Repeated mating and unmating of connectors in medicine quickly wears out the parts, even in cases where disposable parts are used. Hyperboloid wire basket connectors deliver a solution, offering reliability and high mating cycle life.
By Tom Kannally
As more and more electronics are designed into medical applications, high-reliability has become more and more important in interconnect solutions. Connectors used in harsh medical environments typically need to be able to deal with fluids, withstand various chemicals, perform electrically and mechanically after many mating cycles, offer EMI/RFI protection, and withstand shock and vibration. In addition, applications such as operating room equipment need to use inert materials that withstand various sterilization methods such as autoclaving, ethylene oxide (EtO), and gamma radiation.
 FIGURE 1. Connectors designed for medical applications need to take into account both reliability and ergonomics. |
In medicine, connectors are mated and unmated over and over again, even in cases where disposable parts are used (see Fig. 1). In this case, the connector on the patient side is typically discarded along with a single-use probe. However, the instrument side of the connector is fixed and will need to offer a high cycle life with many different disposable mates.
 FIGURE 2. The hyperboloid design is constructed from a series of beryllium copper wires that stretch to accommodate the pin upon insertion. |
Cost is also an issue in medical electronics, but it is important to note that field maintenance is inherently expensive as is the equipment. Consequently, reliability and long life would alleviate such cost-of-ownership.
A connector using a hyperboloid-shaped wire basket design offers many benefits to demanding medical applications (see Fig. 2). Key to the design is a socket constructed from a series of beryllium-copper spring wires. When the pin is inserted into the socket, the wires stretch and wrap themselves around it (see Fig. 3). The design addresses the main connector requirements in medical environments--reliability and a high number of mating cycles.
The key factor in ensuring high reliability is the multiple points of contact inherently provided around the pin by each wire in the hyperboloid design. In an environment that subjects the connector to possible ingress from dried bodily fluids or chemicals along with mechanical duress, these multiple contact lines ensure that signal integrity is maintained under the worst conditions, especially in hospital or ambulance applications.
 FIGURE 3. The sleeve of the hyperboloid-shaped wire basket expands during mating to enable a high-reliability connection. |
A smooth, light wiping action inherent in the contact design also produces little wear of the contact surfaces, making connectors based on the hyperboloid contact suitable for applications that require long life with high numbers of mating cycles. Such contacts have been tested for up to 100,000 insertion and extraction cycles with minimal wear and no loss in functionality or specified electrical performance, contributing to reduced costs in downtime and repairs. In addition, the smooth wiping action cleans the contacts as they are mated and unmated.
Sensitive electronics
The multiple lines of contact in the hyperboloid socket deliver a much greater contact area than other types of contacts of comparable size, resulting in about half the contact resistance of conventional contact designs (see Fig. 4). This is a useful attribute when dealing with sensitive signals from sensors and transducers where power consumption and heat generation of a connector must be kept to a minimum for the equipment to function as specified.
 FIGURE 4. Multiple linear contact paths created around the pin deliver high reliability and maintain contact integrity under duress. |
Further, the contact resistance of the connector should be constant and not vary when under duress, whether due to vibration or any other external influence. A circuit "seeing" a variable contact resistance due to an unreliable connector could result in erroneous readings on the instrument or, in the case of sophisticated medical equipment such as defibrillators, turn out to be life threatening.
Contact resistance typically depends on the size of the contact, dropping with increasing contact size due to the contact surface area available for the current path. The trade off between contact resistance and the potential connector miniaturization is alleviated with the low-resistance hyperboloid technology. For example, a typical hyperboloid socket features a contact resistance of less than 8 mΩ for a 0.4-mm size contact. This falls to 0.8 mΩ for the 2.5-mm size contact. For high-power larger contacts, 0.08 mΩ is possible.
Other advantages include low insertion and extraction forces, immunity to shock and vibration, and easy design customization. Because these contacts offer much reduced insertion and extraction forces, they allow for higher densities than conventional contacts and can be easily mated even with hundreds of contacts in a single connector.
Hyperboloid contacts are capable of withstanding extreme levels of shock and vibration while guaranteeing signal integrity, one of the main reasons that they are popular in industrial and aerospace applications. Shock and vibration often result in an intermittent loss of contact, which becomes important when dealing with high data rates. Due to the hyperboloid socket design, specifically the 360º wrap provided by the spring wires, immunity to high levels of shock and vibration is guaranteed even when measured over periods as small as 5 ns.
Cost of ownership
The initial price of the hyperboloid technology compared to conventional connectors is typically higher. However, when factoring in the superior reliability, the overall cost over the lifetime of the equipment is considerably lower. This is especially pertinent if the failure is caused by a relatively low-cost component, such as a connector, with respect to the overall equipment price tag.
In medical environments, if a connector fails, not only does this incur expensive repair costs, the downtime of the equipment could seriously compromise medical services. While under normal conditions a certain level of redundancy will be available to fall back on, in emergencies this might not be the case.
Many problems associated with medical equipment can cause a perceived risk in terms of its usage. Doctors and technicians are reluctant to use devices that they see as being unreliable. Some typical connector problems might include difficulties in mating and unmating, intermittent connection due to shock and vibration in mobile medical applications such as air or road ambulances, failure or intermittent connection due to fluid ingress, and damage from rough usage.
From rail to sky
Originally designed for the rail market in the 1950s, hyperboloid contacts proved themselves in situations where vibration was a key concern, especially in high-speed train lines. These contacts carried high power and had to offer a solution with high connection integrity.
The technology spread from trains to aerospace, industrial, and medical markets as its benefits became apparent. It is currently used in a host of military, space, and medical applications. With the ability to manufacture miniature connectors based on this technology today, the hyperboloid contact is finding more applications in all these markets as a more reliable alternative that delivers a low maintenance, long-life solution.
Connectors with hyperboloid contacts, designed for medical applications would typically need to offer a wide range of specific benefits, including user configurability, color-coding, and customer keying, and sealing and shielding options (see Fig. 5). Sealed connectors would typically deliver IP67 protection when mated and EMI/RFI shielding would protect against signal interference in electrically noisy environments.
 FIGURE 5. Color-coding facilitates ease of recognition by medical and emergency staff. |
Color coding and keying provide an easily recognizable system to facilitate quick connections on medical equipment without tying up valuable medical staff time. Color coding facilitates ease of recognition and efficiency, enabling staff to connect or disconnect based on color coded instructions, for example red connector goes to red on the equipment. Not only does this save time in highly stressed situations but enables medical personnel to concentrate on the job rather than technicalities. Another advantage is the ability of firefighters and police to help should the need arise without much explanation, thus facilitating simple interaction between emergency services.
Materials used in connector designs, specifically with medical applications in mind, could include polyetherimide, liquid-crystal polymer, and silicone rubber, all of which enable the connector to handle autoclaving, EtO sterilization, and gamma radiation. The materials also withstand cleaning and working chemicals found in medical environments.
Medical connectors based on the hyperboloid contact incorporate all of these requirements while delivering a flexible design that is easily customized for a particular application or offered as a standard product. Modifying the socket diameter or the number, diameter, or angle of the spring wires, various signal and power contacts can be integrated into the same housing. This flexibility, along with a wide range of benefits, makes the hyperboloid contact suitable for a wide variety of applications, especially when taking the cost of faulty operation into account. Currently, hyperboloid connector technology is capable of meeting or exceeding the electrical and mechanical requirements of demanding applications from aerospace to industrial and medical markets.
Tom Kannally is medical industry manager at Hypertronics, 16 Brent Drive, Hudson, MA 01749. Tel: (800) 225-9228; email: tkannally@hypertronics.com.
