Medical connectors can be divided into two broad groups based on how they are retained by the devices to which they are connected: locking – also referred to as latching – and non-locking. Each offers advantages and disadvantages and design and manufacturing challenges.
Locking and Non-Locking Connectors
 USB connectors are examples of non-locking connectors |  Custom locking defibrillator connector latches securely |
 |  Example of “MS” – mil-spec type connector commonly used in early medical devices |
Early medical cable assemblies used connectors adopted from military and industrial applications. These “MS” (mil-spec) metal connectors had knurled threaded couplers. Once screwed down to the receptacle, this connector would not come loose inadvertently. While this type of connection met the requirement that the cable not be disconnected unintentionally, they were difficult for clinical users to connect and disconnect and provided no safety disconnect feature which is often desirable for medical applications.
 Custom overmolding of RJ connector helps prevent the latch mechanism from breaking off | |
The Registered Jack Connector
An example of a locking connector used in medical devices is the Registered Jack, or more commonly called the “RJ” connector. While some RJ plugs are rated for up to 1,000 mating cycles, the latch is typically exposed and easily broken. Once the latch breaks, the plug is poorly retained by the receptacle and the cable requires replacement.
One solution to improve the service life of an RJ connector is to overmold the body and include a flexible hood. The hood should allow the latch to be depressed but should also prevent the latch from being bent upwards and breaking off.
Locking Connector with Safety Disconnect
It is often desirable for a medical cable assembly, particularly those that connect between a patient and a stationary device, to lock to avoid unintended disconnection. In some instances, it is equally desirable for the connector to safely disconnect if axial force is applied to the cable such as when it is caught in a moving bed.
|  A safety release has been designed into the plug (red feature) which will allow safe disconnection above a pre-established load |
This design requires that a minimum retention force be established for the locked connector over which the plug will disconnect. By carefully designing the locking mechanism, the connector will be held securely in the connected condition until the specified axial force is applied. Above that load, the connector will disconnect without causing harm to the patient, device or cable assembly.
Non-Locking Connectors
Medical cables require a positive connection between the plug and receptacle. Any looseness in the connection may cause intermittent contact, resulting in unwanted noise or poor signal quality which may make diagnosis or therapy difficult, if not impossible.
How firmly the plug is held by the receptacle is referred to as retention force and is a controllable characteristic. Pin and socket selection, as well as the physical design of the plug and receptacle, allow control over both insertion and retention force. If the connector is expected to have a high number of mate and un-mate cycles, it is generally desirable to achieve retention force from the friction between metallic pins and sockets rather than plastic parts that wear more easily.
Connector Retention Force
 The interface and friction between pins and mating sockets plays a large role in the retention force of a mated connector | |
Retention force of a connector pair – plug and receptacle – is nominally made up of the sum of the retention force of each pin and socket as well as any friction between the plug and the receptacle. For connector pairs with few contacts, friction, whether intended or unintended, between the insulator and receptacle wall may be the largest factor in determining the total retention force. For units with a larger number of contacts, little or no friction may be needed between insulator and receptacle wall.
An additional factor to be considered is that in a connector with more than a few contacts, the total retention force is greater than the sum of each pin to socket retention force. This characteristic is detailed in a paper by Robert S. Mroczkowski, Sc.D “The Mating Game” in “Connector Specifier” magazine, December, 2001. In the article Mroczkowski states that “mating force will always be greater than that value (if all contacts mate at the same time) because of tolerance and housing interaction effects.”
Enhancing Connector Retention Force
If the retention force achieved by pin-to-socket and friction of the connector housing is insufficient, one method to effectively increase retention force is to design the connector so that axial force applied to the cable does is not directly applied to the removal-axis of the connector.
 Custom, non-locking connector can be disconnected by axial force applied to the cable |  Custom, non-locking right-angel version resists disconnection when axial force is applied to the cable |
Retention Force Specification
For non-locking connectors, one of the specifications established early in the project is the retention force of the plug to receptacle. The amount of retention force as well the required number of mate and un-mate cycles are factors considered in contact selection and part design.
Once mating and retention forces are established and documented, molds are designed in a “tool safe” manner. Tooling is designed to produce plastic parts that have retention force below the desired level. By removing metal from the tool, the connector becomes larger and retention force is increased. Done in very small increments, this method allows retention force to be “dialed in.” Sharing mold trial parts the design team allows insertion and retention force to be evaluated and adjusted before production parts are manufactured.
Connector Retention Force Testing
Once production parts have been manufactured, verification testing will confirm that all specifications, including connector retention force, are met. For non-locking connectors, Design Verification Testing will typically include measuring retention force at pre-established intervals during mate and un-mate cycle testing. This will confirm that retention force is maintained over the design life of the connector.
 Engineering technician, Eric Yamane, performs Mate/Un-mate cycle testing by hand to more closely simulate actual use |  David Moreno measures retention force at pre-defined intervals of mate/un-mate testing |
Summary
Choosing to use a locking or non-locking connector for a medical cable assembly is a decision that should be made early in the life of the project and should consider the user and also how the cable may be used.
The Affinity engineering team has decades of experience designing both locking and non-locking medical connectors and the associated cable assemblies. Let us partner with you on your cable or connector project.
For additional information, contact your local Molex Sales Engineer or Account Manager or call us at +1 949.477.9495 or email us at custcare2@molex.com.
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Read More From The Connector by Molex: http://www.connector.com/2014/06/locking-and-non-locking-medical-connectors/#ixzz34oQyiTPp