Final Design

Main Assembly Overview

Figure 1: CAD of Final Design

The final design solution consists of a customized off the shelf control box, a custom Handheld Flow Control Device, an NE-550 Syringe Pump and a Transpac IV medical-grade disposable pressure transducer and is outlined in the figure above. The custom control box houses the system’s: Arduino Mega microprocessor, main circuitry, NE-550 syringe pump and user interface hardware (screens and buttons which allows the user to navigate the on-screen menus and make selections). The functions of the modules of the system can be described as follows:

·  The Control Box is used to: parameterize the procedure, display real-time injection pressure signal from the transducer, display volume of drug injected by the syringe pump, and to establish communication between the syringe pump and handheld flow control device.

·  The Custom Handheld Flow Control Device houses the flow rate control sensor, a joystick potentiometer, in an ergonomic handheld design. Additionally, it is designed so that a Braun 17 Gage Needle can be snapped on to the front and connected to the system tubing. This allows the anesthesiologist to manipulate the needle, which is injected into the patient, and control the drug flow rate with a single hand. It also houses a tare button that allows the user to zero the injected drug volume and set a reference injection height for pressure sensing purposes.

·  The NE-550 Syringe Pump is an off-the-shelf unit that can actuate syringes of various sizes for infusion and withdrawal at a variety of flow rates and is controlled by the Arduino.

·  The Transpac IV Pressure Transducer is an off-the-shelf disposable medical-grade pressure transducer distributed by ICU Medical. It was originally designed for intravenous pressure monitoring and adapted for in-line pressure sensing in our design. An analytical model was developed such that the pressure at the outlet of the needle could be calculated based on a pressure reading from the transducer at an arbitrary location in the fluidic system.

Figure 2: Procedure Block Diagram

Individual Component Overview and Design Solutions

Control Box

• Houses the NE-550 Syringe Pump

• Displays live pressure readings in units of psi up to 0.1psi precision

• Displays live injected drug volume readings up to 0.01cc precision

• Incorporates easy user interface through ergonomic button placement and straightforward menu system

• Houses reliable connections for pressure transducer and handheld flow control device

• Securely houses microprocessor and other circuitry

Figure 3: Control Box Overview

Syringe Pump

• Holds a single syringe

• Compatible with syringe sizes ranging from 10cc to 60cc

• Analytic fluid model  restricts maximum infusion rate to 1cc/s

• Capable of communicating with Arduino Microprocessor

• Capable of being externally controlled

  • Flow rates are  variable and capable of changing during a procedure

• Infusion and withdrawal capability

Figure 4: NE-550 Syringe Pump (from www.syringepump.com)

Handheld Flow Control Device

• Integrates snap-on snap-ff feature for rigidly securing a Braun 17 Gage Tuohy Needle for injection and removal from patient

• Needle is held in the device in such a way that it allows the anesthesiologist to see blood backflow upon withdrawal

• 5 inches long and 1.1 inches in diameter

• Houses a potentiometer joystick, that can send a signal to the Arduino Microprocessor. This signal controls the flow rate output by the syringe pump

  • Sensor is self centering

  • Syringe Pump flow rate output varies with degree of travel of sensor

• Device houses a button in the rear of the device in such a way that it cannot be accidentally pressed. This tare button sends a signal to the microprocessor which zeros the injected volume readout and sets a reference height for pressure purposes. 

Figure 5: Custom 3D Printed Handheld Flow Control Device Overview

Pressure Transducer

• Pressure signal is sent from the transducer to the Arduino microprocessor in real time

• Transducer is calibrated with a 2nd order polynomial function in a range of -1psi to 30psi with an average accuracy of 0.78%

  • Linear range of transducer is -1psi to 6psi

• Measurements must be repeatable and accurate such that research can be done with the completed device

• Transducer is capable of interfacing with the Arduino Mega Microprocessor

• Transducer is disposable (Market Price ~$6 depending on health care institution)

Figure 6: Transpac IV Disposable Pressure Transducer Overview (http://www.icumed.com/products/critical-care/pressure-monitoring-system/transpac.aspx)

Analytic Fluid Model Pictorial Overview

Figure 7 shows an overview of the components of the fluidic system. The syringe pump and handheld device are not shown for simplicity. Figure 8 shows an overview of the Extended Bernoulli Equation used in the analytical model of the fluid system. The fluidic system incorporates this model to calculate the pressure at the point of injection based on a pressure reading from the Transducer, which is placed at the outlet of the syringe. In order to use the model, it was necessary to parameterize the terms in the Extended Bernoulli Equation. The area term, which describes the loss, or gain or pressure due to the area difference between the p calculation point (P2) and the pressure sensing point (P1) is parameterized by measuring the components directly. Furthermore, the flow rate of the system is determined by the pumping rate, a value which is obtained from the syringe pump software.

Figure 9 shows the logic and method used to determine the height difference between P2 and P1. Essentially, once a point of injection is chosen, the handheld is placed at the same height. This creates a column with fluid aligned with the earth's gravitational field which creates a constant pressure difference due to height difference between P2 and P1. As such, a signal is sent once the handheld device is aligned with the point of injection so that this pressure difference due to height is recorded and accounted for. Once the procedure is underway, the height of the device is static enough that the errors due to vertical displacement of the handheld are negligible (less than 3% over 0.5m).  

Finally, the head loss terms account for up to 98% of the pressure difference between P2 and P1 when there is a flow of drug through the system. The head loss sources are primarily the medical tubing and the needle. Component data that is necessary to analytically determine the head loss due to each component as a function of flow rate is unavailable. It was therefore necessary to experimentally determine flow rates. An experiment was run such that data could be collected and Figure 10 shows the collected data. The data is significant because it constrains the maximum flow rate that would allow accurate pressure sensing at the point of injection up to 20psi to be 1cc/s. 

Figure 7: Fluidic Setup Overview

Figure 8: Analytical Model Overview

Figure 9: Height Component Determination Overview

Figure 10: Head Loss Term Determination Experimental Results, Head Loss Vs. Flow Rate (cc/min)