CHRONIC leg ulcers affect 1% of the adult population in

Proceedings of the 2011 IEEE International Conference on Mechatronics and Automation August 7-10, Beijing, China Measurement of Interface Pressure Applied By Medical Compression Bandages Jawad Al Khaburi, Abbas A. Dehghani-Sanij, E. Andrea Nelson and Jerry Hutchinson Abstract Medical compression bandages (MCBs) are widely used to treat venous leg ulcers. Interface pressure measurement devices like PicoPress R transducer are used occasionally to measure the pressure applied by MCBs to train nurses to apply MCBs with the correct pressure profile to patient s leg and for quality assurance purposes. However, most of these devices provide only single point pressure measurement. Some of them like the Medical Stocking Tester (MST) and the Oxford Pressure Monitoring MK can provide multiple points of pressure measurement simultaneously. However, the current available systems cannot map the pressures applied by MCBs to a leg at 16 or more points simultaneously. Flexible sensors like FlexiForce R seem to provide a good alternative to current pressure measurement system. This paper will report on the work carried out to compare the pressure measured using FlexiForce R force sensors with the pressures measured using PicoPress R, which is a pneumatic transducer, and FS01 force sensor, which is an ultra profile rigid MEMS force sensor. In addition, it will compare the pressures measured using the three sensing transducers with the pressures computed from the amount of extension in the MCB, which is applied to the medium which these sensors are attached to. Index Terms Ulcers, Compression, Pressure Measurement, Pico- Press, FlexiForce, Force Sensor I. INTRODUCTION CHRONIC leg ulcers affect 1% of the adult population in developed countries, and the majority of leg ulcers are due to venous disease [1], [2]. Venous ulcers impact on the quality of life is significant, and it costs the NHS 300-600m annually [3]. Medical compression bandages (MCBs) are the cornerstones in the treatment of chronic venous ulcers [4]. MCBs should be applied with an optimum pressure profile of 40 45mmHg (5.3 6kN/m 2 ) at the ankle, reducing gradually to 15 20mmHg (2 2.7kN/m 2 ) at the knee [5], [6]. These values have been agreed by experts rather than through systematic study [6]. Insufficient or non-sustained compression therapy will be less effective than sufficient and sustained compression due to an impaired hemodynamic effect [5], [6]. Excessive bandage pressure can lead to tissue damage, pressure sores and necrosis [5], [6]. Reverse gradient compression is likely to worsen the condition as it increases This work was supported by ConvaTec Limited, Deeside, UK and the corresponding author was sponsored by ConvaTec Limited and Ministry of Higher Education in Sultanate of Oman. J. Al Khaburi was a PhD student in the School of Mechanical Engineering, University of Leeds, LS2 9JT, Leeds, UK (Corresponding author phone: 968 92154894, e-mail: j.alkhaburi@gmail.com) A. A. Dehghani-Sanij is with School of Mechanical Engineering, University of Leeds, LS2 9JT, Leeds, UK (e-mail: A.Dehghani@leeds.ac.uk) E. A. Nelson is with School of Healthcare, University of Leeds, LS2 9JT, Leeds, UK (e-mail: e.a.nelson@leeds.ac.uk) J. Hutchinson is with ConvaTec Limited, CH5 2NU, Deeside, UK (email: jerry.hutchinson@convatec.com ) 978-1-4244-8115-6/11/$26.00 2011 IEEE 289 the pressure in the veins [6]. Limb damage or treatment failure may result in limb amputation [5]. A number of interface pressure measurement systems have been developed in the last few decades to help researchers study the effectiveness of various types of MCBs [7], train nurses and clinicians to apply MCBs with the required pressure profile [6], [8] and for quality control purposes in clinics [9]. PicoPress R (Microlab Electronica, Ponte S. Nicolo PD, Italy), Air-Pack Type Anlayzer model 3037 (AMI Techno, Tokyo, Japan), Kikuhime R (TT Medi Trade, Soro, Denmark) and SIGaT R (Ganzoni-Sigvaris, St Gallen, Switzerland) are examples of pressure measurement transducers which are currently used by researchers and clinicians [10] [12]. Most of these devices can provide a single point pressure measurement. Some of them like Medical Stocking Tester (MST) (Salzmann AG (SAG), ST. Gallen, Switzerland) and the Oxford Pressure Monitor MK II (Talley Medical Group Ltd, Hants, UK) can provide pressure readings at number of points [13], [14]. However, the latter devices were found by other researchers to be inaccurate [10], [14]. In addition, all the mentioned devices fail to meet the probe thickness specifications recommended by experts in the field of compression bandages and venous ulcers to avoid the problem of overestimating the pressure [15] [18]. In fact, no evaluation of the impact of these transducers probes thickness on the interface pressure has been reported in the literature. Previous experimental investigation by the authors found that there is a significant effect of the sensor thickness on the measured interface pressure [18], [19]. Flexible sensors have been around for number of years and they have been utilized in a number of biomedical applications. Some researchers have utilized FlexiForce R force sensors (Tekscan, Boston, USA) to