Scalp Skin Tensiometer
Introduction
This project describes the “Scalp Skin Tensiometer” that I have designed and built under the guidance of my mentor, Dr. Poul Nielsen, Professor at Auckland Bioengineering Institute, The University of Auckland, New Zealand.
The instrument measures and displays the tension in scalp skin around the wound area when it has been excised. It gives quantitative information of the scalp skin tension while incision is being closed and sutured, along with the displacement b/w the two edges of the incision.
Firstly, I tell you why it was required to make such an instrument. The human skin basically shows varied mechanical properties and is a complex organ in itself, which is non- homogeneous, anisotropic (non-uniformity in different directions), non-linear (has highly non-linear stress-strain relationship) and visco-elastic (time-dependent elastic behavioural) material [Source: (Hendriks, 2001)]. This behaviour gets intensified for the scalp skin, as it is effectively a membrane stretched over the skull, tethered at the periphery by muscles and tendons, especially at the brow and top of the neck. Because of this high tension, and highly nonlinear mechanical characteristics of skin (as shown in figure 1), it is frequently difficult to predict how much tension the sutures will have to bear, especially when a section of skin has to be removed. This is necessarily important for the surgeon to know because over-stressed sutured conditions may affect the wound healing, cause necrosis along edges or even facilitate dehiscence (wound breaking-open along suture) if loosely sutured. Although many researchers have developed experimental constitutive models for determining in-vivo mechanical characteristics of scalp skin and then used these models to generate
simulation programs (using finite element analysis, etc) for characterisation and optimisation of reconstructive surgery procedure in order to minimise stresses and foster healing , but these models have limitations in that they are generated from scalps of limited number of people (thus, effects of variation in individuality and age are not entirely incorporated in these models) and their results will vary depending on the extent of undermining (as more you undermine for your constitutive model, less is the holding response of the underlying skin layers and thus tension varies and hence varies the corresponding simulation model), hence different simulation models are required for different cases; generalisation is not achievable here. So, an instrument can be made that can be employed every time when surgery is being done on the individual, which will provide the tension values along with the displacement/gap between two edges of the incision.
This quantitative knowledge will be of more use to surgeons who have begun their practice lately. Also, such a setup is required in robotic surgery, which requires tactile and haptic feedback from scalp skin for proper suturing.
The instrument consists of two barbed pads that anchor to the skin on either side of the incision; mechanism by which the force pulling the pads apart is transferred to the small force transducers (Honeywell FSS-SMT force transducer) which measure the tension force; a linear bearing (P1U120120 NSK linear bearing rail and PAU12TR NSK linear bearing carriage) with locking mechanism to control the separation of pads; a linear slide potentiometer (Panasonic — EVAJQLR15B14) hooked up to linear bearing to measure displacement b/w the two pads; signal conditioning circuit- using Texas Instruments INA-126P for necessary amplification of force signals; a National Instruments USB-Based Data Acquisition System- NI USB-6009 to record the force voltage and displacement voltage measurements; and the laptop running graphical programming software- LabVIEW to process and display these signals. I also used AutoCAD software for designing the instrument. The unit is small enough to be used in a surgical environment.
Instrument Design
Here, I explain the different parts and working of the instrument. As shown in figure 2, the pad ‘A’ is made up of Acrylic and is detachable from the rest of the instrument so that it can be sterilised after surgery. 6 pins-‘I’ of length 2mm are provided with mutual gap of 1.5mm within the row and 1mm within the column, and the gap of 1st row from edge of the pad is 1mm. These pins are inclined at an angle of 45˚ from the horizontal and they go inside the skin for holding purpose. Dimensions of the pad are 20mm×6mm×1mm. Pad ‘A’ can be attached to part ‘B’ and other ‘A’ to part ‘C’ of the instrument using two bolts. Its height is kept low (1 mm) so as to allow both pads easily slide over each other. This Pad is flexible in vertical motion but stiff in horizontal. This is made so as to eliminate the unwanted effect of torque (on the force transducer- ‘H’) generated about the two bolts when skin gets displaced parallel to the wound’s axis. Also, Part ‘B’ is vertically attached to the pad ‘A’ and not horizontally so as to minimise the torque effects.
