell

manipulation and the measurement of dielectrophoretic velocity. To prevent the

aggregation of blood cells by positive DEP and electrostatic aggregation, we

needed to minimize the size of light spot which is the same with the size of

virtual electrode. If we could utilize negative DEP for blood cell manipulation,

single cell trapping and manipulation would be easier. For inducing the

negative DEP of biological cells, higher conductivity of the buffer media is

required. To apply the buffer media which has higher conductivity for cell

manipulation without electrolysis, the conductivity of photoconductive layer

should be larger than that of the buffer solution. However, in the case of our

OET device which is composed of hydrogenated amorphous silicon, the

photoconductivity is too low to apply high conductive buffer media such as

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phosphate buffered saline (PBS) solution. If we utilize a material which has

higher photoconductivity, the negative dielectrophoretic manipulation of

biological cells without additional processes for buffer change will be possible.

The particle adsorptions by vertical DEP forces in OET device was also a

problem. Of course, the adsorptions between manipulated particles and device

surfaces were reduced by using a 3D OET device in chapter 2. However, we

could not utilize the 3D OET device to the lens-integrated LCD OET platform.

It was caused by the light focused into only one photoconductive layer by the

condenser lens as we mentioned above. Because we use the condenser lens of

microscope for focusing LCD images, the background light for observation of

microparticles in the OET device was intercepted. It can also be a problem of

power our integrated LCD OET platform, when we observe the manipulated

particles using an objective lens of high magnifying power.

However, we could easily set up the lens-integrated LCD OET system

as positioning an LCD module and an OET device on the light source and on

the stage of a microscope, respectively. In addition, high performance for

microparticle manipulation was obtained with the simple structure. The

interactive control program which we developed was also helpful to manipulate

single cells or microbeads interactively as seeing the target particles through a

68

computer monitor. The higher performances and well-integrated structures of

our new OET system can make it a wide usable platform for optoelectronic

manipulation of microparticles including live cells in many biological

applications.

69

Summary in Korean

본 연구에서는 미세입자 및 혈액세포를 자유자재로 구동하기

위한 액정 디스플레이 (LCD; Liquid crystal display) 기반의 새로운

광전자집게 (OET; Optoelectronic tweezers)를 제안하였다.

광전자집게란 움직이는 디스플레이 영상을 광전도성 층에

조사함으로써 가상의 전극을 형성시키고, 이로 인해 발생한 불균일한

전기장을 이용하여 유전영동 (DEP; Dielectrophoresis)을 유도,

미세입자를 자유자재로 구동하는 기술이다. 이러한 광전자집게는

세포와 같은 미세입자들을 제어하여 여러 가지 생물학적 또는

화학적 실험을 진행하는데 매우 유용하다. 그러나 기존의

광전자집게의 경우 2 차원 상에서 형성되는 가상 전극을 이용하기

때문에 미세입자의 수직 운동을 제어할 수 없다는 한계가 있다.

그러므로 수직방향으로 작용하는 유전영동력에 의해 현미경

초점으로부터 벗어나게 되는 현상이나 정전기적 표면-입자 반응에

의한 미세입자의 표면 흡착 현상 때문에 미세입자의 포획 및 구동

효율이 감소하는 문제점이 존재하였다.

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본 연구에서는 이러한 문제점을 해결하기 위하여 두 개의

광전도성 층으로 이루어진 3 차원 광전자집게 (3D OET; Three-

dimensional optoelectronic tweezers)를 제안하였다. 3차원 광전자집게는

하나의 광전도성 층 및 하나의 투명한 접지전극 층을 사용하는

기존의 광전자집게와 달리, 두 개의 광전도성 층을 이용하기 때문에

미세입자를 중심으로 상하로 형성되는 가상의 전극을 이용할 수

있다. 이 때 상하로 형성된 가상의 전극은 음의 유전영동력을

발생시켜 3 차원 상에서 미세입자를 포획할 수 있도록 하며 이로

인해 미세입자의 흡착과 같은 현상이 제거되며, 결과적으로

미세입자의 포획 및 구동 효율이 크게 향상될 수 있다.

3 차원 광전자집게를 형성하는 광전도성 층은 투명한 전도성

물질인 ITO (Indium tin oxide)가 코팅된 유리웨이퍼 위에 PECVD

(Plasma enhanced chemical vapor deposition)를 이용하여 접촉 저항

감소를 위한 n 형의 도핑된 비정질 실리콘 (n+ doped amorphous silicon),

광전도성 물질인 진성의 비정질 실리콘 (intrinsic amorpous silicon) 및

샘플의 전기분해를 막기 위한 질화 실리콘 (silicon nitride)을 차례로

증착시킨 후, ITO 에 전압인가를 하기 위한 일부 영역을 RIE (Reactive

71

ion etch)를 이용하여 식각함으로써 제작되었다. 이렇게 제작된 두

개의 광전도성 층을 미세입자가 들어있는 미세유체 방울을 사이에

두고 서로 마주보도록 포개어 3 차원 광전자집게를 완성하였다.