measure the pressures applied by medical compression stockings to healthy legs [20], [21]. However, they have not evaluated the errors and uncertainties associated with such transducers, knowing that many researchers have shown in their published work that Flexible force sensors like FlexiForce R and FSR R (Interlink electronics, Camarillo, USA) suffer from number of deficiencies and cannot be used for accurate measurements [22] [24]. This paper reports on the work carried out by the authors to assess the feasibility of using FlexiForce R force sensors to measure the interface pressure applied by MCBs to a dummy leg by comparing FlexiForce R measured pressures when MCBs are applied to a rigid uniform cylinder to the pressures reported by PicoPress R, which is a pneumatic transducer, and FS01 (HoneyWell International Inc, New Jersey, USA) force sensor, which is a thin rigid MEMS force sensor, and pressures computed from the amount of extension in the MCB

fabric which produced that interface pressure. II. MATERIALS AND METHOD A. PicoPress R PicoPress R is a portable pneumatic measuring system fitted with an ultra thin probe. The probe thickness is 0.2mm when it is not inflated and 3mm when it is inflated and no pressure is applied to it. The probe diameter is 50mm. Before the measurement, the probe is inflated with 2cc of air by means of electronically controlled syringe integrated in the system [10]. The transducer can be calibrated under the bandage [10]. The sensor was found to have a nonlinearity error of 1.09 ± 0.18%FS, a hysteresis error of 1.77 ± 0.49%FS and a repeatability error of 0.90 ± 0.19%FS, where the sensor full span (FS) is defined as 16kN/m 2 [19]. Two of the transducer s probes were attached to the cylinder used in the setup at Levels A and B of the cylinder (see Fig. 1). The distance between levels A and B is approximated to the distance between the ankle and just below the knee which is normally covered by the MCB when nurses apply MCBs to venous ulcers patients. (Equation 1). However, the model proposed by Vinckx et al. [17] is used in this reported work to estimate the pressure perturbation when PicoPress R sensor is used to measure the interface pressure applied to a cylinder, which is used in the apparatus, as no other mathematical model is currently available to estimate this perturbation. The estimated value of perturbation was, then, used to apply a correction factor to the pressure values measured by PicoPress R (Equation 2). C pp = sin(α/2+γ) sin(α/2) α = D S R R γ = arccos( R + d ) Where, C PP is the coefficient of pressure perturbation, R is the limb radius in meters (m), d is the sensor thickness in (m) and D S is the sensor length or diameter in (m). (1) Correction Factor = 1 C PP (2) Fig. 1. Arrangement of the sensors used in the experiment. C. FS01 Force Sensor FS01 Force sensor is a low cost, peizoresistive-based force sensor. It is a high-level voltage output, calibrated, and temperature compensated sensor and it gives an accurate and stable output over a 5 C to 50 C temperature range. It can be operated from a single 5V DC supply [25]. The sensor is about 25 17 8mm. The sensor was found to have a nonlinearity error of 2.89 ± 0.43%FS, a hysteresis error of 0.97 ± 0.26%FS, a repeatability error of 4.65 ± 0.71%FS, where the sensor FS is defined as 14.6kN/m 2 [26]. Four of these sensors were embedded in the cylinder used in the setup at Levels A and B, where each cluster contained four sensors (see Fig. 1). The sensors were calibrated using an aneroid sphygmomanometer on the same cylinder used in the experiment. D. FlexiForce R Force Sensors B. PicoPress R Correction Factor FlexiForce R ultra-thin (0.203mm) flexible sensor is constructed of two layers of substrate film with silver conductive The authors previous investigation showed that material applied over the substrates, followed by a layer of PicoPress R pressure sensors will overestimate the pressure applied by the bandage due the thickness of the sensor which creates a local curve over the measured area which subsequently result in overestimating the pressure [19]. Vinckx et al. [17] proposed a mathematical model (Equation 1) to estimate the amount of perturbation in the measured pressure caused by flexible thin plate sensors when these sensors are placed under a pressure garment applied to a curved surface like the lower limb. Experimental results published by the authors [18] confirmed that increasing the thickness of the sensor will result in overestimating the pressure and increasing the sensor length will reduce the overestimation problem. Nevertheless, the experimental data did not fit with the mathematical model proposed by Vinckx et al. [17] pressure sensitive ink. Applying force to the sensitive area of the sensor results in a decrease in the resistance of the sensor and increase in its conductance. The sensor comes in three force ranges with the lowest range of force is 0 4.4N [27], which means that the smallest range of pressures the sensor can be utilized to measure is 0 57.7kN/m 2. The sensor was found to have a nonlinearity error of 4.17 ± 0.53%FS,ahysteresis error of 3.27±0.81%FS, a repeatability error of 8.07± 1.62%FS, where the sensor FS is defined as 14.1kN/m 2 [26]. Sixteen FlexiForce R sensors were distributed on the cylinder in four clusters at Levels A and B of the cylinder (see Fig. 1). All sensors were calibrated individually using an aneroid sphygmomanometer over a cylinder with the same dimensions as the cylinder used in the experiment. 290

