Trilogy Part 1 -
by Shuvo Roy & Aaron Fleischman
The Cleveland Clinic Foundation
The recent progress in microfabrication and micromachining technologies is transforming the field of solid-state transducers into what has become known as microelectromechanical systems (or MEMS, for short).
In general, MEMS refers to the integration of sensors, actuators and electronics using techniques originating in the semiconductor (silicon) microelectronics industry to realize miniature, high-performance, low-cost, electromechanical systems with minimum feature sizes on the order of microns. Miniaturization of mechanical systems is particularly attractive since micromechanical devices and systems are inherently smaller, lighter, faster and usually more precise than their macroscopic counterparts. Furthermore, because of batch processing, another major benefit of MEMS technology is its ability to drive down component cost.
The extension of MEMS technology to biomedical applications offers the potential to realize small and compact devices with sophisticated functionality. Consequently, there is a lot of research activity in universities, research institutions, government laboratories and corporations. Biomedical application areas include diagnostic tools, surgical instrumentation, artificial organs and drug delivery devices.
MEMS for Biomedical
Two ubiquitous biomedical applications of MEMS include the sensing of pressure and acceleration. Typically, the sensing element consists of a micromachined beam or diaphragm, which deflects in proportion to the measurand. The extent of the microstructure deformation is then converted to an electronic signal that is then sent to microprocessor circuitry for further processing or display.
The foremost medical application for MEMS pressure sensors is the measurement of blood pressure. In one implementation of an external blood pressure monitoring system, the pressure sensor is an integral part of an instrument that consists of a saline solution bag and tubing, to which the sensor is attached. The components are then connected to the patient to provide signals to a monitor. Fluid passes through the tubing into the patient and when the heart beats, a pressure wave moves up the fluid path and is detected by the sensor. In other implementations, especially when blood pressure must be measured internally, the sensing element is coated with an inert, compliant gel that transmits the pressure signal, but avoids direct contact between the MEMS device and body fluids and tissues.
Other biomedical systems that use pressure sensors include monitors for respiratory ventilation, kidney dialysis machines, sleep disorder monitors for apnea detection and ophthalmologic equipment to measure vacuum and barometric pressure during eye surgery. Pressure sensors also can be used to regulate the hospital beds of burn victims. In this case, sensors monitor the inflation levels of a series of chambers within the bed. Subsequently, macroscopic actuators are used to adjust pressure in the various sections, thereby alleviating pain and augmenting the healing process of the patient.
Major processing steps in the microfabrication of
Accelerometers are used to measure frequency and amplitude of vibrations, or under steady-state conditions, to determine tilt angle or the amplitude of a single shock or pulse. Acceleration measurement is performed in sleep and motion studies as indicators of patient activity. The sensitive microsensors can be mounted along multiple axes to detect small changes in patient position. Recent designs of implantable pacemakers also incorporate accelerometers to monitor the activity level. For pacemaker wearers, the heart must beat faster during periods of increased exertion. Consequently, pacemaker systems use accelerometers in conjunction with microprocessors to determine patient exertion level and suitably adjust the output signal rate, respectively.
Limitless Potential will enhance Performance, Portability
The potential of MEMS technology for biomedical applications extends well beyond the realm of pressure sensors and accelerometers. A variety of MEMS devices have been realized by combining one or more of the micromachining techniques with microfabrication processes. Biomedical systems that incorporate MEMS have already been demonstrated including precision drug delivery devices using integrated microvalves and micropumps, fast-response PCR reactors for DNA amplification using low thermal-mass microchambers, and portable biochemical analysis instrumentation using microfluidic networks.
Although most of these MEMS components are still at the research and development stage, their eventual integration into larger biomedical systems will revolutionize performance and enhance portability within the medical environment.
Fabrication by Micromachining
The fabrication of MEMS devices is based on a merger of microelectronics fabrication (microfabrication) processes and micromachining techniques to create the desired microstructural components. The ever-increasing demand for microelectronics systems with increasing computational ability, sophisticated embedded control, and high-speed networking has advanced microfabrication through the development of improved and/or new processes to realize sub-micron size features. Therefore, the development of MEMS devices has usually resulted from combining the standard microfabrication processes with innovations in micromachining techniques. This combination of microfabrication and micromachining has provided a powerful set of tools for batch processing and miniaturization of systems into a dimensional domain not accessible by conventional machining techniques.
