Like most modern electronic devices, small, smart, wearable (or implantable) medical devices are becoming much more common in the medical profession. As technology evolves, so have the methods design engineers use to develop medical devices that eliminate end-user guesswork and improve treatment methods.
Seeking new ways to advance a medical device is an arduous part of the R&D process. It often involves making investments in new test and measurement technologies—like sensors—to uncover opportunities for improvement. Nevertheless, design engineers may inadvertently think of sensing technology in two separate categories. One category is the aforementioned use of sensors as a testing or research tool, while the other category uses sensing technology as an embedded component in a device design.
Considering the important role that force- and pressure-feedback plays throughout the medical profession, tactile-sensing technology represents a sensor category that can lay the framework to enhance an R&D or clinical research process while being embedded as a component to differentiate a device.
Tactile Sensing: Solutions for Discovery and Integration
There are several ways to apply tactile-sensing technology in R&D or medical research labs. From an R&D-design perspective, tactile-sensing technology provides engineers insights on how a device’s components fit or connect, addresses potential durability concerns (e.g., drop testing), and provides a better understanding of how a physician or patient may physically handle the device. In the medical field, clinicians and researchers use tactile-sensing technology to monitor a patient’s treatment process (e.g., injury recovery), quantify force application by the physician or the patient, and as an actionable training tool for different types of medical or surgical procedures.
The uses for tactile sensors embedded as components within a device design are equally as diverse. Depending on the needs of the application, embedded tactile sensors can detect and measure relative change in force or applied load, identify force or pressure thresholds to trigger an appropriate action by the device, monitor rate of change in force, and other uses.
Naturally, embedding any sensor technology into a device design can prompt mechanical and electrical challenges within a design. Given the size and power limitations of most medical devices, it’s important to select the right sensor that will deliver desired results without adding complexity.
What are Your Tactile Sensor Options?
The three most common tactile-sensing technologies are load cells, strain gauges, and force-sensitive resistors (also known as piezoresistive sensors).
Load cells are the most well-known type of force-sensing technology. They offer the highest level of accuracy, which can be important for applications that require exact force measurements. As an embedded component, load cells often require a significant amount of space in a device design due to their large size and weight, making them difficult to embed into small, compact spaces.
Strain gauges are a common tactile-sensing technology that are much smaller in size than load cells. Unlike load cells, though, strain gauges measure force as a result of an indirect measurement correlated to the strain of fine conductive wires. Strain gauges also need complex electronics to function, which can be difficult and expensive to embed on a large scale.
Force-sensitive resistors consist of semi-conductive material contained between two pieces of thin, flexible polyester. As their name suggests, force-sensitive resistors are passive elements that act as a resistor in an electrical circuit. When unloaded, the sensor has a high resistance (about <2MΩ) that drops when a load is applied. Considering the inverse of resistance (conductance), the conductance response of touch sensors is linear as a function of force within the sensor’s designated force range.
Force-sensitive resistors can come in two different forms: single-point sensors or multiple-point matrix sensors. Single-point sensors are specifically used for force detection on a specific spot, whereas matrix sensors are a collection of single-point sensors that independently scan pressure distribution across a larger sensing region.
Above: As shown in these two diagrams, both single-point and matrix force-sensitive resistors are constructed with the same materials. On the matrix sensor, however, each intersection on the conductive layers represents a single, independent sensing point.
Above: The sensor on the left is an example of a force-sensitive resistor matrix, while the image on the right represents a single-point force-sensitive resistor. Both sensors have flexible form factors, and can be customized in different shapes and in different force or pressure ranges.
Given the thin, flexible characteristics of this technology—along with their ability to function on simple electronics—force-sensitive resistors are a prime example of a sensor that has no boundaries, whether used as an R&D/research tool or as an embedded component.
Transitioning Clinical/R&D Force-Sensitive Resistor Technology into an Integrated Device
It takes engineering ingenuity to merge an R&D tool into an embedded technology, but when done successfully, design engineers can essentially create new categories of medical devices and systems that set higher standards for the industry. Here are a few examples of how force- and pressure-testing efforts mapped the path to an improved or first-of-its-kind device.
Example 1: Foot Mapping Science that Wins Over Customers
Pressure-sensitive platforms employing a force-sensitive resistor matrix have long been used by podiatrists and clinical researchers in a variety of processes. These platforms, which are typically connected to real-time analysis software, identify asymmetries between a patient’s left and right foot, monitor effectiveness of a treatment program (as in the case of a limb replacement or an injury recovery process), and other uses. Moreover, these platform systems are useful tools to educate patients about pathologies with a clear visual representation of biofeedback.