E. Computing Pressure from Extension Many researchers have used Equation 3 (below) to estimate the interface pressure from the tension developed in MCBs [28] [30]. Equation 3 was based on Law of Laplace, which is defined as the tension in the walls of a container being dependent on both the pressure of the container s content and its radius [28], [29]. The authors have revised the model (Equation 3) and proposed an improved mathematical model (Equation 4) to overcome some of the limitations of Equation 3 [31]. Equation 4 is used in this work to convert the tensions developed in the MCB into the equivalent pressures. However, it is very difficult to measure the tension developed in the bandage while it is applied to the limb or in this experiment to a cylinder. Nevertheless, when a force is applied to the MCB, it results in extending the MCB. If the MCB tension-extension relationship is known, the tension developed in the bandage material can be calculated by measuring the extension and then converting the measured extension values into the equivalent tension values. P = T (3) R Where P is the interface pressure in (N/m 2 ), T is the tension in bandage in (N) for (1m) width of fabric and R is the curvature radius in (m). T (D + t) P = 1 2 wd2 + wt(d + t) Where P is the internal pressure in (N/m 2 ), T is the tension force in (N), D is the limb diameter in (m), t is the bandage thickness in (m) when it is extended and w is the bandage width in (m) when it is extended. An Instron 4301 (Instron, High Wycombe, UK) was used to measure the tension developed in two types MCBs while they were extended at constant speed of 100mm/min. A 100N load cell was used to measure the tension in the MCBs. Data obtained were used to obtain the 4 th order polynomial fittingline for the 1 st cycle loading side of the tension-elongation curve for the two MCBs used in the apparatus. MATLAB R2009b (The MathWorks Inc, Massachusetts, USA) was used to obtain the 4 th order polynomial fitting-line. Details of the method can be found in [32], [33]. In this experiment, the extension in the MCB used was measured at eight different locations: four sides of both level A and B. F. Mechanical Rig The rig was composed of a cylinder, with a 0.114m diameter and 0.55m length and a wooden base Fig. 2. A slot, oriented 8 with the vertical axis of the cylinder, was designed in the cylinder wall. The slot was used to fix the bandage to the cylinder using medical textile based tapes and to enable applying the bandage using a spiral wrapping technique with 50% bandage layers overlap. (4) Fig. 2. The rig used in the experiment G. Condition Circuits, Data Acquisition and Pressure Measurement Routine FS01 sensors were powered using a processing circuit which supplied the sensors with constant 5V and the FlexiForce R sensors were powered using a condition circuit which powered the sensors also with constant 5V, filtered the sensors output signals using a hardware low-pass filter with cut-off frequency of 10Hz and amplified the output filtered signal. The output signals from the two processing circuits were passed to a Mass Term 6225 USB DAQ card (NI, USA) through a screw terminal board (LPR-68, NI, USA). A program was written in LabView 8.6 (NI, USA) to acquire and display the signals, convert them to the equivalent pressure values, perform averaging for FlexiForce R pressure values, display the measured pressure values for FS01 and FlexiForce R sensors and the averaged pressure values for FlexiForce R sensors numerically and store the voltages, the measured pressures and the average pressures for further processing. The signals were sampled at 1kHz and a software based 2 nd order low-pass filter with 10Hz cut-off frequency was used to filter out the signals. H. Measuring the Extension and Pressure: Prof. E.A. Nelson (a trained bandager) was asked to apply eight pre-marked MCBs for two different types to the cylinder with a constant pressure of 5.3kN/m 2. Pressure measurements were acquired throughout the bandage application. After applying the MCB, PicoPress R transducer was used to obtain the pressure at levels A and B. Pressure readings for each PicoPress R measurement point was obtained three times and then averaged to report the pressure. The amount of extension was recorded using a measurement tape at eight points: four sides of the two levels A (ankle) and B (Below knee) (Fig. 2). MATLAB R2009b was used then to compute the theoretical pressures from the measured extension values. MATLAB R2009b and Excel 2007 (Office 2003, Microsoft, USA) were used to compare the theoretical and measured pressure values. The average computed pressure for the four sides at levels A and B was used to report the computed pressure for 291

