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.

Successful Strain Gauge Technology Applications

Insulin Pump Fluid Flow

For flow monitoring, an OEM approached HBM to develop a custom strain gauge that was to be used to monitor and control the output of an insulin pump. The pump is designed for on-body continuous patient use and provides insulin incrementally as determined by feedback from an integral blood glucose monitor, while still allowing the patient to override as needed for insulin therapies upon demand. The application environment itself is highly compact and would be subject to the ongoing shock and vibration inputs of movement caused by normal patient use. Thus, a highly rugged, precise, lightweight strain gauge sensing technology was required. For this application, HBM designed and developed a unique subminiature strain gauge sensor assembly, positioned and strategically weakened to allow for desired control of insulin delivery, both manual and automatic. The success and overall reliability of this technology integration has led to the incorporation of strain gauge sensors as an industry standard for many of the industry’s leading insulin pump manufacturers.

Medical Infusion Pumps/Syringe Pumps

A unique 1.5 lbf strain gauge sensor assembly, developed by HBM, has been successfully used to ensure accurate and repeatable liquid flow control within a syringe pump.
For integration into a medical infusion pump, HBM was asked to design a series of custom strain gauges that could monitor and control fluid flow of intravenous medication received via the tubing clamp. The device design had to allow for uniform fluid delivery without coming into contact with the fluid itself. For this application, HBM designed and developed a unique 1.5 lbf strain gauge sensor assembly, positioned and strategically weakened to form a blade-shaped configuration. With its blade shape, the assembly mimics the behavior of a perfect spring, returning to zero when liquid stops flowing and with highly repeatable performance. The strain gauges were applied in several strategic locations of the pump and also on the tubing clamp, allowing the device to measure not only the weight of the fluid, but also to sense an increased fluid loss or empty bag — signaling the need for replacement, as well as alerting the user to any potential flow interference from external factors, such as the tangling of a tube line or patient vein blockages. Numerous pump OEMs have adopted use of this specific technology in various formats as a cost-effective means of monitoring and controlling flow for critical therapies such as intravenous anesthesia, pain management therapy, and blood transfusion equipment, meeting both in-hospital and out-patient care requirements.

Kidney Dialysis Machines

Kidney dialysis machines incorporate strain gauge sensing technologies to ensure uniform fluid flow and fluid circulation of proper rate, proportion, and frequency according to the parameters set by their accompanying electronic controller devices. Timing within this flow measurement system is of critical patient importance, to ensure that as blood leaves the body and is filtered, the recirculation process occurs in a synergistic timeframe. In these applications, strain gauged parts are used to monitor not only medical pumps, but also the weight of canisters of both blood and waste, and flow of hanging intravenous fluids, all with the goal of ensuring consistent therapy delivery. Typically, the electronic controller device managing the dialysis process uses data from these sensors. For this application, HBM designed a variety of custom strain gauge sensor assemblies with accompanying load cell technologies for canister weight and measurement. With this comprehensive technology integration, the OEM was able to ensure that both pumps and canister systems incorporated the necessary checks and balances to ensure a smooth-running dialysis process.

Conclusion

With its relatively simple construction, low cost, and design flexibility, the incorporation of custom strain gauge technologies into OEM medical pump and device applications presents many advantages. The high accuracy and repeatability of the strain gauge and its ability to mimic the behavior of a simfple, repeatable spring allow for seamless integration into a variety of device designs, with minimal to no service requirements. The extensive capability of the strain gauge for modification to meet specific customer requirements facilitates its growing use within the industry.

This article was written by Robert Chevalier, Director of Sensor Sales for HBM, Inc. (Marlborough, MA) and Molly Chamberlin, president & founder of Embassy Global PR & Marketing Communications, LLC (Orchard Park, NY). For more information, Click Here 


Medical Design Briefs Magazine

This article first appeared in the March, 2011 issue of Medical Design Briefs Magazine.

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