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AuthorChe-Chang Yang (2006-06-21)recommendYeh-Liang Hsu (2006-07-04).
Note: This paper is published in Biomedical Engineering, Applications, Basis, and Communications, Vol. 21, Iss. 1, pp. 9-16, February 2009.

Development of a Rapid Prototyping System for Custom Contoured Foam Cushion Using the Pressure Mapping Method

Abstract

Prevention of pressure sore is an important health care issue for people with limited mobility. To relieve pressure at buttocks, foam cushion enveloping specific shape or contour matching with an individual’s buttocks has commonly been employed. This paper presents a complete rapid prototyping system to generate custom contoured foam cushion using pressure distribution at the buttock-cushion interface. A pressure sensor mat is placed on top of a flat polyurethane (PU) foam to measure the pressure distribution at the buttock-cushion interface. Based on the load-deformation characteristics of the foam material, this pressure distribution is converted into the corresponding deformation of the foam. This conversion is followed by a surface smoothing process to generate a bi-cubic surface, which approximates all discrete data points. ASCII NC codes are then generated for the precise CNC fabrication of the cushion with the desired surface contour. The shape and pressure-relief performance of the custom contoured foam cushion is validated. This system is completely automated and provides a quick and economical way to fabricate custom contoured foam cushion.

Keywords: pressure sore, custom contoured cushion, pressure mapping method

1.     Introduction

Decubitus ulcer (pressure sore) is one of the most common sitting-induced problems caused by prolonged sitting and unrelieved pressure. For people with limited mobility such as the wheelchair users, prevention of pressure sore is an important health care issue. Cushion filled with fluid or soft bulk material has been developed for this purpose. Cushion filled with air or gel in interconnected grid cells provides satisfactory effect of pressure relief [Hong et al., 2002]. However, it does not provide sufficient sitting stability and is relatively expensive. On the other hand, foam made from polyurethane has a combination of visco-elastic and resilient properties, which give abilities to hold, or “memorize” the buttock’s shape while providing sufficient dynamic stability through resiliency [Brienza et al., 1999]. Compared with fluid-filled cushion, foam cushion provides similar performance in pressure relief at a relatively lower cost.

To further relieve pressure at buttocks, foam cushion enveloping specific shape or contour matching with an individual’s buttocks has commonly been employed [Chow et al., 1978]. There can be a uniform pressure distribution over the resilient foam with a pre-contoured shape [Springle et al., 1990]. A comparative evaluation also indicated that the contoured cushion has more uniform pressure distribution and lower peak pressure compared to the pressure distribution and peak pressure of flat foam cushion [Chung et al., 1987]. In order to produce such a matching shape, pressure and shape measurements for the buttock-seat interface are used clinically to evaluate and prescribe support surfaces for wheelchair users. Brienza et al. [1993] developed the computer-automated seating systems (CASS) for this purpose. The CASS has the ability to adjust the seating surface shape by measuring the pressure applied to the buttocks. The device comprises an 11×12 force sensing array of spring-loaded elements coupled with linear potentiometers. Pressure sensors were mounted on the swiveling head atop of each element. The deflection (height) of each element can be independently controlled to vary the loading conditions and surface shape in evaluating soft tissue deformation. Both pressure distribution and deflections measured are incorporated altogether to generate an optimized custom contoured shape.

There is an alternative method, the so-called “pressure mapping method” to approximate the shape of an individual’s buttocks only by using the pressure distribution at the buttock-cushion interface. The intuited hypothesis is that the pressure distribution at the buttock-cushion interface could be used to generate a contour equivalent to that obtained by deflection sensors. Contour generation with this approach yields more economical and simplified instrumentation. A preliminary study conducted by Carmen et al. [2000] indicated that cushion with contour generated by this method reveals similar pressure-relieving characteristics compared with other types of cushions. Brienza et al. [1996] also concluded that custom contour can be generated using interface pressure measurements without the need of a contour gauge.

This study presents a complete rapid prototyping system to generate custom contoured foam cushion using the pressure mapping method. A pressure sensor mat (BPMS, Tekscan Inc.) is placed on top of a flat polyurethane (PU) foam to measure the pressure distribution at the buttock-cushion interface. Based on the load-deformation characteristics of the foam material, this pressure distribution is converted into the corresponding deformation of the foam. This conversion is followed by a surface smoothing process to generate a bi-cubic surface, which approximates all discrete data points. A spherical object (a basketball filled with air) is used to validate the contour generated by this process. ASCII NC codes are then generated for the precise CNC fabrication of the cushion with the desired surface contour. The pressure-relief performance of the custom contoured cushion fabricated using this process is evaluated. The whole process is completely automated, which is a more economical, effective and convenient technique to fabricate custom contoured foam cushion for clinical or residential uses.