Three Major Categories
There are three primary categories of micromachining: bulk micromachining, surface micromachining and micromolding. MEMS devices are typically fabricated using one or a combination of these micromachining techniques in conjunction with standard microfabrication processes.
Bulk micromachining designates the technique in which the bulk of the silicon substrate is etched away to leave behind desired micromechanical elements. Wet and dry etching processes are used in conjunction with suitable etch masks and etch stops to sculpt a variety of microstructural components from silicon and/or glass substrates such as beams, diaphragms and nozzles. The patterned substrate could form the MEMS device by itself. Alternatively, two or more patterned substrates could be bonded together to construct complex three-dimensional microstructural components.
In surface micromachining, the substrate is used primarily as a mechanical support on which multiple, alternating layers of structural and sacrificial material are deposited and patterned to realize micromechanical structures. Surface micromachining relies on encasing specific structural parts of a device in layers of a sacrificial material during the fabrication process. The sacrificial material is then dissolved in a chemical etchant that does not attack the structural parts to realize freestanding components. Surface micromachining is highly compatible with microelectronics manufacturing processes and equipment, and consequently, it enables the fabrication of complex, multi-component, integrated micromechanical structures that would be impossible with traditional bulk micromachining.
Micromolding refers to fabrication of microstructures using molds to define the deposition of the structural layer. After deposition of the structural layer, the mold is dissolved in a chemical etchant to realize the final microfabricated components. The mold can be realized using a variety of processes including lithography and etching. One interesting approach to mold formation uses high-intensity, low-divergence, hard x-rays as the exposure source for the lithography. These x-rays are usually produced by a synchrotron radiation source to pattern polymethylmethacrylate (PMMA) layers into features with extremely high depth-to-width aspect ratios that are not possible using conventional bulk or surface micromachining techniques.
After creation of the mold, the openings in the mold pattern are preferentially plated with metal (usually nickel or copper) to yield a highly accurate complementary replica. Finally, the mold is dissolved to leave behind plated microstructures. It is also possible to use the plated metal structures as mold for plastic injection molding or hot embossing. After curing, the metallic mold is removed leaving behind micro-replicas of the original pattern.
For more information, contact Shuvo Roy, Dept. of Biomedical Engineering, The Cleveland Clinic Foundation, Cleveland, OH 44195. 216-445-3243. http://www.lerner.ccf.org/bme/biomems Circle 707.
Sailing into the Future
Michael Sailor, award-winning chemist and molecule explorer from University of California, San Diego, is conducting experiments that may result in microscopic robots that provide feedback about the environment or an entire chemistry lab on a chip. Sailor is collaborating on MEMS projects with Sandia National Labs, who is expending a lot of effort on the "lab on a chip" idea. "The vision is to produce something the size of a cell phone or smaller that can replace an analytical lab the size of a room," says Sailor. In a medical setting, this could mean the creation of a device that could take a saliva, blood or urine sample, process it and perform an analysis for pathogens or biochemical markers for certain diseases. This device could sit in a doctor's office and provide analysis while the patient waits.
Professor Philippe Fauchet at the University of Rochester is trying to take microelectronics-based medical diagnostics even further. Together with an MD from U of R's med school, he has founded the Center for Future Health, whose goal is to place miniature electrical devices such as those mentioned above into the home. For example, a personal medical computer could take pictures of you every morning in the shower, use pattern recognition to see if any of your freckles are changing appearance and alert your doctor if it looked like you were developing skin cancer. The possibilities of various sensors and feedback devices are endless.
Another of Sailor's contemporaries, Kris Pister of UC Berkeley, is working on building very small MEMS, also called "smart dust." "One vision of smart dust is a collection of dust-size computers with little propulsion units that wander around a room, building or field and reporting back to a central office on what they see, hear or smell," says Sailor. These devices could potentially be injected into humans, too. For example, smart silicon implants could detect when a diabetic patient has high glucose and could then activate an "onboard" mini-injector or biomedical synthesis plant to provide insulin.
For more information on Dr. Sailor's research and publications, please visit http://firstname.lastname@example.org/sailor/