**12. Creation of a novel epidural simulator**

406 Practical Applications in Biomedical Engineering

**Figure 17.** The syringe connected to the computer as an input device.

**Figure 18.** Binary serial data transfer protocol.

update according the pressure applied by the operator's thumb on the plunger of the syringe. This has the advantage that the user can control the visualization with the same equipment that would be used in-vivo, which is a more natural interface than simply using keyboard or mouse. Additionally, since the saline line separates the hardware device from the needle, the user can move the needle around since it is attached only by the saline line.

The hardware device runs at 8MHz. Data is transmitted from the hardware device to the computer using the serial RS232 port. The serial bit rate is running at 22000 bits per second. The serial data transfer protocol uses -12V DC as a positive bit and +12V DC as a negative bit. The serial transfer cycle starts with a negative start bit, followed by 8 data bits sent consecutively and finished with a positive stop bit. As shown in Figure 18, the following

The 8 data bits are received and interpreted as binary and converted into a decimal number from 0 to 255 for use in the software. The decimal value represents the pressure of the saline between 0 to 70 kPa, which is 0 to 550 mmHg. The 256 possible values give an accuracy resolution to within +/- 0.14 kPa. This can be easily increased to 1024 with 10 bits data transfer which will then provide accuracy of within +/- 0.03kPa. The speed could also increase beyond the current 22000 bits per second but it does not seem necessary since no delay is noticed between pressing the plunger and seeing the results on screen. Currently at 22000 bits per second the time delay between bits is 45μS so the start bit is identified by

start bit can then occur either immediately or after a pause of arbitrary length.

The presented biomedical engineering ideas have enabled us to develop a simulator with a combination of engineering, computing and clinical technologies as discussed in previous sections above. Data from the developed measurement devices have been used to configure a realistic force feedback epidural simulator [25]. Numerous improvements have been identified that could enhance existing epidural simulators. Manikin models are generally static and only able to represent one or two patient variations, such as normal and obese. An advanced simulator would be able to simulate insertions on a variety of body mass indices because excess fat deposition has the potential to generate very different changes in patient characteristics.

The developed system offers a virtual reality based epidural simulator (Figure 19) incorporating a 3D graphically modelled spine complete with skin, fat and tissue layers, supraspinous, interspinous ligaments and ligamentum flavum. In the current prototype, a Novint Falcon haptic device is used in combination with a Portex LOR syringe connected as a human-computer interface via a custom made electronic serial interface. As the haptic stylus is moved, the needle follows on the screen in 3D in real time. When pressure is applied to the plunger by the operator's thumb, this is displayed in the graphic model. As the needle is advanced through the tissues, the forces are generated by the haptic device to reconstruct the feelings of needle insertion through each tissue layer. The forces of the needle insertion are based on the recorded forces measured during the clinical trial, and this data based approach is more accurate than previous simulators which have used a user evaluation approach to configure the forces.

Novel aspects of our epidural simulator include stereo graphics, modelled vertebrae, spine flexibility, patient variation, haptic force feedback based on measured needle insertion data, custom made syringe interface. The simulated needle can be inserted at any spinal position

Biomedical Engineering in Epidural Anaesthesia Research 409

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**Figure 19.** Prototype 3D graphics epidural simulator with haptic device interface

from T2 – L5 and needle direction from midline to paramedian. The 3D graphics allow a close-up real time view of the needle internally during insertion. The virtual patient can adjust to various body shapes, weights and heights since body size considerably affects insertion force. These all have roots in biomedical engineering that can potentially enhance many clinical procedures.
