Magnetic Microactuators for Liver Collagen Removal

MAE 268/MATS 254: MEMS Materials, Fabrication and ApplicationsProfessor Bandaru, Professor Jin, Professor Talke

 Shalini Ananda, Dor Y. Ashur

Introduction

Liver fibrosis:

Excessive accumulationof collagen that occurs in most types of chronic liver diseases.

IntroductionCauses:

- alcoholism- hepatitis B and C - fatty liver disease

Risks:

- Cirrhosis - liver failure- portal hypertension

Current Cures: - liver transplantation, - Spread prevention through immune suppressants which endanger the body to infection and

disease during recovery.

Size = 5 microns

Actuation TypesElectrostatic Actuation

•Electrostatic force clamps the flexure devices to an insulated ground plane

•An off-chip magnetic field actuates unclamped devices.

Magnetic Actuation

•Coils around each device produce a magnetic force

•A ferromagnetic core could also be integrated into the flexure, though fabrication is complicated

Electro/magnetic Actuators

A combination of both magnetic and electrostatic forces in would exploit the advantages of each actuation force

Electrostatic forces:

• Powerful over short distances and large areas

• Consumes minimal power

Magnetic forces

• Powerful over longer distances

• Can achieve large displacements both in and out of the plane of the wafer.

Design

• This design will employ a torsional flexure remove collagen scar tissue, to gain better leverage towards removing tissue.

Fabrication Process

1 micron

1. Clean <100> mm wafer with HF

2. Pattern nitride with photoresist, nitride layer etched with advanced oxide etcher, plasma etch protective photoresist.

3. Second pattern photoresist, to expose areas for metal adhesion. Deposit metal with an e beam evaporator, and lift off to remove photoresist and metal.

4. Titanium is evaporated over the entire wafer to connect nickel and chrome seed layers. A thicker layer of photoresist is plated to define a mold for a nickel magnet. An HF dip removed the titanium at the base of the photoresist to expose nickel layer for electroplating.

5. Remove photoresist with, methanol, isopropanol and then water. Titanium layer is removed by dipping in HF solution.

6. Plate wafer in nickel solution and pass a current of 10mA/cm2 to seed layer for 25 minutes.

Magnetic Microactuators for MEMS Enabled Ventricular Cathers for Hydrocelpahus, Lee et al, IEE EMBS Conference on Neural Engineering. 2007

Fabrication

1. To release the microactuators, a 1um thick silicon dioxide on the backside, and 6 microns of polyimide on the front side, serve as an etch stop for fast DRIE.

2. The backside is etched by downstream plasma etching.

3. The polyimide is removed by etching thrice consecutively in downstream plasma etching. The 100mm diameter substrate is then diced into 16 dies, each with a single array of 16 devices

4 Parylene C polymer is added to improve biocompatibility.

Testing Procedure 1. Design a test apparatus to measure tensile strength of collagen.

2. Photograph tested collagen to study the effect of the cantilever on the cells

3. Expose new collagen to the micro actuators and measure the quantity and quality of interaction

4. Photograph tested collagen samples to assess homogeneity and efficacy

- Homogeneity of collagen removal may have an effect on the potency of the treatment

4. Test the designed actuators in a cell simulated environment

Testing the tensile strength of scar (collagen) tissue

Pitch of screw = 0.5mmMotor driven by 200steps /rev400 steps/mm of displacement

Force = weight of saddle x sine of angle of tilt.

Sensitivity = 0.1micrometer

To tilt

Biddulph et al , Meds Sci. Techno 5 (1994) 9-11

Laser interferometer

Scar tissue can be simulated with cell culture techniques

Cells will be grown over the actuator surface for different time spans to achieve varying levels of scar formation.

www.medscape.com/liver_fibrosis

Imaging actuation in a cellular environment

High resolution stereoscopic digital camera

Actuation at several frequencies around 50 Hz

Images will be taken before and after actuation

Images will be taken at time points: t= 0, 5, 10, 15, 20, 25 minutes.

Future directions- Further miniaturization is needed for

actual human useIf the devices are scaled by a factor of s-1, then the magnetic torque and angular stiffness will scale by S-3. The scaled device will achieve the same angular deflection.

- Further design of combined magnetic and electrostatic actuators could improve efficacy.