Senior Design Project
Project Background

As a final test of the engineering material that my fellow undergraduate students and I had learned through our college years, we were assigned groups and sent out to tackle an engineering problem chosen by a sponsor. Naturally, project ideas were presented to us through sponsors willing to invest in the projects, and we were allowed to choose those that we were interested in before groups were decided. We had the opportunity to present our own project ideas if we could supply funding, but there were a few available options that captured my interest, particularly the self-locking syringe problem.

The problem of syringes being re-used by other people, which spreads disease among millions of people per year, was one that my design group wished to solve by changing the mechanism within cheap plastic syringes to prevent them from being used more than once. Our group of four members met multiple times per week to discuss design ideas and make decisions, following a Gantt chart that we had designed in our first meeting. Gantt charts are essentially detailed calendars which predict the time that each task will take to complete, giving them each a sufficient amount of extra allotted time, and ensuring that everything is done before deadline if followed properly.

We began by determining the exact goal of the syringe that we were to design and by doing a bit of research into existing syringes including other attempts to make locking syringe mechanisms. Preemptively, we decided that we would most likely apply for a patent on our final design if it were feasible to manufacture cheaply and worked well. The generalized end goal, as dictated by our sponsor, was to design a syringe that could compete in price, manufacturability and usability with generic syringes, but with a mechanism that would prevent the syringe from being used more than once.

Detailed Description

Our brainstorming led to three designs, two of which were based on the same concept. Our first and least plausible design took advantage of hinged plastic “teeth” produced with the rest of the syringe in a single mold from ductile plastic. The teeth were to be preloaded during manufacturing against their natural state, then flip back into their natural state when the syringe was pulled back by any amount and lock behind a wall in the syringe’s plunger, preventing it from being pulled back. This idea was difficult to manufacture and relied on multiple extremely small, unreliable moving parts for it to function well with high repeatability, as we desired in our final product.

Of our other two designs, which were similar in structure, we settled on the third and most physically complex and chose to develop it further. Our final design incorporated a slot mechanism into the plunger wall to constrain its motion along a set path, with a peg protruding from the chamber wall into the slot to act as a cam follower. As the plunger is drawn back by the user, the track pushes the follower tangentially to the syringe wall which forces the plunger to rotate which shifts the peg into a new motion track and barring it from re-entering the initial track.

This is done through the use of a ramped barrier which allows the follower to move into the new motion track with ease by sliding past its incline and slightly deforming the chamber like a spring, as the plastic that it’s formed from is thin and flexible, but then prevents the follower from shifting back over the blunt end of the ramp. If the plunger is fully reinserted into the chamber after it’s been pulled past the ramp so-as to expel all of the liquid within it, clips that became aligned with their holes during the rotation of the plunger fall into place, preventing the plunger from being pulled back out.

We found that we could produce this syringe for around 1.36 times cost of producing generic syringes, which is currently around four cents per combined set of plunger and chamber. Later in the project, we also considered the cost of producing molds to be used in the manufacturing of our syringes as part of ensuring that our design was feasible to manufacture with existing technology. The last step in the senior design project required that students present their final product and design process in a poster-style exhibit.

Upon presenting our sponsor with a satisfactory final design, our work was completed, however we decided not to pursue a patent for our design. My own reason for making that choice is that while our final design is worlds better than our original design, and better in many ways than existing attempts at solving the syringe problem, I was not satisfied with how our mechanism performed. I have since done some of my own conceptualizing and modeling of other feasible mechanisms, which are already superior to those that we came up with within our project thanks to the time that we spent troubleshooting and prototyping.

If I settle on a satisfactory design that I believe can be manufactured easily enough to be considered in real products, I do plan to move forward with applying for a patent. I will be revisiting this project in the next year or so but postponed it along with many others to prepare this portfolio. Overall, my senior design experience taught me about realistic design workflow and time management and allowed me to work with an enthusiastic and capable group and contemporary technologies to find an innovative solution to a real problem.

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