Executive Summary

Figure 1 Final design of the pedal controlled syringe.

The pedal controlled syringe gives anesthesiologists complete control of their procedure. The current project builds on the concept developed from previous devices of automated injection and aspiration  and extends to incorporate a pedal interface that will allow for the operator control of the rate of injection and aspiration. The device is compact enough (about the size of a laptop)  to easily be implemented at the operating room and ease the jobs of physicians. The device can also accommodate and secure various syringe sizes from different manufactures. A pressure transducer is then used to both easily connect to a built-in monitor displaying the pressure and encoded for automatic shut off of the system at 270 mmHg, an exceedingly high and unsafe pressure for patients.

            After going through a different number of designs and prototyping, the final design can be seen as a CAD model in figure 1. The syringes are loaded inside the syringe holder and attached to the plunger holder which is controlled by a motor powered lead screw. Many types of designs were considered such as a spring-pulley system and linear actuators. Although those designs had their benefits, such as precise location control with linear actuators, the size of the final design would not be ergonomic. The configuration with the lead screw gives the device a very fine control over the speed. The speed requirement was 1 mL/min to 25 mL/min, and depending on the size of the syringe, varies from 1mL/min to 40mL/min. This configuration also provided for the necessary torque of 0.9 Nm. For precautionary reasons, the stall torque of the motor chosen is significantly larger than the required torque at 1.8 Nm.

As for the foot pedal layout, the original 3 pedal layout was simple and precise but a number of layouts were considered. A tilting pedal and strap pedal was considered to seamlessly and intuitively move the syringe forward and reverse. Although it would be very beneficial to have an intuitive design which anyone can pick up, safety is the prime aspect to consider. In the end, the original 3 pedal design was the correct choice although it was altered from the original proposal from the sponsor. This design consists of one potentiometer pedal which controls speed, a latching foot pedal switch to switch the control of the syringes, and another latching switch to control the forward and reverse direction of the motor. 

To provide system feedback to the user, LEDs were added to the top syringe pump to clearly specify settings at any given time of both syringe selection and direction. The entire user interface ,including some of the LEDs, are angled upwards towards the operator to give the user a better view. An LCD monitor is also on the angled base that reads out pressure for the user to monitor. One glaring issue that kept arising was the loading and unloading of different  sized syringes. Since the loading and unloading of syringes would require a specific distance of the plunger holder, it would be very tedious to manage solely with foot pedals. To counteract this, a manual button control was added to the design. This gives the user manual control when loading  new syringes of different sizes without the necessity of using a foot pedal.

Considerations in quality of material and feasibility of machining were inputted to the structure of the system. An aluminum frame supported  the required loads and  also composed the plunger holder and syringe end support. Delrin was used as the primary material for the casing of the whole design as well as the layout of the pedals.  Testing was performed by injecting different fluids (water, applesauce mixture, tomato juice, and yogurt) from both the 10 ml and 20 ml syringes. On average, the component was able to inject/expel 4 ml of fluid per second. Testing yielded above satisfactory results and was given very positive feedback from the sponsor.

The final product met all the requirements that were asked for, but in order for this product to be commercially ready, a number of components may need to be redesigned.  For instance, the plunger holder may need to be redesigned to incorporate a releasing mechanism off of the lead screw to make it easier to move around. Other safety alterations can be considered such as an ejection mechanism to release the syringe in case of any errors detected by the pressure transducer or the encoder. Another handy addition would be a motorized valve controller which would make it easier for the user to switch syringes without needing to manually switching the valve him or herself. Prototyping and development of this product was completed at a cost of about $1075.