Magnetic Microactuators for Liver
Collagen Removal
MAE 268/MATS 254: MEMS Materials, Fabrication and Applications
Professor Bandaru, Professor Jin, Professor Talke
Shalini Ananda, Dor Y. Ashur
Introduction
Liver fibrosis:
Excessive accumulation
of collagen that occurs in
most types of chronic
liver diseases.
Introduction
Causes:
- alcoholism
- hepatitis B and C
- fatty liver disease
Risks:
- Cirrhosis
- liver failure
- portal hypertension
Size = 5 microns
Current Cures:
- liver transplantation,
- Spread prevention through
immune suppressants which
endanger the body to infection and
disease during recovery.
Actuation Types
Electrostatic 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
Magnetic Microactuators for MEMS Enabled
Ventricular Cathers for Hydrocelpahus, Lee et al, IEE
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.
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
To tilt
Testing the tensile strength of scar
(collagen) tissue
Laser
interferometer
Pitch of screw = 0.5mm
Motor driven by 200steps /rev
400 steps/mm of displacement
Biddulph et al , Meds Sci. Techno 5 (1994) 9-11
Force = weight of saddle x sine of angle of tilt.
Sensitivity = 0.1micrometer
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
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.
High resolution
stereoscopic digital
camera
Future directions
- Further miniaturization is needed for actual
human use
If 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.