미세입자를 구동하기 위한 영상은 액정 디스플레이를 통해

조사되었으며, 렌즈가 없는 랩온어디스플레이(Lab-on-a-display)

플랫폼을 이용함으로써 상하 두 개의 광전도성 층에 똑 같은 모양의

가상의 전극을 형성시킬 수 있었다.

3 차원 광전자집게를 이용한 미세입자의 구동 속도 및 구동

효율을 측정한 후 기존의 광전자집게와 비교한 결과, 3 차원

광전자집게에서 음의 유전영동력에 의하여 유체 층의 중심으로

포커싱된 미세입자의 경우 기존의 광전자집게에서보다 구동 속도 및

구동효율이 증가함을 알 수 있었다. 특히 기존의 광전자집게에서는

절반 가량의 미세입자들이 흡착되었던 반면, 3차원 광전자집게에서는

그러한 흡착현상이 거의 완벽하게 제거됨으로써 90%이상의 매우

높은 구동효율을 얻어 낼 수 있었다. 이러한 3 차원 광전자집게는

완벽히 비접촉(non-contact)된 구동환경을 제공함으로써 흡착이나

72

외부에서 가해지는 힘에 특히 민감한 생물학적 세포의 구동에 매우

적합하다.

광전자집게는 영상을 조사하는 장치의 종류 및 렌즈의 유무에

따라 다양한 종류가 존재한다. 지금까지 개발된 광전자집게

플랫폼으로는 디지털 미세거울 장치 (DMD; Digital micromirror

device)를 이용한 DMD OET, 프로젝터를 이용한 iDEP (image DEP),

액정 디스플레이를 이용한 lab-on-a-display가 있다. DMD OET 와

iDEP 의 경우에는 빛이 반사되어 분산되는 형식의 영상표시장치를

이용하기 때문에, 광전도성 층에 정확하게 상을 맺히게 하기 위해서

복잡한 렌즈 및 거울 장치를 필요로 한다. 반면에 lab-on-a-display의

경우에는 복잡한 광학계 없이 액정 디스플레이 위에 바로

광전자집게 장치를 위치 시킴으로써 매우 단순한 구조와 휴대성

(portability)을 갖추고 있다. 이는 빛을 반사시키는 디지털

미세거울장치나 프로젝터와 달리 빛을 통과시킴으로써 영상을

만들어 내는 액정 디스플레이 특징 때문에 가능한 것이다. 그러나

lab-on-a-display 의 경우에는 렌즈와 같은 광학장치가 없기 때문에

빛이 분산되는 현상이 발생하고 이로 인해 흐릿한 영상이

73

광전자집게에 맺혀 흐릿한 가상의 전극이 형성되었고, 그로 인해

미세입자를 구동하기 위한 성능이 다른 장치에 비해 현저히

감소하는 문제점이 있었다.

본 연구에서는 사용하기 편리하고 단순한 구조를 지닌 동시에

뛰어난 미세입자 구동성능을 가진 새로운 광전자집게 플랫폼으로서

렌즈가 통합된 새로운 액정 디스플레이 기반의 광전자집게 시스템을

제안하였다. 본 연구에 따른 새로운 플랫폼은 빛을 통과시켜 상을

맺히게 하는 액정 디스플레이의 특성을 이용하여, 단순히 현미경의

광원 위에 액정 디스플레이를 올려놓음으로써 현미경에 통합되어

설치된 집광렌즈를 거쳐 재물대 위의 광전자집게 장치에 정확하게

상이 맺히도록 하는 원리를 이용하였다. 이러한 단순한 구조의 통합

플랫폼은 현미경과 액정 디스플레이, 광전자집게 장치만으로 우수한

구동성능을 보이며 사용자 입장에서 그 실용성이 매우 뛰어나다고

볼 수 있다.

