Micro-scale actuators and motors provide a vast range of potential applications
in broad areas. One promising area is biomedical applications, including
active "smart" implants and surgical tools. In such devices,
actuators need to fit in extremely limited physical spaces typically available
in them, yet has to offer high operation reliability as well as high manufacturability
for their production. Furthermore, wireless operation is essential for
implant applications. We are investigating novel micro actuators that fulfill
these requirements, aiming to drive technology innovations in biomedical
areas and beyond.
RF Actuation of Hydrogels
Thermoresponsive hydrogels such as poly(N-isopropylacrylamide), or PNIPAM,
exhibit the phase transition temperature above which they shrink and deswell
the fluid. Toward controlled wireless actuation of thermoresponsive
materials such as PNIPAM, we have developed a frequency-selectable wireless
resonant heater powered using external RF electromagnetic fields. The PNIPAM
structures micropatterned on the planar heater were activated to eject
the liquid stored in them, simply by tuning the field frequency to the
resonant frequency of the heater, demonstrating wireless RF control of
hydrogel actuation with high frequency selectivity. This novel technique
was applied to the wireless control of microvalves for an implantable drug
delivery chip (refer to Medical MEMS).
Synchronous Wireless Control of Micro Actuator Array
The ability to control multiple micro actuators selectively in a wireless
manner will be a key to achieving multi-functionality in smart implants.
For wireless drug delivery implants, for example, this ability is essential
to perform synchronous operation of micro pumps and valves embedded within
them. Moreover, it potentially enables selective delivery of different
types of drugs at specific mixtures. In light of these, we have developed
an RF technique to selectively control micro thermal actuators. Each of
the actuators is designed to respond to RF excitation at a unique frequency,
and FM waves are used to synchronously control them. This technique was
applied to a microsyringe device that can eject controlled amounts of fluid/gel
from its deformable reservoir through selective activation of multiple
shape-memory alloy (SMA) micro actuators.
Nitinol Micro-Coil Actuation via Direct RF Power Transfer
The RF-to-thermal energy conversion should be more efficient if the power
transfer is made directly to a thermoresponsive material. Following this
hypothesis, we designed and fabricated inductive spiral coils made of Nitinol,
a biocompatible SMA, that could be used as RF receivers. In partiular, resonant circuits were
formed by integrating planar capacitors on the coils, so that the circuit
functioned as not only wireless resonant heaters but also actuator structures
that were driven by heat produced directly in the Nitinol. Fabricated devices
showed more power efficient and faster actuations. One type of them (shown
below) was applied to an implantable drug delivery chip for wireless control
of the microfluidic pump embedded within the chip (refer to Medical MEMS).
Micro-Scale Ferrofluid Manipulation and Application
The magnetic fluid known as Ferrofluid belongs to a category of smart fluids
that can be manipulated using magnetic fields. In the presence of a magnetic
field gradient, this type of fluids flows toward the location with the
highest magnetic flux density. We are studying the methods to achieve controlled
manipulation of this fluid in the micro domain and exploring its potential
in MEMS. This study has led to, for example, a method that uniquely enables
bidirectional actuation of the fluid using micropatterned planar coil array
and its application to an active mirror array. Another application that
we have reported is a planar variable inductor, whose inductance is controlled
via actuation of integrated ferrofluid that is used as a movable magnetic
core for the inductor.
Ferrofluid Levitated Micro Actuators
One challenge in realizing practical micro actuators/motors is the development
of a reliable bearing. Although various antifriction methods (active fluidic
bearing, passive fluidic/solid bearing, microball bearing, etc.) have been
proposed toward this goal, they pose practical issues related to integration
complexity, robustness and reliability, and high packaging cost. We have
developed a novel micro bearing based on ferrofluid to address the problem.
When a ferrofluid is applied to the rotor/slider magnet of rotary/linear
micro actuator, the fluid accumulates around and levitates the magnet,
allowing for its friction-less motions on the substrate. Here, the ferrofluid
layer serves as a self-sustained liquid bearing as well as a lubricant
that follows the magnet as it moves, eliminating the need for any precision
alignment/assembly to maintain the bearing in the actuator construction.
The device shown below is a micro linear actuator developed through this
Diamagnetic MEMS: Micro Graphite Rotor
We are also investigating another type of micro levitation method based
on diamagnetism for MEMS applications. This study has shown the first micromachined rotor of highly oriented pyrolytic
graphite (HOPG), a strong diamagnetic material. The HOPG rotor is levitated
above a permanent magnet structure to enable its friction-free rotation.
The rotor was driven by gas flow to demonstrate stable continuous revolutions
with the rates up to 500 rpm. The results suggest a promising application potential of the novel mechanism in friction-less micro sensors and actuators.
Collaborator: Prof. Yufeng Su, Zhengzhou University, China
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