Part ‘B or C’ of the instrument is a very crucial part, as it provides the mechanism to deliver the force exhibited by skin on the pads to the Honeywell FSS-SMT force transducer- ‘H’. The design of part ‘B’ basically is a form of lever-fulcrum system with lengths ratio of 1:3 (5.5mm:16.5mm) about the hinge (fulcrum). This is done because the force exhibited by skin on the pad usually varies b/w 0–40N when skin is stretched to 0–30mm (as shown in figure 1) and the transducer that we are using has a maximum force rating of 14.7N. So, 1:3 length ratio on part ’B’ makes the operating force on the transducer go within its range. Now the base of the part ‘B’, over which transducer’s base is kept, is attached firmly to the PAU12TR NSK linear bearing carriage- ‘D’( by screwing through ‘B’ into two bolt holes of ‘D’) which provides one-dimensional motion to the pads by sliding over the P1U120120 NSK linear bearing rail- ‘E’. The linear bearing rail ‘E’ is provided with holes along its length and thus,
is mounted firmly on the part ‘C’ by screwing in through the bolt hole of the rail. The carriage ‘D’ is provided with a clamp so as to lock the sliding and allow the force exerted on the transducer to be measured at the required displacement when the clamp is locked. The voltage output from the force transducer is sent to data acquisition system for recording and display.
The parts ‘A’, ‘B’ and ‘C’ were made in the machining lab to their very accurate dimensions. The height of both legs of part ‘B’ and part ‘C’ are 25mm and 30mm, their thicknesses are 3mm and width being 6mm each. The hinge has width of 0.30mm and its two constituent concave arc forms have radius of 1.75mm each. Thus, gap b/w two legs of part ‘B’ is 3.5mm. Now, the height of force transducer is 3.76mm, so fixing it in b/w 3.5mm gap is necessary so as to aid in the measurement of slight increment or decrement of force exerted over it. The 13.7mm×5.6mm base of the force transducer is paced on the base of part ‘B’ as shown in figure, so as to allow room for soldering after installation.
The linear slide potentiometer (Panasonic — EVAJQLR15B14) — ‘G’ is mounted at the back of the acrylic plate (whose dimensions are 128mm×27mm×2mm) using bolts screwed through the holes of the acrylic (as shown in figure 2) which go and fit inside the bolt holes on the potentiometer, which (bolt holes) are already provided by the manufacturer of potentiometer on its top face. Its slider is attached to the linear carriage ‘D’ using a L shaped acrylic plate ‘J’ that screws in at the bolt holes of the carriage at one end and attaches to the slider by bolts at other end. The rail is also provided with bolt holes throughout, so it is placed at the front bottom of the acrylic plate ‘F’ by screwing in through these holes and the acrylic plate. Thus, as the carriage moves on the rail of the linear bearing, slider of the potentiometer moves adjacently and its voltage drop is recorded and calibrated using NI-6009 and LabVIEW to give the displacement value.
In short, the working takes place as this: After hooking both ‘A’ parts of the instrument into the skin around the wound edges, surgeon moves the carriage ‘D’ close to or farther from the end of the rail that is mounted to the part ‘C’ and locks it when he/she wants to measure the force at that position. So, when carriage is locked and skin edge is forcing the pad away from the wound edge, one-third of that force acts on the steel ball of the force transducer (so that operating force acts within its rating-14.7N) due to the lever-hinge system, which is measured using data acquisition system and LabVIEW, after providing necessary signal-conditioning to the voltage output of force transducers. The displacement value is being measured all through by the potentiometer, whose slider is in contact with the carriage.
Data Acquisition and Display
- Honeywell FSS-SMT Force Sensor: Being Honeywell’s smallest force sensors, these are basically low profile and low cost force sensors, which are very reliable and efficient for the medical instrumentation. They basically incorporate the wheatstone bridge principle and piezoresistivity of the silicon crystal for strain measurement. The steel ball as you see in figure 3 is very hard which provides the place for strain input. The base area of this sensor is 13.7mm×5.6mm and the height is 3.18+0.56 (steel ball) = 3.76mm. The absolute maximum operating force is 14.7 N, so I used lever-fulcrum system to input maximum of skin’s 40 N force into it. Excitation Voltage (5V and GND) is provided to it at Pins 1 and 3, from the +5V power supply screw terminal and GND terminal of the NI USB-6009 Data Acquisition Device. Now the output range (0–180 mV) and sensitivity of these sensors (ranging between 10.2–14.3 mV/N) are very low, so before we input the output signal of theses sensors to NI USB-6009, we have to provide signal conditioning to them for the required amplification. As shown in figure 4(a), the output from pins 2 and 4 of the force sensors (V+o and V-o) are inputted to the instrumentation amplifier INA-126P for the required amplification and then, the signal is inputted to the Analog Input pins 2 and 3 of NI USB-6009 for one force sensor and pins 4 and 5 for the other.
- Instrumentation Amplifier: INA-126P that I have used are Texas Instruments’ low-cost precision instrumentation amplifier for accurate and low noise differential signal acquisition. Their major applicability is in bridge circuits and medical instrumentation. As shown in figure 5, they have two op-amp design that provides better performance with low quiescent current. Its operating voltage range is 1.35V to 18V, which is fine for our 5V power supply. Its greatest advantage is that it provides a huge range of gain: 5V/V to 10000V/V which is calculated by the formula: Gain= 5+80kohm/Rg. I needed the gain of about 10V/V, so I used Rg=15kohm for 10.33gain. The connections to two INA-126P amplifiers for my project are shown in figure 4(a). It was necessary to use these differential signal amplifiers for force sensors, because NI USB-6009 doesn’t take signals of mV order of amplitude. So the output signal of force sensors is firstly conditioned using these instrumentation amplifiers and then, inputted to NI USB-6009 for data acquisition.
