Home : Case Studies : Mechanical Intravenous Infusion Pump

Summary: A prior vendor had created a design for the pump, but had been unable to demonstrate a working prototype. Convinced that the design would only function with production materials, prototype tools were constructed but the pump still did not work. Creative Engineering was approached by the client and asked to debug the design and get it working. We determined that the design was fundamentally flawed and offered to redesign it completely. We did so and the pump is now on the market. In redesigning the pump we were able to improve the performance, use less exotic resins in its manufacture, and introduce many aesthetic, ergonomic and functional improvements to the product.

Prior Design: A constant force spring was used to deliver about 16 lb. of force to a custom shortened plunger in an otherwise standard 60 cc syringe. In a fashion similar to other products on the market the fluid was then passed through very small diameter tubing in order to choke the flow to the desired rate. The constant force spring (S) was concentrically wound with a metal tape (T) very similar to that found in common tape measures. The use of the metal tape resulted in a unit that was very compact, except for the fact that the entire syringe projected away from the housing. An escapement-type governor mechanism was used to prevent the unit from running too quickly under no-load conditions.

Problems: The main problem with this design was that the spring had to deliver the full 16 lbs of force at the outer diameter of the pulleys. Such a spring is quite thick, heavy and exerts such high forces that the housing was having trouble containing it and the winding mechanism would not function. Other issues:

• The escapement governor was unreliable and very loud
• The design required tight manufacturing tolerances and glass-filled resins
• The syringe was very vulnerable to being knocked off the unit
• A standard IV set could also work with the device (with dangerous consequences)
• The winding torque was too high
• The winding handle (much like a fishing rod) was awkward to use

New Design Solution: Working with another mechanical designer, we came up with a solution that used a much lighter (but longer) constant force spring along with a pulley and cable system to provide the ratio needed to get the output up to 16 lb.

In our improved design a standard syringe is used. The cable (C) is driven by a small diameter pulley, which multiplies the force of the constant force spring (S). We introduced many other improvements, such as a custom leur lock connector design keyed to the pump housing to ensure that only the proper IV set is used, and an innovative "tension sensing" swing gear which disengages the governor gear train (not shown) during a run in order to ensure smooth drug delivery. Winding the pump is simplified because the lighter spring allows the main pulley to be wound directly and we implemented a large easy to grasp knob rather than the winding handle of the previous design.

The images shown here are 4 iterations of the process. Missing from the sequence is the SLA and Urethane Cast parts stage wherein we provided working cast samples that accurately represented the production product. The first image shows the earliest Proof Of Concept model which was constructed and tested on a flow bench. This prototype demonstrated the validity of our design and gave our client the confidence to proceed to the next step, which was to design and prototype all of the other subsystems and consider the form factor and packaging issues. The Fully Functional Breadboard Model was built based on the mechanical layout developed in Pro/ENGINEER and demonstrated all of the functional aspects of the design. More refinement followed ultimately resulting in a complete CAD database ready for tooling, debugging and production.