본 논문에서 사용한 광전자집게 장치는 앞서 서술한 것과

똑같은 방식으로 제작된 광전도성 층과 ITO 로 만들어진 접지전극

층으로 이루어져 있다. 액정 디스플레이는 최소한의 가상 전극

74

크기를 얻기 위해 픽셀 크기가 14 µm 인 프로젝터용 흑백 액정

디스플레이를 사용하였으며, 현미경에 통합된 집광렌즈는 개구수가

0.9, 배율이 5 배인 것을 사용하였다. 이로서 우리는 최소 2.8 µm 의

크기를 지닌 가상 전극을 광전도성 층에 뚜렷하게 형성되도록 할 수

있었다. 이는 기존의 lab-on-a-display에 비해 약 100 배 정도 더 작은

크기이다. 뚜렷한 영상에 의한 가상의 전극과 흐릿한 영상에 의한

가상의 전극에 의해 각각 형성되는 전기장의 기울기 및

유전영동력을 시뮬레이션을 통해 확인한 결과 영상이 뚜렷할 경우

약 1.5 배 정도 그 크기가 컸다. 실제 광전자집게에 조사되는 빛의

세기를 측정한 결과 렌즈를 이용하였을 때 빛의 세기는 약 10 배

정도 크게 측정되었다. 미세입자를 구동한 결과 렌즈를 사용한

새로운 플랫폼에서는 구동 속도가 10 배 이상 빠르게 측정된 것도

앞서 서술한 몇 가지 요소들이 미세입자에 작용하는 유전영동력의

크기에 영향을 미쳤기 때문일 것이다. 뿐만 아니라 광전자집게의

유체 층의 높이가 유전영동력에 미치는 영향에 관한 실험도

수행하였다. 유체 층의 높이가 낮아질수록 더 작은 미세입자를 더

75

빠르게 구동할 수 있었다. 이는 유체 층의 높이가 낮아질수록 전기장

기울기가 커지고 따라서 유전영동력도 증가하기 때문이다.

본 연구에서는 위의 새로운 액정디스플레이 기반의

광전자집게 시스템에 미세입자의 인터랙티브한 제어를 위한 플래쉬

기반의 컨트롤 프로그램을 적용하여 미세입자 및 혈액세포의

인터랙티브한 구동에 성공하였다. 혈액은 쥐로부터 채취하여

원심분리 한 후 상층액을 제거하고 등장성 용액으로 10:1 로

희석시켜준 것을 최종적으로 사용하였다. 인터랙티브 컨트롤

프로그램을 이용하면 현미경과 카메라를 통해 컴퓨터 전송되는

실시간 영상을 보면서 원하는 세포나 미세입자를 키보드나 마우스를

이용하여 선택하고 움직일 수 있다.

본 연구에서 제안한 렌즈 및 컨트롤 프로그램이 통합된 액정

디스플레이 기반의 광전자집게는 기존의 광전자집게 플랫폼에 비해

뛰어난 성능은 물론, 단순하고 사용하기에 편리한 구조와

인터페이스를 제공함으로써 보다 실용적인 통합 시스템을 구축할 수

있도록 하였다. 이러한 새로운 액정 디스플레이 기반의 광전자집게

시스템은 세포나 미세입자를 구동하는 여러 가지 생물학적

76

응용분야에 이용될 수 있을 것이다. 예를 들어 세포와 세포간의

상호작용 및 결합력 등을 측정하거나, 단백질이 코팅된 미세입자를

이용하여 세포 표면의 특이적으로 결합하는 단백질의 농도를

모니터링하는 등의 단일 세포, 단일 미세입자 기반의 생물, 화학적

실험과 세포들을 일정한 패턴으로 고정화하여 단일 세포 어레이

등을 제작하는데 응용될 수 있으리라 기대된다. 뿐만 아니라, 영상

장치를 이용하여 가상의 전극을 형성시키는 광전자적 구동원리는

자동화된 미세입자 및 세포 분석 실험에 좋은 도구로서 이용될 수

있을 것이다.

77

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Acknowledgements

이 논문이 완성되기까지 직·간접적으로 도움을 주신 모든

분들께 감사드립니다. 가장 먼저, 지난 1 년 반 동안 부족한 저를

맞아주시고 지도해주신 박제균 교수님께 감사드립니다. 교수님의

가르침을 밑거름 삼아 실망시켜드리지 않도록 열심히

노력하겠습니다. 또한 바쁘신 와중에도 논문 심사에 기꺼이

응해주시고 조언을 아끼지 않으신 남윤기 교수님과 정기훈

교수님께도 감사드립니다. 본 연구가 잘 수행될 수 있도록 도움을

주신 경희대학교 장진 교수님과 김세환 연구원님께도 감사드립니다.

