The myriad of devices used in surgical, interventional, imaging, diagnostic and therapeutic, sensors, and single-use medical applications use some form of transmission medium to transport electrical signals. While the construction of a cable seems relatively straightforward compared with the complexity of the apparatus it is connected to, the environmental considerations and regulations behind these constructions all bring with them additional design constraints. This article provides an overview of particular characteristics found in medical device cables and connectors along with some respective design considerations.
Medical settings are saturated with electrical equipment to supplement patient care. This equipment can vary from portable defibrillators to MRI machines. Connector heads used in these applications require a high resistance to dirt and moisture ingress as well as the ability to be easily mated and unmated. A pre-certified connector with an Ingress Protection (IP) rating of at least IP67 — a ranking that indicates a device is dust-proof and can withstand immersion in water for an extended period of time (e.g., 30 minutes) — would be favored because they would function regardless of time in storage or in case of accidental liquid entry. As an additional note, an IP69K rating would indicate further protection against high temperature and high-pressured spray downs at a close range. Connector heads at this ranking would be able to withstand both washdowns and submerges.
Table 1. Standards and regulations applicable to the design and integration of connectors and cables in medical instrumentation. Push-Pull Connectors
A push-pull connector can be used for a number of cable configurations including coaxial, fiber, and multi-conductor cables. The type of connector utilized depends on whether the wire or cable must carry data, a high-frequency signal, power, or a combination. The latching is generally accomplished through a straight, axial motion where a spring-loaded contact becomes engaged. Disconnections are done similarly where the outer sleeve is pulled back to rapidly disengage, or in other words, pop off. The push-pull mechanism can be broken down into two main interfacing types: male or female. The female-based push-pull type connectors involve a female cavity that includes threads extending radially outward to be engaged by a male connector head. The male-based push-pull type connector typically has threads that project radially inward that can be engaged by the female cavity. These types of connector heads can have pin counts ranging from 1 to 400 depending on the number of data lines and amperage per line.
Certain materials used in the push-pull mechanism must be compressible materials in order to enable thousands of mating cycles without performance degradation. The image shown in Figure 1 provides an example of a keyed IP69K push-pull M12 connector with five pins that attach to a cable containing five unshielded wires. This type of a cable assembly would typically be used for data and communication, the type of signal transmission that is most often used for medical devices. Push-pull connectors are often leveraged in medical applications for a relatively simply connection and disconnection as compared to a threaded or screw down connector head.
The main benefit of a push-pull connector is the ability to generate a rapid connection. This, however, does come with some drawbacks depending upon the application. Certain segments of the connector require specific dimensions of compressible material that may not fit industry standards. Moreover, these connector heads are often keyed to enable a very specific mate. While this simplifies mating, it limits the interoperability of push-pull connector heads. As an additional note, there is often an unshielded gap left at the mating surface, which leaves that connection open to electromagnetic interference (EMI).
As made evident by the name, screw-in connectors involve thumbscrews, or a threading mechanism to accomplish a screwed mate. This could range from d-subminiature connector heads to custom high-voltage connectors. More often than not, connectors such as d-sub or USB would be used for instrument control within portable and non-portable equipment such as ultrasound or patient monitoring machines for data transfer. High-voltage connections would be necessary for high-powered equipment such as imaging and laser equipment as well as defibrillators. These connectors are contrary to the push-pull connector types that would be leveraged by a physician or technician on a daily or weekly basis.
Fig. 2 - I/O data connectors come in a large range of configurations but there is almost always an IP-rated variant that would be beneficial in medical applications.
Parameters of high flexibility or high mating cycles are not paramount but rather supplementary for these types of connectors. These types of connectors are necessary for almost any equipment utilizing a computer regardless of application. For medical applications in particular, however, the IP rating is relevant. As shown in Figure 2, a rating of IP67 and above can be accomplished on d-sub connectors with kits where the backshell can conform to a variety of cable diameters with an adjustment to the sealing nut. Custom connectors for particular equipment would have specific latching mechanisms and materials that are often proprietary.
Screw-In Connector Considerations
Connectors with thumbscrews are generally not meant for frequent mating/unmating due to the time it takes to screw and unscrew. They are also generally not an ideal connection for situations where the cable is flexed frequently because the lateral strain can bend the pins that are exposed in the shell. There are alternatives that take the strain off the port such as socket savers for d-sub connectors or additional strain relief boots could add flexure to any cabling. High-voltage rated connectors can range from 1 to 100k VDC, when leveraging powers as high as this, it is best to avoid a simple friction fit. In this case, screw-in is ideal.
A Myriad of Connector Topologies
While these are some commonly used connector architectures, the medical industry is not limited to them. High-frequency signals will use a coaxial interface with one conductor, a dielectric, and shielding surround it. These types of signals would require a coaxial connector that introduces another realm — signal dynamics. The burgeoning field of Medical Body Area Networks (MBAN), for instance, would have to consider high-frequency electronics and their respective interconnections. There are also single-use connector heads that would have to be constructed of materials such as PVC that can be disposed of inexpensively.
Ultimately, the many different medical instruments demand a fairly large range of connections and wires. The general rule is that these cable assemblies must be safe for technician and patient use. Overmolded cables are popular because, similar to strain relief boots, they add flexibility. Overmolding also adds a watertight, or even a chemical tight, seal to an assembly as the mold extends over to the connector head. This, however, may not be necessary for a connector that has already been IP rated.
Medical Cables and Connectors Standards
The medical industry has no global cable standard that can be specified for any medical application. This does not mean that there are no regulations applicable to medical cable assemblies. For example, the ANSI/AAMI 53 standard from the Association for the Advancement of Medical Instrumentation includes minimum safety and performance specifications for ECG cable and lead wires. The standard is designed to prevent inadvertent connection of the patient leads to a power source. The standard specifies that all controls, switches, and connectors be clearly labeled for their function. Warnings and current rating labels must also be placed in areas where maintenance personnel could get shocked. The maximum output display noise is also specified where cables are a direct contributor to this and are therefore a significant consideration.
There are no other cable-specific standards. However, Table 1 identifies additional relevant medical device standards. The practices for ensuring that any of these standards are met could extend to the interconnect. For instance, FDA 21 CFR 820.30 subpart for medical devices defines requirements for sterility to prevent the growth of microorganisms. ISO 14971, which is a risk management standard, encourages manufacturers to use specific connectors that cannot be connected to the wrong component, thereby mitigating any human error. All of these are considerations that have a direct impact on connector and cable design.
The medical industry has very stringent regulations compared with commercial and even military standards. This, in turn, affects the design and selection of the cables and connectors attached to medical devices globally. With the wide variety of connector and cable configurations available, it is important to consider the end use and function to determine which is right for the application. These considerations should be valued according to the specific instrument and application.
This article was written by Dustin Guttadauro, Product Manager for L-com, North Andover, MA. For more information, visit here.