- Linear Taper Slide Potentiometer: As I used linear bearing rail of 120mm length and carriage of 35mm length, so I was allowed the track length for motion of 85mm. Keeping this in mind, I used Panasonic-EVAJQLR15B14 linear slide potentiometer to measure the displacement/gap between the two pads, which have total travel of 100mm. This potentiometer is hooked up to an acrylic plate [which has properly positioned-made holes (so that these holes come just above the bolt holes of potentiometer for screw-attachment) and 100mm×1mm gap for allowing slider of potentiometer to move through it] as shown in figure 2. The track resistance of this potentiometer is 10kohm and maximum operating voltage is 30V, so by using 5V power supply from the NI USB-6009 I get the change in voltage per mm displacement of about 0.05V/mm. As shown in figure 4(b), +5V and GND pins of NI USB-6009 are connected to the Vin+ and Vin- terminals of the potentiometer and the remaining one terminal of potentiometer which is giving the travelled displacement’s corresponding voltage output, is connected to Analog Input pin 8 of NI USB-6009 for data acquisition.
- NI USB-6009 Data Acquisition System and LabVIEW 2011: This is a very crucial part of whole instrumentation task. National Instruments manufactures a huge range of data-acquisition devices ranging from module-chassis based data acquisition systems with Ethernet, USB or WiFi connections which are most of the times sensor type-specific to the basic data acquisition devices which take voltage signals from any sensor. To keep the budget of the project in limit and to understand how to make user-defined LabVIEW VI programs for any task, I chose to use NI USB-6009 Data Acquisition Device (over module-based data acquisition systems) which is the most basic and cost efficient data acquisition device that takes voltage as input. Now I chose NI USB-6009 among other basic data acquisition devices because as shown in figure 4(a) and 4(b), I needed just two differential signals (from two force sensors, after these signals were conditioned using INA-126P) and one single-ended signal (from potentiometer) to be acquired, give +5V and GND connections to these devices (force sensors and potentiometer), provide reasonable resolution to input signals of atleast 10bit and aggregate sampling rate of atleast 10kS/s. This was efficiently achievable with NI USB-6009 as it provides 8-Analog Input pin terminals which can be used as both differential and reference single ended (RSE) terminals, both +5V and GND connections, 14-bits resolution for differential signals, 13-bits resolution for single-ended signals and maximum aggregate sampling rate of 48kS/s. Resolution and sampling rate are important parameters for the selection as they basically determine the efficiency of analog-to-digital conversion in the data acquisition device, before the signal is transmitted to the computer USB ports. As the process of analog to digital conversion consists of converting a continuous time and amplitude signal into discrete time and amplitude values, optimum resolution and sampling rate are required to efficiently reconstruct the signal digitally. Thus, NI USB-6009 caters to all needs of my project task and hence used here. Now as shown in figure 4(a) and 4(b), the required connections of the setup are made and then USB cable connecting to NI USB-6009 is connected to the USB port of the computer having NI LabVIEW and NI-DAQmx driver software installed in it. Now, we open the Measurement and Automation Explorer from All Programs>National Instruments and see that NI USB-6009 is being displayed under the Devices and Interfaces tab. Right clicking the device name, we get an option to have ‘self-test’ which when left-clicked, tests if the device is ready to use or not. Then, from the Softwares tab, LabVIEW 2011 is chosen. After configuring the NI USB-6009 device and making the required VI for it on LabVIEW, we run the program and as the pads attached to skin are moved close to or away from each other (alongwith providing locking at every step where we want to get tension reading display), we get the corresponding force and displacement measurements’ display on LabVIEW VI. Figure 8 shows the Front Panel and Block Diagram of the VI that I made and used for the project.
Conclusion
In the course of my research project, I came to understand how to tackle the real-world biomedical challenges using bio-instrumentation. I have learnt to make 3D designs using AutoCAD, make VI programs on LabVIEW, understood the working of USB-based data acquisition devices and basically learnt how to use these devices and softwares to make instruments of bio-medical usage. The instrument that I have made, under the guidance of my mentor Dr. Poul Nielsen, is easily reproducible, easy to handle in surgical environment and very cost-effective too as it is made of common materials, tools, devices and software. The instrument will be of more use to surgeons who have begun their practice lately. This setup can also be employed in robotic surgery, which requires haptic and tactile feedback from the scalp skin while suturing.
Thank you for interest in the blog. Please leave comments, feedback and suggestions if you feel any.