Given the success of this pressure-sensitive technology in a clinical setting, combined with the fact that the orthotic market is crowded with options for consumers, Schering-Plough (now Bayer) realized an opportunity to integrate a platform system into a unique sales tool. The end result—the Dr. Scholl’s Custom Fit Orthotics Kiosk—turned a successful research tool into an educational experience that connects a customer with a product that addresses the needs of their exact footprint. The quantifiable information captured by these kiosks goes a long way in building consumer trust in the Dr. Scholl’s brand.
Above: The design for the Dr. Scholl’s Custom Fit Orthotic Kiosk originated from a pressure-sensitive platform used by podiatrists to treat injuries and evaluate patient progress, among other uses.
Example 2: CPR Training Device Becomes an In-Field Aid
Even for some of the most seasoned emergency medical personnel, there is always some unease associated with applying CPR to an individual struggling to breathe. Not only is proper application cadence important, but when adrenaline is pumping and a life is on the line, there’s the risk of putting too much force on a subject and potentially causing a serious injury. Pressure-sensitive dummies, or training pads, are common training tools used to help EMTs and medical professionals hone their CPR administration technique. In the field, however, there may be little time to think about whether too much or too little force is being applied.
One medical device company saw an opportunity to develop a portable assistive CPR system to quantify applied force. Its idea was to create a pressure-sensitive pad system—embedded with a single-point force-sensitive resistor—that would connect to a handheld or wearable monitor to capture and display force applied to a patient.
First, the R&D team used a force-sensitive resistor matrix to determine where the embedded sensor would ultimately be placed on the device. From there, a custom force-sensitive resistor was designed in a shape that would effectively capture force across the administrator’s hands. Using a simple analog-to-digital converter, the sensor’s force readings were transmitted to a portable viewing device that would instruct the administrator to apply more or less force.
Example 3: Helping to Standardize Knee Replacement Procedures
As most orthopedic surgeons may tell you, every knee replacement procedure is unique. Achieving proper balance in an orthopedic joint is a guess-and-check technique that is learned over time. In some research or training labs, tactile-sensing technology has been used as an instructive tool for surgeons learning their joint balancing technique on cadavers. Force feedback captured through a force-sensitive resistor matrix and relayed to a digital monitor can identify unevenness across the joint, helping the surgeon-in-training get a better understanding of how the knee replacement will maintain stability over time.
In an effort to reduce pain and other complications after a procedure, a medical device company began developing a minimally invasive device to confirm proper knee balance while in surgery. A hand-held device embedded with a customized force-sensitive resistor inserted in between the joint surfaces presents a digital output, quickly confirming evenness of the joints. This tool provides actionable information that takes the guesswork out of the procedure and helps patients onto a faster path to recovery.
Above: This diagram provides a concept for a force-sensitive device used by surgeons to confirm proper knee balance while in surgery.
Example 4: On-the-Fly Inspection of Medical Product Packaging
Packaging for medications or medical supplies must follow strict quality control standards. In the past, pill blister packs were evaluated for seal leaks through a dated and time-consuming ink test. In this process, a testing ink would be applied to the outside of the blister packs after a production run, which would then seep into any crevices in the blister cavity to identify faulty product.
One pill pack machine designer used a force-sensitive matrix sensor while testing the alignment of a new machine design. It was at this point that the R&D team determined this same technology could be a useful application within the machine interface to help operators streamline machine setup processes.
As a result, a force-sensitive resistor matrix was designed and embedded into the lid of the machine to capture pressure distribution while the machine creates a vacuum across each cavity of the blister pack. Cavities with a low pressure were clearly shown on the machine’s display, using a software API. This unique application help the machine designer set their design apart from the competition, while saving their users time, money, and medication loss.
The Path to Innovation May Be Clearer Than You Think
Force-sensitive resistor technology offers design engineers a wealth of opportunities to separate their devices from the competition thanks to their thin size and simple electronic requirements. However, tactile sensors represent just one sensor category that has diverse uses in and out of the R&D or clinical research labs.
Chances are your next great product innovation to enhance the structure, usability, and functionality of your medical device may be the same tools you use to evaluate your device design.
To learn more, visit Tekscan at Booth #324 at the upcoming Atlantic Design & Manufacturing 2018 show June 12-14, co-located with MD&M East in New York City.
[All images courtesy of TEKSCAN INC.]