Fig. 3. The computed and measured pressures for each bandage application. On x-axis: I is a symbol for the first MCB type, II is the symbol for the second MCB type, A refers to level A, B refers to level B and the number refers to the bandage number each bandage. The average of the three subsequent measurements III. RESULTS AND ANALYSIS for each PicoPress R transducer was used to report the Fig. 3 illustrates the average pressures computed and mea- PicoPress R measured pressure. A correction factor of 0.58 sured at levels A and B for each MCB application. Results was also applied to the PicoPress R averaged pressure. The show that at both levels A and B, computed pressures, average of two FS01 sensors located at each level was used to corrected PicoPress R measured pressures, FS01 measured report the pressure measured by FS01 for that particular level. pressures and FlexiForce R measured pressures are much The average of the two clusters of four FlexiForce R sensors closer to each other compared to pressures measured using at levels A and B was used to report the pressure measured PicoPress R transducer. by FlexiForce R sensors for that level. Table I demonstrates that the PicoPress R corrected measured pressures and the pressures measured using I. Statistical Analysis: FlexiForce R sensors are within the tolerance values for agreement (±667N/m 2 )in50% of the cases. The agreement is Percentage frequency counts were used to check the levels of agreement between: higher between the corrected PicoPress R sensors measured pressures and FS01 sensors measured pressures (56.25% of the pressures measured using PicoPress R sensors and the the cases). The highest levels of agreement was found to be pressures measured using FlexiForce R sensors, between FS01 and FlexiForce R sensors (68.75% of the cases). the pressures measured using PicoPress R sensors after In addition, the levels of agreement between the computed applying the correction factor and the pressures measured pressures and pressures measured using FS01 sensors is higher using FlexiForce R sensors, than the levels of agreement between the computed pressures the pressures measured using FS01 sensors and the pressures measured using FlexiForce R sensors, and the pressures measured using FlexiForce R sensors, which is in turn higher than the levels of of agreement between and the pressures computed from extension and the the computed pressures and the pressures measured using pressures measured using FlexiForce R sensors. PicoPress R transducer after applying the correction factor. the pressures measured using PicoPress R sensors and the pressures measured using FS01 sensors, TABLE I the pressures measured using PicoPress R sensors after PERCENTAGE FREQUENCY COUNTS FOR THE LEVELS OF AGREEMENT BETWEEN THE DIFFERENT METHODS OF PRESSURES ESTIMATION, WHERE applying the correction factor and the pressures measured THE LEVELS OF AGREEMENT IS DEFINED AS THE DIFFERENCE BETWEEN using FS01 sensors, ANY TWO METHODS TO BE LESS OR EQUAL TO ±667N/m 2 the pressures computed from extension and the pressures measured using FS01 sensors, Corrected Computed PicoPress FS01 FlexiForce PicoPress the pressures computed from extension and the pressures Computed 100.00 6.25 43.75 56.25 50 measured using PicoPress R transducer, PicoPress 6.25 100.00 0.00 0.00 0.00 and the pressures computed from extension and the Corrected pressures measured using PicoPress R transducer after 43.75 0.00 100.00 50.00 50.00 PicoPress applying the correction factor. FS01 56.25 0.00 50.00 100.00 68.75 In each test the measured pressures were said to be in agreement if the difference between them was within ±667N/m 2. FlexiForce 50.00 0.00 50.00 68.75 100.00 Excel 2007 was used to carry out the statistical analysis. These values indicate that even though FlexiForce R sensors 292

suffer from low accuracy, they not only report less error in pressure readings than PicoPress R sensors, they also produce pressure values that are more in agreement with the computed pressures and the pressures measured using FS01 than PicoPress R sensors even after applying the correction values, given that FlexiForce R sensors are used in arrays i.e. the pressure reading is for the average of number of FlexiForce R sensors and not for a single sensor; as taking the average measurement reading of number of sensors will reduce the error in the pressure measurement. It might be argued that in the absence of a gold standard for measuring the interface pressure [6], [26], there is no proof that FS01 force sensors provide more accurate pressure measurements than PicoPress R for example. The counter argument is that having computational pressures, measured pressures using FS01 and FlexiForce R sensors, both calibrated using aneroid sphygmomanometer, and corrected PicoPress R measured pressures all close to each other, and all much lower than the pressures measured using PicoPress R sensors without correction, suggests that those values which are in agreement are much more likely to report the actual pressures than PicoPress sensor without correction. These results indicate also that the medical pneumatic transducers like PicoPress R transducers might not provide accurate pressure measurement even after using a correction factor. However, it was previously reported that the model, which is used to calculate the correction factor [17], does not agree with experimental findings [18]. 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