Microelectromechanical Systems (MEMS), as the name implies, are miniature devices composed of mechanical (springs, deformable membranes, vibrating structures) and electrical (resistors, capacitors, inductors) components that, together, are able to sense and report on physical properties of the environment in which they are deployed, or, when signaled to do so, are able to perform some kind of controlled actuation.
In the medical applications market, MEMS components are used in diagnostic, monitoring, surgical, and therapeutic devices. These devices employ MEMS as pressure sensors, temperature sensors, flow sensors, accelerometers, optical image sensors, and silicon microphones, among other uses.
The techniques used to fabricate MEMS devices are similar to the techniques used to make silicon-based integrated circuits (i.e., microprocessors or memory chips), which means that MEMS manufacturing can enjoy the kind of scaling advantages and manufacturing efficiencies that have long been a hallmark of integrated circuit (IC) fabrication.
Additionally, MEMS devices have several advantages for medical applications that make their use attractive: the 2 to 100μm-sized features typically found in MEMS devices are compatible in size with living cells and fine physiological structures in the human body. MEMS devices are manufactured with materials such as silicon, silicon dioxide (glass), precious metals like titanium, gold, and platinum, and organic materials like Parylene, that are biocompatible.
With the close family resemblance to integrated circuit manufacturing, MEMS devices are easily integrated with electronics components to include signal processing and analysis capabilities and wireless telemetry functions for ingestible or implanted MEMS devices. This enables the powerful combination of integrated performance sensing with wireless data transfer, all in a compact form factor.
Table 1 presents the world top five medical device makers by revenue and by selected MEMS-based activities associated with them.
Why the Need for MEMS-Enabled Medical Devices?
According to GE Sensing, Inc., the reason is that “Today’s patient care demands the ultimate in accurate and reliable monitoring of critical parameters.” MEMS sensor-enabled devices are one proven way to deliver that result. (www.ge-mcs.com/download/temperature/Sensors_for_healthcare.pdf )
MEMS pressure sensors have a long history in medical applications, and have been used in non-invasive medical equipment, such as respiratory gear and blood pressure cuffs, since the 1980s. More recently, MEMS pressure sensors have been used for invasive applications as well, including as catheter tip sensors, or as an implant reporting on blood pressure (systolic and diastolic), heart rate and cardiac output, around the clock. Figure 1 shows an optical photograph of a MEMS pressure sensor.
Again, per GE, “Disposable pressure sensors are used primarily during surgical procedures and in the ICU to monitor patient heart rate and blood pressure. Pressure sensors in ventilator machines watch patients with respiratory illness or people on life support, helping control the amount of air pressure delivered to a patient’s lungs. They also monitor people with sleep disorders. During neonatal care, pressure sensors help physicians check the pressure within an expectant mother’s uterus.”
Transparency Market Research estimates that “The global MEMS medical applications market was valued at USD 1.8 billion in 2012 and is expected to grow at a CAGR of 20.2% from 2013 to 2019, to reach an estimated value of USD 6.5 billion in 2019.”
MEMS pressure sensors have the largest single piece of the medical applications market, at $486 million in 2012, and are expected to grow at a compound annual growth rate of 18.7% from 2013 to 2019 as they find their way into even more medical devices.
Customizing MEMS Pressure Sensors
Given the expected growth of MEMS enabled medical applications, the medical device engineer has three general options for sources of MEMS pressure sensors. One is to use a commercially available, off-the-shelf component. A second option is to undertake a development program to create a custom MEMS pressure sensor. The third is a semi-custom development option. Let’s take each of these approaches in turn.
Using a commercially available MEMS pressure sensor is the easiest solution, if the correct pressure sensor is available. Off-the-shelf pressure sensors are generally designs that have been tuned to either automotive applications, such as control of internal combustion engines, or consumer applications, such as altimeters and barometers. As a result, it is possible that a commercially available MEMS pressure sensor may fit a medical application, but the fit is largely coincidental.
MEMS pressure sensor manufacturers have addressed medical applications only for the largest markets, such as blood pressure monitoring. In contrast to the automotive or consumer electronics markets, the market for medical devices that could use MEMS pressure sensors is fragmented. For a particular medical device, designed to treat a specific condition, volumes may be on the order of 10,000 to 100,000 units per year. Because of the economics of semiconductor fabrication, volumes of this size are typically viewed by traditional MEMS manufacturers as being too small. The result is that there are fewer off-the-shelf MEMS pressure sensors optimized for medical applications.
If an off-the-shelf MEMS pressure sensor is not available, the medical device design engineer may consider developing a custom MEMS pressure sensor. The business case for this development must justify several things. First, the typical investment to develop a custom MEMS pressure sensor begins at $500,000 and can require a minimum of two years. Second, after the development is completed, semiconductor foundries that can manufacture the chips have minimum order quantities that begin in the range of several hundred thousand devices per year. Given these constraints, one can begin to understand why MEMS pressure sensor manufacturers have focused on large markets.
A third option of “semi-customized” MEMS is now emerging. This approach can customize MEMS devices to the required specification, while avoiding the large investments and the minimum die volumes required by fully custom products. As a result, the semi-custom approach has the potential to enable medical applications that were previously inaccessible.
As the name implies, the semi-custom approach offers some ability to make design changes, but with less flexibility than the fully custom approach. The primary constraint, and the primary opportunity, lies in the fact that the greatest driver of cost, time, and risk for a custom MEMS sensor is in the development of the manufacturing method. The advantage of a semi-custom approach is that it preserves a known manufacturing method, while enabling customization of many other aspects of the device.
In the case of MEMS pressure sensors, this level of customization can address a wide range of applications. The sorts of device parameters that may be customized include: pressure range, pressure sensitivity, device impedance, device equivalent circuit, sensor size, pad size, pad placement, and other parameters.
Because the manufacturing method remains the same across device designs, multiple device designs may be fabricated simultaneously on the same wafer. As a result, the minimum required production volumes may be shared across multiple applications. This enables applications that may require only a few thousand devices per year to be manufactured. Figure 2 shows an optical photograph of several MEMS pressure sensor designs fabricated simultaneously on a single silicon wafer.
When ordering semi-custom MEMS pressure sensors, clients provide the required specifications, and then receive completed MEMS pressure sensors several months later. The manufacturer will translate the customer’s specification to a chip design, and then fabricate it at a silicon foundry.
MEMS devices have wide application in the medical space, and innovation in MEMS-enabled medical applications is expected to grow the market quickly. Semi-custom sensors are an emerging option that gives designers the ability to get the MEMS chips they need without the expense and risk of a custom sensor development.
This article was written by Paul Werbeneth, Consultant, and Charles Chung, Associate, A.M. Fitzgerald & Associates, LLC, Burlingame, CA. For more information, Click Here .