In today’s competitive marketplace, medical pump OEMs and their associated design engineering personnel are incorporating strain gauge technologies into both small- and large-volume portable and ambulatory medical infusion pumps, such as insulin and syringe pumps and kidney dialysis machines, as a means of more accurately monitoring and predicting fluid flow rates. The pumps themselves are considered “mission critical” components within these medical device and equipment applications. Their accuracy and reliability remain of the utmost importance for ensuring a constant infusion of vital medication or fluid to the patient.

Successful integration of strain gauge technology has long been field-proven within these applications for greater pump accuracy and extended service life, allowing a pump OEM to add that “extra design edge‚” thereby increasing sales and overall “hit rate” for major contracts.

A typical dialysis machine often includes multiple force sensing technologies to ensure proper blood filtering and recirculation.
Within the medical device marketplace, strain gauge technology is also known for its contributions to a decrease in required man-hours for maintenance costs due to fewer field failures and improvement of overall patient care quality. As such, strain gauge technology is often considered a technological “secret of success” among OEMs. To truly understand the benefits of strain gauge technology, it is important to first understand the strain gauge itself. This article will provide an overview of strain gauge technology and its practical applications; explain the typical processes by which the technology is introduced; and offer examples of improvements achieved via successful integration.

Understanding the Strain Gauge

The levels of mechanical strain most typically measured with strain gauges are small and precise. Consequently, changes in resistance are also very small and cannot be measured directly with an ohmmeter. The strain gauge must therefore be included in a measurement system where precise determination of the gauge's change in resistance is possible. To do this, a Wheatstone bridge circuit must be created. The first component in the system is formed by the strain gauge itself. It converts mechanical strain into a change in electrical resistance. Both the strain gauge and the measuring circuit are, in the physical sense, passive components. Each strain gauge is then wired into a balanced bridge, consisting of two portions of an equal resistive value, forming a Wheatstone bridge circuit.

HBM, Inc. (Marlborough, MA), a supplier of custom strain gauge sensing technologies for medical devices and equipment, also constructs gauges with 1⁄4- and 1⁄5-bridge designs that require a fixed resistor to complete the Wheatstone bridge. Regardless of bridge configuration, energy must be passed through the gauge to excite the circuit. The circuit must have an auxiliary input energy source, typically external, to obtain a useful signal. A constant electrical voltage is used, though a constant current power source can also be utilized. With a change in strain gauge resistance due to strain, the bridge circuit loses its symmetry and becomes unbalanced. A bridge output voltage is obtained, proportional to the bridge's unbalance. If there is no change in value to the balanced resistance, the electrical output is null or zero. A typical strain gauge on average can measure 1/10,000 micro strain, or enough to detect a small 1 dB vibration across a 10-ft. room. Thus, measurement possibilities have an infinite range. An amplifier must be included in the measurement process to amplify the bridge output voltage to a level suitable for compatibility with indicating instruments. Sometimes amplifiers are designed to give an output proportional to the bridge output in voltage.

Strain Gauge Technology in OEM Medical Pump Designs

Strain gauge technology incorporated into an on-patient insulin pump must be highly rugged and unaffected by shock and vibration inputs during normal patient use.
The process of incorporating strain gauge technology into OEM medical pump designs can be highly specialized. Implementation is often subject to regulatory compliance and federal government regulations, both of which may directly impact final design. To successfully incorporate the technology into a finished part, it is important to partner with a manufacturer that is well-versed in such compliance matters; that builds samples and performs necessary verification and validation checks within extremely short timeframes; and that has the necessary in-house expertise to troubleshoot more complex areas of technology integration.

For example, at HBM, after a technology briefing and customer identification of applicable regulatory standards, a prototype sensor design is created. This process includes a detailed application analysis and recommendations for the best gauge for intended performance, with varied geometries, holes and cutouts, resistances, threads, and other options. Using finite element analysis (FEA) and other advanced design tools, the appropriate location for the strain gauge is identified and then incorporated into the prototype under specific conditions. As part of this process, the designer will strategically weaken the structural member to allow specific deflections under applied load, enabling the member to mimic the structural behavioral properties of a real-use condition. It is then calibrated and adjusted to perform a perfect, accurate, and repeatable measurement. In-house testing must also be completed in accordance with industry standards. Once a successful prototype is built, tested, and customer-accepted, it is sent to production. A medical pump OEM may opt to self-manufacture finished sensors from a gauged prototype part. However, a strain gauge manufacturer’s in-house expertise allows a customer to have complete assurance in the quality and uniformity of manufactured sensors, tested according to the same rigorous in-house standards as the prototype, while ensuring accuracy, reliability, and timely delivery to meet OEM manufacturing schedules. This mitigation of in-house risk can ultimately save time, cost, and resources.