제가 이 자리에 있기까지 많은 가르침을 주신 존경하는

바이오및뇌공학과 교수님들과 동아중학교, 부산과학고등학교

은사님들께도 늦게나마 감사의 말씀을 드립니다. 제가 학위과정을

무사히 마칠 수 있도록 지원해주시고, 학문적으로, 정신적으로

성숙할 수 있도록 도움을 주신 나노바이오공학 실험실 식구들께

감사드립니다. 힘들 때면 항상 믿고 의지 할 수 있었던 부산과학고

동기 친구들과 존경하는 선배님들, 사랑하는 뮤즈 식구들, 함께

도전하고 노력해온 바이오시스템학과 동기, 선후배님들에게도

감사드립니다. 마지막으로 항상 저를 걱정해주시고 응원해주시며,

아낌없는 사랑을 베풀어주시는 사랑하는 가족들, 할머니, 어머니와

누나, 그리고 멀리서나마 항상 저를 지켜봐 주시고 힘을 북돋아

주시는 아버지께 이 논문을 바칩니다.

87

Curriculum Vitae

Name in Full : Hyundoo Hwang

Date of Birth : November 16, 1985

Place of Birth : Busan, Korea

Education

March 2001 ~ February 2003 Pusan Science High School

(Currently, Busan Science Academy)

Busan, Korea

March 2003 ~ February 2006 B.S. Department of BioSystems

(Currently, Bio and Brain Engineering)

Daejeon, Korea

March 2006 ~ August 2007 M.S. Department of Bio and Brain

Engineering

KAIST

Daejeon, Korea

88

Publications

1. W. Choi, H. Hwang, H. Kim, W. Lee, S. Kim, J. Jang, J.-K. Park (2006),

Automatic optical manipulation of microparticles using a lab-on-a-display

system, 한국바이오칩학회 2006 춘계학술학회, 고려대학교, 서울 (2006.

6. 23).

2. H. Hwang, Y. Oh, J.-J. Kim, Y.-J. Choi, W. Choi, J.-K. Park, S.-H. Kim, J.

Jang (2007), Z-axis focusing and manipulation of microparticles using 3-

dimensional optoelectronic tweezers, 제 9 회 한국 MEMS 학술대회, 제주 (2007. 04. 5-7), pp. 122-125.

3. H. Hwang, Y.-J. Choi, Y. Oh, J.-J. Kim, W. Choi, J.-K. Park, S.-H. Kim, J.

Jang (2007), Real-time interactive manipulation of microparticles using

optoelectronic tweezers, 한국생물공학회 춘계학술발표회, 인천 (2007. 04.

26-28).

4. W. Choi, H. Hwang, J.-K. Park (2007), Study on frequency-dependent

phenomena in microscopic particle manipulation using lab-on-a-display

platform, International Scanning Probe Microscopy Conference 2007, Jeju

Island, Korea (June 10-14, 2007).

89

5. H. Hwang, Y.-J. Choi, Y. Oh, J.-J. Kim, W. Choi, J.-K. Park (2007), A real-

time interactive control system for optical manipulation of microparticles using

liquid crystal display, IEEE International Conference on Information

Acquisition (ICIA 2007), Jeju Island, Korea (July 9-11, 2007).

6. H. Hwang, Y. Oh, J.-J. Kim, Y.-J. Choi, W. Choi, J.-K. Park, S.-H. Kim, J.

Jang (2007), Adsorption-free microparticle manipulation using 3D

optoelectronic tweezers composed of double photoconductive layers, Micro

Total Analysis Systems 2007 Conference, Paris, France (Oct. 7-11, 2007).

7. H. Hwang, J.-J. Kim, Y. Oh, Y.-J. Choi, J.-K. Park (2007), Electrostatic

particle-particle interactions in optoelectronic tweezers, Micro Total Analysis

Systems 2007 Conference, Paris, France (Oct. 7-11, 2007).

8. 박제균, 황현두, 최원재, 3차원 광전자집게 시스템 및 이를 이용한

미세유체 구동 방법, 대한민국특허 출원중. 10-2007-0017774 (2007. 2.

22)

9. 박제균, 황현두, 최원재, 이명권, 평판 디스플레이 및 렌즈를 이용한 미세입자 구동기 및 방법, 대한민국특허 출원중. 10-2007-0048690

(2007. 5. 18)

10. 박제균, 이명권, 최원재, 황현두, 발광 소자를 이용한 미세유체 및

미세입자 구동기 및 방법, 대한민국특허 출원중. 10-2007-0051974

(2007. 5. 29)

90

11. H. Hwang, Y.-J. Choi, W. Choi, S.-H. Kim, J. Jang, J.-K. Park (2007),

Interactive manipulation of blood cells using a lens-integrated LCD-based

optoelectronic tweezers, Electrophoresis, Submitted.