Thin Film Force Sensors: Changing the Medical Device Design Game

Data drives results. Today, medical devices give feedback and insight like never before. Advances in engineering medical devices has led to smarter devices, improved consistency amongst practitioners, and faster recovery times for patients. Force feedback is an increasingly valuable feature in the medical device market, providing doctors and patients with quantifiable data. This data allows for a more systematic approach to treating patients efficiently and effectively.

Fig. 1 – Infusion pump utilizing a custom force sensor to detect potential blockages.

One of the most important elements of a medical device is the feedback it provides the person using the tool, whether it is a primary care physician, surgeon, or patient. The device design must support a flow of communication between the patient’s body, the tool used, and the doctor reading and analyzing the output. There are a few ways that force can be measured, but depending on the context of the application, some force sensing technologies prove more ideal than others do. Load cells, strain gauges, and piezoresistive elements are popular devices used to measure force.

Load cells can use a variety of technologies to sense loads, but are bulky in size; making them difficult to design into an application where lightweight and small size are priority. Strain gauges are smaller than load cells, but require highly skilled technicians to install and yield measurements that are a result of indirect force drawn by correlating the strain of an assembly with a load.

Microelectromechanical systems (MEMS) sensors are also smaller than load cells and measure force indirectly. These sensors typically require a plunger-type of load device embedded into the MEMS package. Additionally, MEMS require a large upfront investment and price per piece is only cost effective with very high volumes.

In recent years, a different approach to force sensing technology has become increasingly popular and commercially available. The generic term for this device is the tactile force sensor. Thin film tactile force sensors consist of a special, proprietary, piezoresistive material sandwiched between two pieces of flexible polyester. The sensors are resistors that vary linearly in terms of conductance vs. force under an applied load, and can come in off-the-shelf standard shapes for test and measurement, as well as proof of concept. In addition, they can be customizable for specific original equipment manufacturer (OEM) applications.

Tactile force sensors are easier to integrate into medical products and systems as compared to the other force sensing options due to their thin, flexible nature. This type of component is ideal for a design engineer looking to design a lightweight, unobtrusive medical device that provides feedback to its user.

Force feedback is important in various medical settings, such as in a hospital, operating room, as well as the patient’s home or during hospice. Tactile force sensors are used in a variety of medical applications including: infusion pumps, robotic surgery, prosthetics, and shoe insoles. Below are a few applications highlighting how force sensor integration enhances the design of the medical device.

Drug Delivery: Infusion Pumps

Custom force sensors, designed into wearable, drug-delivery infusion pumps, help detect potentially life-threatening blockages. These automated pumps continuously deliver vital drugs to the patient on a daily basis. When designing the delivery system, engineers concluded that the detection of blockages and functional problems within the pump was critical. When a blockage occurs in the pump, the tubing within the pump expands. The custom sensor, located where the tubing meets the housing, in turn detects this expansion by monitoring the force applied to the sensor by a section of the tubing. The sensor then triggers an alarm to alert the user of a detected blockage, and to take the necessary steps to correct the problem in order to reduce any negative effects. (See Figure 1)

Robotic Surgery

A key contributing factor to a successful surgery is sensory feedback. In recent years, with the help of modern surgical tools, robotic surgical procedures have become increasingly less invasive. Today, surgeons using robotic controls must depend on tactile cues and visual confirmation to direct the robot.

Design engineers are challenged with creating devices containing sensory force feedback that ultimately relay force measurements to the operator, so he or she can properly control the robot performing the surgery. For example, some robotic systems have grippers used to hold very small and extremely sensitive parts of the body, such as veins and soft tissues. This tool allows surgeons access into parts of the body not easily accessed by the operators themselves. Integrating force sensors allows the surgeon to detect how much force is being applied during surgery. This insight is key.

Prosthetics

Fig. 2 – A variety of commercial prosthetic hands, all shown without cosmetic glove. (Credit: U.S. Department of Veterans Affairs)

According to the Amputee Coalition, approximately 185,000 amputations occur in the United States each year. This large number drives the need for enhanced prosthetics. Medical design engineers aim to create prosthetic devices that provide force feedback to the user. The engineer’s goal is to create a communication between the prosthetic and the user, allowing them some kind of sensory ability. Sensors can be used to accomplish this communication.

Sensor feedback allows users to know how much force they are applying to objects and to practice day-to-day activities. For example, force sensors are used in an artificial hand; the sensors are located at the tip of the thumb, index finger, and middle finger. The sensors help the user understand how much force is applied when grasping and releasing objects. The hand is connected to a PC that is also connected to a nerve stimulator. The nerve stimulator sends electrodes to the user’s upper arm. This system allows the prosthetic to relay back to the user, creating a smart medical device. (See Figure 2)

Shoe Insole

Monitoring systems are a way for doctors, patients, and their loved ones to keep constant track of a patient’s day-today activities in the comfort of their own natural environment. New medical devices are allowing doctors 24-hour access to this insightful information regarding a patient’s progression and everyday routine. Real-time monitoring systems are becoming increasingly popular in geriatrics, specifically keeping a close eye on elderly patients. For example, force sensors can be designed into non-invasive medical devices either worn or used by the patient. For instance, sensors can be designed into shoe insoles. (See Figure 3)

The force sensors designed into these products provide continuous data for relay to the doctor or the patient’s family members via a wireless hub. This data provides insight into the patient’s daily activity or lack thereof, which, in turn, would alert doctors or family members to check in on that patient. These medical devices allow doctors to stay informed and connected to their patients, resulting in more precise treatment methods.

Foot Drop System

Fig. 3 – Shoe insole utilizing a force sensor to help assess balance issues in patients.

Another type of assistive living device is one whose main purpose is to communicate with the patient’s body itself. Functional electrical stimulation is a technique that uses electrical currents to activate damaged nerves, due to a stroke, neural damage, etc. Foot drop systems are wearable devices to help those affected by muscle and nerve damage to regain mobility and improve their quality of life. These devices senses when your foot is on or off the ground and helps the foot adjust to changes while walking.

The device consists of a lightweight cuff worn below the knee, a gait force sensor attached to the inside of the shoe, and a portable, wireless control unit. The force sensor placed within the shoe relays wireless signals to the leg cuff, which then produces electrical stimulation to specific leg muscles. This stimulation of nerves and muscles signals the foot to lift off the ground and helps the user walk more naturally. The user also has the ability to adjust the level of stimulation with the hand-held portable unit, which is small enough to carry in a pocket or bag.

The combination of this lightweight, flexible sensor and the wireless design results in a noninvasive device that can be used by the patient in their natural environment and easily integrate into their daily routine. This type of assistive device is liberating to its users because they have the ability to not only adjust the stimulation levels to meet their personal needs, but it gives them a sense of control, which they did not have before.

Conclusion

As stated before, data drives results. In the medical field, medical devices equipped with force feedback capabilities ultimately result in better medical practices, improved patient outcomes, and patient experiences. The quantitative and qualitative data provided by these devices enhanced by force sensors is invaluable. These smart devices give practitioners and patients better insight into their overall health, bodies, and rehabilitation. As the medical market evolves, tactile force sensors provide OEM design engineers a durable, costeffective solution that helps them create an intelligent and valuable product.

This article was written by Jeannine Croteau, Marketing Specialist, Tekscan, Inc., South Boston, MA. For more information, Click Here . MD&M West, Booth 3255