Working principals behind an infusion pump
What is an infusion pump?
An infusion pump is a medical device that is used to administer a steady flow of medications and fluids to a patient over a set amount of time.
In the hospital setting, infusion pumps are commonly found in Intensive Care Units (ICUs), Neonatal Intensive Care Units (NICU) and Surgical and Oncology Centers.
Different infusion pump involves different mechanism in liquid transfusion, such as syringe pump which involves a piston mechanism while a balloon-pressure elastomeric pump would utilise pressure from a balloon to control liquid flow rate.
For our project, we will be exploring the use of peristaltic pumps (which contains rollers that compresses the tubing) to control the liquid flow rate.
How peristaltic infusion pumps work
- Fluid reservoir: between 50ml – 1000ml
- Catheter / Plastic tubing: delivers fluid from reservoir to the patient; can be as long as 48” (~1.2m)
- Peristaltic pump
- User interface
- To program & monitor fluid delivery
Commonly used infusion liquids
- Common infusion solutions:
- Aqueous solutions of glucose (5 cg/g) (for treatment of carbohydrate & fluid depletion)
- Aqueous solutions of sodium chloride (0.9 cg/g) (also known as saline, used to treat dehydration)
- Aqueous solutions of sodium chloride (0.45 cg/g) (used for patients where there is an incompatibility of drugs to be administered / hypertensive patients)
- The above has similar viscosity and density to that of water, hence our pump which we calibrate using water as the reference liquid would be mainly applicable for the above solutions.
- For other commonly used infusion liquids that has much higher density and/or viscosity than water (such as Hespan® and Dextran 40® which are 4 times more viscous than water), this is an area we can explore in the future as our pump is mainly calibrated to the viscosity and density of water.
Precision & Accuracy of infusion pump
The accuracy of an infusion pump is based on how close the actual flow rate is to the programmed flow rate. Peristaltic infusion pumps used in hospital and ambulatory setting generally has an accuracy of ±2.5 – 6%, with a maximum flow rate of 250ml/hr.
Bicycle powered generator (to be checked)
- Presence of copper coils between 2 sets of magnets in the white component of above picture
- Pedaling rotates black gear which in turn causes magnets in white component to spin as they are connected
- Rotation of magnets result in a changing magnetic field which induces a current in the copper wires which can be used for recharging reusable batteries
Infusion pump delivery mechanisms – Stepper motor or traditional DC motor
Stepper Motor
DC motor
Traditional DC motors are much easier to code with. Only requires PWM to regulate the amount of current going into the motor to affect how fast it will spin and therefore the flowrate.
//Therefore, similar to the stepper motor, we will also be testing a variety
To further increase the accuracy and reliability of the DC motor, a PID tuning system.
PID : Proportional, Integral, Derivative.
These 3 aspects work together to ensure the accuracy of the of pump.
Proportional takes into account the current speed of the motor to decides whether it is enough to push the required flowrate
Integral looks at the performance and data from the start of operation, and then decides whether it is necessary to further tune and adjust the flowrate.
Lastly derivative aims to look into the further based on the current and past information and performance. This allows for the system to preemptively make any chances it might need before the any errors occur.
A PID controller requires a feedback mechanism, which will allow for some sort of output value to be recording, therefore allowing for a relation between the output and input, which can be detected by the arduino. A rotary encoder will be used in this project to fulfill this rolew. Essentially, it is a very simple device that records the amount of rotations made by the pump and motor. Based on the specifications of said rotary encoder, we are then able to deduce the angular velocity of the motor too! Combined with experimently achieved results, we are able to make now draw a relation between angular velocity of the motor shaft with the flowrate of the pump.
