Embedded Files
The final design is a tabletop device with a 3D printed case which houses all the components. The device uses a printed circuit board (PCB) to provide electrical connections to an Arduino Nano, 2 pressure sensors, and a piezoelectric pump. The plumbing system for the device uses plastic tubing and corresponding tube fittings to make it compatible with existing medical equipment such as epidural Tuohy needles and saline bags.
How it Works
The device operates by:
(1) Drawing saline from a reservoir and dispensing it through the device and to an epidural needle.
(2) Measuring the pressure drop within the tubing system as the needle advances through various tissue layers.
(3) Converting the pressure variations to obtain live-feed flow rate readings of saline leaving the needle.
As the needle advances from the ligamentum flavum layer to the epidural space, a significant difference in the saline flow rate can be detected. This is a clear indicator that the epidural space has been reached and is the objective of this device.
Project Performance Summary
The performance of the device was evaluated at the Simulation Training Center at UCSD Medical School. A spine model made of ballistic gel simulated the various tissue layers (skin, subcutaneous fat, and muscle) that the needle will pierce through. A silicon block running through the model simulates the ligamentum flavum and the epidural space.
Flow rate and pressure readings collected from a simulated epidural injection is graphed below:
When inserting the needle through the ballistic gel and tubing wall, the flow rate readings were found to be 1.02 ± 0.84 [ μL/s ]. The flow rate readings following puncture of the inner tubing wall (simulated entry into the epidural space) were found to be 29.7 ± 5.3 [μL/s ].
The difference in the mean flow rate between the two layers was found to be statistically significant at p << 0.001. These results indicate that the device has a high probability of accurately identifying when the needle tip has moved from a region of high resistance to flow to a region of low resistance. Initially, the flow rate drops once the needle is inserted into the ballistic gel and the pressure within the tubing builds during this time. Once the needle punctures into the simulated epidural space, the flow rate spikes and levels out as the pressure inside drops.
Plumbing System
Saline is drawn to the device through the pump inlet and outlet, to which the saline travels across the pressure sensor system and ultimately to the epidural needle. The pump is controlled by the Arduino Nano inside, and the flow rate can be set using pulse width modulation (PWM) code.
As the needle advances through a tissue layer, the device measures the pressure readings and flow rate and displays it onto the LCD screen. A significant drop in the flow rate value will indicate to the practitioner that the epidural space has been reached, and the green button can be pressed to turn off the pump.
Electronics
PCB Design
The PCB takes in power through a barrel jack connector which will power the entire device. The Arduino Nano is used as a microcontroller to control the pump and to collect data from the pressure sensors and display the flow rate onto the LCD screen. Each pressure sensor has a signal conditioning system comprised of resistors, capacitors, instrumental op-amps and op-amps used to amplify the gain and filter the measured readings.
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