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Working with Complicated Disposable to Instrument Interfaces

Lab on a Chip, Microfluidics, and Sample to Answer are terms we hear a lot when designing medical devices.  For years the trend has been to integrate more and more laboratory functions into small disposable chips or cartridges with the ideal goal of being able to perform all the functions from sample preparation to detection in a single disposable on one instrument.  This has led the instrument to disposable interfaces to be increasingly complex.  Sometimes there are hundreds of electrical connections for internal sensors or actuators, multiple heaters for PCR amplification, magnets for magnetic bead capture, pressure connections for fluid control, actuators for dispensing stored fluids, and external sensors  just to name a few.  The complex interfaces required can cause many design challenges, but after working on several of these types of projects over the years we’ve learned there are some ways you can make your life easier, and end up with a better design.

Proper alignment between the instrument and disposable is key to making reliable connections.  Ensuring proper alignment with the large quantity of connections is often one of the most difficult challenges.  The size of the disposables makes it tough to fit all of the connections in the available space.  This drives the design towards smaller interface components that are less tolerant of misalignment.  Also, to provide an optimum user interface, usually the user inserts the disposable, and then the instrument must move the interface connections to engage with the disposable.  This usually results in a large tolerance stackup between the interface and the disposable.  Adding alignment features to the disposable and the moving interface portion of the instrument (e.g. two alignment pins on the instrument and a hole and slot in the disposable) can significantly reduce the stackup and improve alignment.  Adding compliance (e.g. spring loading the interface) in the direction of motion of the interface can eliminate the effects of the tolerance stackup in that direction.   In addition, assembly jigs can be beneficial in ensuring that the connections are positioned correctly relative to each other, especially when the components are mounted on something like a PCB where holding tight tolerances on placement might not be possible.

The large quantity of connections can also result in fairly large forces that the disposable and instrument need to withstand.  For instance you might be using a 50 gram spring loaded pogo pin to make electrical connections, which doesn’t seem high, but when you need to make 200 connections the forces add up quickly.  The wide variety of connections that need to be made can also cause problems, especially when they fight each other.  For example, you might need to press on the top of the disposable to dispense a stored fluid that takes 20 lbs to dispense, but that force might cause electrical connections on the bottom to disengage.  Keeping the forces in mind early in the design can help make things easier as the design progresses.  The correct amount of compliance can be added to the interface and the connections can be laid out so they work together as opposed to fighting each other.

Another challenge with these types of projects is that often different teams/departments/companies are designing the instrument and the disposable.  Dealing with communication between the teams and working to come up with a layout that meets everyone’s requirements exacerbates some of the challenges mentioned above.  Keeping as much inter-team communication as possible is crucial.  On time critical projects even having daily meetings between teams can be worth it.  In the early stages of these projects, requirements documents or specifications are often evolving and not finalized.  At least writing interface specifications to document connection locations and other information on how the disposable and instrument will interface can help significantly.  Even if they eventually get folded into larger instrument and disposable requirements documents/specifications, having an interface specification early helps facilitate communication between teams and makes sure that everyone is on the same page.

Every project with these types of interfaces will have its own set of unique challenges.  Keeping these general challenges and lessons learned in mind though has helped us be more efficient and produce better designs when working with these complicated disposable to instrument interfaces.

Eric Schneider


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