Originally Published MEM Fall 2008
CONNECTORS
The medical industry has seen a rapid increase in the use of digital electronics in items such as catheters, defibrillators, and monitors. New medical chip sets are capable of handling multiple input data channels from analytical instruments. These chips also provide a wide range of display information, machine orders, and pump signals. Medical equipment is sending and receiving more electrical signals than was perceived possible just a few years ago. As a result, patient monitoring, analysis, and life-sustaining services have improved significantly.
New medical treatments based on electronic systems are emerging all the time. Examples include:
- The neurological measurement of brain waves to treat cerebral palsy and aneurisms.
- Both pH and glucose level monitoring in the blood system for diabetes.
- Ultrasound scanning to determine liver, lung, prostate, and other internal organ damage.
New probes, sensors, and detectors, together with all their associated wiring, cables, and connectors, have become an integral part of these medical systems. Until the last three years, cable harnesses, with connectors, were one of the larger and more difficult portions of the instrument system to handle. Planning for the sterilization or disposal of the parts of the system exposed to the patient was also difficult. To address these problems, integrated connector and cable systems have become an essential component in electronic medical equipment.
Cell phones, laptop computers, and digital cameras all contain electrical connectors molded into the housings. Cables are hidden inside hinges and shells and behind displays to maximize use of space, reduce weight, and save money. In much the same way, medical instruments are designed such that the standard elements of high-reliability connectors, pins, or sockets are molded directly inside their components.
Medical applications using built-in connectors include laser tool handles, probes, sensors, electronic catheters, extended optical inspection devices, and even robotic instruments. Bone conduction hearing aids, for example, demand connectors that are lightweight, small, and easy to mount. Surgical tools and ultrasound equipment can use integrated connectors to enable quick tool and head replacement.
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Figure 1. Built-in connectors shown are designed to be molded directly into medical equipment. |
Connector Construction
Built-in connectors (see Figure 1) are designed to be molded directly into medical equipment. Connector insulators are first assembled with pins or sockets that have been prewired or that have solder lugs on the back. Solder lugs are more common on connectors that have a larger pitch (0.050 in. and above). However, the latest-generation miniature connectors—termed nanominiature on a 0.025-in. pitch—are so small that there is often no room to use a soldering tool. Therefore, these are supplied prewired, having been crimp-wired by the connector supplier.
Mechanical assembly drawings are used to size and specify the internal dimensions of the housings, handles, and probes that will contain the connectors. Injection overmolding processes complete the final step of fabrication to form the final assembly. There are many material options for the outer shell material, and selection is based upon the application. Softer silicones are preferred if the application is for an instrument in which the cable is likely to come into contact with the patient's skin. Patient well-being is right at the top of current thinking, and the cable must not cause discomfort because it is too hard or abrasive to the touch.
Conversely, devices that require autoclaving will require the toughest protection. In these instances, the housing material selected must be cut-resistant, durable, and able to withstand temperatures of up to 150°C. Here, materials such as Teflon and other commercially available fluorocarbon plastics may be the best choice. In many instances, instrument designers choose to use proprietary compounds that combine some of the advantages of both silicones and Teflon-style materials.
Medical designers can check electrical functionality quickly by using off-the-shelf connectors that are available on next-day delivery. Overmolded parts usually require a 30–60-day turnaround time.
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Figure 2. This perfusion monitor uses built-in miniature connectors in the probe assembly. |
The perfusion monitor shown in Figure 2 includes built-in miniature connectors in the probe handle assembly. The microcircular connectors used in the design enable quick-change probes and sensors to be used for medical diagnostics. In this case, the probe contains a thermistor element that changes electrically with variations of temperature. A surgeon can insert the probe into the body and continue to monitor a patient's blood and tissue temperature during procedures. After the procedure, the patient end of the cable assembly is disposed of while the mating instrument cable can be sterilized for the next user.
Standard building sets are available that comprise circular insulators with alignment keys in the insulators to ensure error-free mating and easy attachment. Socket shrouds protect pin and socket elements. Such low-cost connectors (typically costing less than $1/pin) are designed to deliver high reliability and ruggedness for integration into medical instrument technology. Standard sets are used at the start of the integration process. It is important to seek connector designers that have experience in USP Class VI materials and experience in helping customers achieve FDA approvals.
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Meeting Requirements
For medical applications, it is essential to use a connector designer that can provide very fast prototyping at the equipment level by hand wiring off-the-shelf sets of the circular insert connectors prior to designing the mold. Connectors are generally available for next-day delivery. Such prototyping ensures that after the final electrical and mechanical configuration is complete, insulators fit their final shape and mating arrangement.
Built-in circular designs are available in three different diameters and offer up to 27 positions in mated pairs on a 1.27-mm pitch. Standard wiring includes Teflon-insulated 26-gauge stranded wire to achieve maximum cable flexibility. Custom wire and mechanical assemblies are done routinely for new instrument design—especially if the product is being developed by a small design house.
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Overmolded nanominiatre connectors are preferred in a wide range of medical applications. |
Built-in connectors are produced using materials that meet the highest test standards in the industry and are constructed using military-grade materials—such as Teflon, silicones, beryllium copper, and various epoxies—to guarantee that the medical industry's quality and reliability demands are met. Pins and sockets are made of annealed beryllium copper that is spring-tension controlled and then plated with nickel and gold. Circular insulators are molded using a glass-filled liquid-crystal polymer that exceeds most temperature and chemical requirements in the industry.
Teflon-insulated wires are preassembled or have solder cups to allow attachment in the factory during equipment configuration. Elements and materials as described earlier perform up to and beyond the standards used in MIL-DTL-32139 (nanominiature 0.025-in. pitch) and MIL-DTL- 83513 (0.050-in. microminiature connectors). Specific application testing and certification for performance is all that remains to ensure that the final design meets medical application requirements.
Conclusion
New materials and processes have combined to improve and speed up the design and fabrication of medical instrument cable systems. Materials such as liquid-crystal polymers and others have evolved and are significantly improving the chemical, physical, and temperature stability of connector insulators. When used with high-reliability pin and socket elements, overmolding becomes the production technology of choice for a wide range of hand tools, probe systems, and electronic catheters. Many of those have already passed the reliability and quality tests that are necessary to achieve approval by FDA and other medical regulatory bodies.
Robert Stanton is director of technology and market development for Omnetics Connector Corp. (Minneapolis).
