hands-free syringe pedal
Project Highlights
The Team
I served as product designer on this foot pedal project with assistance from a machinist and electrical technician.
The Pedal Work
Research
Reasearch indicated that utilization of doctors' feet to manipulate the fluid delivery device would prove very beneficial, freeing their hands to accurately place the needle and guide the ultrasound probe.
While many foot pedal designs already existed, our choices were limited for a disposable application.
Concept development
Brainstorming began.
Standout concepts
Some great ideas came out of the brainstorming sessions, many of which were explored further with Solidworks mock ups or additional sketches.
Plastic pedal
Pedal has integrated living hinge for pedal resistence and spring back which was molded into lower section.
Also, battery compartment could be molded into lower section.
Pedals would be sold in pairs (aspirate & infuse) with syringe and are designed to pack efficiently together. They are also lightweight to reduce shipping costs.
Parts are designed to be vacuum formed for low cost manufacturing.
Rolled up plastic tube
I had particular fun with this one.
Step one: Print conductive parallel zigzag ink circuits on plastic film and punch mechanical interlock tabs in opposed long edges of sheet.
Step two: Roll up plastic sheet film into a tubular shape.
Step three: Use punched tabs to interlock and hold plastic in a circle.
Result: Separate circuits now pass over each other, but still don't touch.
Crushing plastic tube with foot causes zigzag circuits to touch. Remove foot and plastic sheets "memory" makes pedal go back to its tubular shape and circuits no longer touch.
Round foam
People loved the way foam felt as it compressed under your foot. The compressive resistence of the foam felt natural, a desirable attribute.
Insulation foam with a hole in it worked well and allowed space for a potential internal switch mechanism.
This concept iterated quickly with idea of color coded sides for easy identification: green for infuse and yellow for aspirate, mimicking traffic lights (i.e., green = GO and yellow = CAUTION).
This concept iterated quickly again with cable exiting through one end cap on one end.
The hollow piece of foam just needed an internal switch mechanism to activate when the foam compressed. Maybe a pressure switch, lightbeam, pressure film or click switch?
I've always been drawn to the simplicity of tactile switches: a great design that is both cheap and super reliable with decades of industry exposure - just what I needed.
So, I explored this vision further and, after creating some napkin sketches, my design took shape - tactile switches on a printed circuit board.
Left side of board runs motor clockwise "aspirate" circuit. Right side counterclockwise "infuse" circuit.
The circuit board would be slid into the tube on center with a deliberate force fit. This force fit causes the foam to distort and become oval as shown. The circuit board edges depress the foam which holds it in position.
This assembly method allows the board to "float", avoiding extreme bending and breakage when pressed.
Self Righting Pedal
This ovality means the circuit board won't sit perpendicluar to the ground when dropped during deployment. Even if not exactly parallel to the floor, as you depress the pedal the circuit board promotes rotation until the switches are facing either up or down.
Uncompressed switch "OFF" Foam in relaxed expanded state facing either up or down.
Compressed "ON" Switch works either way up. Foot compresses foam first then the foam forces click switch(es) to activate.
Foam tube would have hard plug ends to allow doctor to feel when is foot is at each end of pedal without looking.
Prototyping
Crude protypes were made of the systems components to get a touchy feely handle on things.
Living hinge pedal
Rolled up plastic film pedal
Print circuits on plastic with conductive ink and roll up and interlock tab. Extra circuit for redundancy.
Crush the tube with your foot and the circuits are forced in contact with each other: switch is closed and motor runs.
Release foot and electrical circuit broken when resilient flattened plastic snaps back to tube shape. An unplanned bonus of the design was the crackle of the plastic being crushed and snapping back to original its form gave the pedal a nice form of audile feedback.
Tubular Foam pedal
A mockup circuit board was made up with fibre glass (G10) and the switches hard-wired in parallel. Plastic plugs were machined on lathe and glued into the ends of the foam. The foam was sprayed painted with rubberized paint for flexibilty to simulate colored foam.
This prototype was mocked up just to see how having the wire wound in center for storage would work. Pedal diameter had to be big to make it effective. Large foam diameter was not ergonomically friendly for foot.
The final contender by knockout decision
The tubular foam pedal was a hands down winner. People loved the way foam felt as it compressed under your foot. The progressive resistence felt natural.
Design for function manufacture
Different diameter and density foams in different colors were played with.
Design for manufacture
How to build the circuit board and switches
Pedal end caps plugs will be molded with barbed ends for interfence fit and glued permanently in position.
The final design for assembly and low cost production in China.
Testing
Pedal was subjected to short and long term routine functionality and had no problems. To test robustness I tried to crush it with a Volvo, and to drown it.
It survived.
What I learned
Designing and manufacturing high volume low cost reliable medical products requires lot of empahsis on marrying both product design for function but also optimization for manufacture and assembly. Saving ounces of plastic on one part amounts to tons on thousands of parts.