2.     Generating the custom-contour foam cushion

In this study, the Body Pressure Measuring System (BPMS, Tekscan, Inc.) was used for pressure measurement. In BPMS, the sensor (SEATMAT #5315) is a flexible pressure sensor mat with a dimension of 622mm(length)×530mm(width). With 42×48 sensing points distributed over an area of 488mm×427mm, this pressure sensor mat has a sensing density of 0.97 per square centimeter. The pressure sensor mat is connected to a handle for signal amplification, A/D conversion, and transmitting the measurement data to a PC. The graphical pressure contour and numerical information are then displayed in the PC. Figure 1 shows an example of pressure distribution at buttocks obtained by BPMS, in which the subject sits on a 75 mm in thick flat foam cushion.

MATLAB Handle Graphics

Figure 1. An example of pressure distribution at buttocks

The visco-elastic foam (memory foam) made from polyurethane (PU) was used in this study. Small foam samples with dimensions of 60mm×60mm×75mm were tested on a universal testing machine to determine their load-deformation characteristics. The foam sample deforms with the distributed pressure load applied onto it. Figure 2 shows the relation between load and the corresponding deformation. In this figure, two materials (high-resiliency foam and memory foam) exhibit similar characteristics. For pressure below 0.1 kg/cm2, the pressure-deformation relationship is almost linear. The amount of deformation at 0.1 kg/cm2 is about 50mm (note that the thickness of the foam samples was 75mm). Beyond this pressure, the deformation of the foam saturates, and dramatically higher pressure is required to produce the same increments of deformation. The average pressures at buttocks obtained from the subjects recruited in this study ranges from 0.04kg/cm2 to 0.06 kg/cm2. In addition, less than 10% of the sensing points are greater than 0.1 kg/cm2. Therefore, most pressure sensing points at the buttock-cushion interface are within the region of linear deformation in Figure 2.

Figure 2. Load-deformation characteristics of two foam materials

The load-deformation relation in Figure 2 can be represented by a function D(p). The pressure contour obtained in Figure 1 is then transformed into the deformation contour of the foam using the load deformation relation D(p). Assume P is a 42×48 matrix, which represents the raw measurement data at the pressure sensing points when the subject sits on a flat foam cushion. Each component pij in P can be converted to the corresponding deformation D(pij) at that point to form a deformation matrix D. It is also assumed that the buttock is a rigid body compared with the foam, and most deformation occurs at the foam. Therefore D represents the shape of the buttock. Figure 3 shows the deformation contour obtained from the pressure distribution in Figure 2.

MATLAB Handle Graphics

Figure 3. Deformation contour converted from the pressure distribution in Figure 2

As shown in Figure 3, the deformation contour directly converted from the raw measurement data at the pressure sensing points contains many discontinuous fragments, which are far from a realistic surface. Therefore an algorithm is designed to smoothen the deformation contour. The strategy is to reduce the size of the matrix D, while the smooth surface generated from the reduced matrix should retain most characteristics of the original matrix. As shown in Figure 4, in this study, the 42×48 matrix D is reduced to a 7×8 matrix D’. Each component in D’ contains the average of the neighboring 6×6 sub matrix from matrix D. Finally a bi-cubic surface is generated to approximate the smooth surface using the data points in D’, as shown in Figure 5. ASCII NC codes are generated for this bi-cubic surface by discretizing the surface (or using a standard CAM software). Custom-contour cushion can then be fabricated using a CNC machine (Figure 6).

Figure 4. The 42×48 matrix D is reduced to a 7×8 matrix D’

MATLAB Handle Graphics

Figure 5. Contour generated after surface smoothing

Figure 6. Custom-contour cushion fabricated using a CNC machine

3.     Validation of the shape and pressure-relief performance of the custom contoured foam cushion

The purpose of the process presented in this paper is to generate a contour on the foam cushion that completely matches the actual shape of a subject’s buttocks, which should result in the preferred effect of pressure-relief. It is difficult to actually measure the shape of a subject’s buttock. Therefore in this section, a sphere (a basketball filled with air) of diameter 240.3mm is used to validate whether the surface generated by this process actually matches the shape of the original object. The sphere was initially placed on a flat foam to yield a pressure distribution as shown in Figure 7. An external force of 120N was exerted. Following the process presented in the previous section, this pressure distribution was then converted to the smooth bi-cubic surface as shown in Figure 8. Note that the sphere is rigid enough that deformation of the sphere is neglected throughout the test.

MATLAB Handle Graphics

Figure 7. Pressure distribution of the sphere

MATLAB Handle Graphics

Figure 8. The smooth surface generated from the pressure distribution in Figure 7

Figure 9 shows a cross-sectional view of the sphere and the deformed cushion surface obtained in Figure 8. The dashed line at the zero depth level represents the flat plane of the foam cushion. The maximum depth, which reflects the maximum pressure is identified at point C. This point is used as a common reference point for the sphere and the deformed cushion surface, by which the common center of circle can be identified. The distances Dn, which are the differences in radii between the sphere and the deformed cushion surface, are calculated for 187 discrete points in two orthogonal directions on the deformed surface. A sphere with radius R which best fitted this deformed contour is calculated by examining the minimum standard deviation STD in Dn. There exists a minimum standard deviation STD=5.18mm in Dn when the radius of this sphere is R=127.15mm. Therefore, the difference in radii between the real sphere and fitted sphere is 7mm.

Figure 9. Cross-sectional view of the sphere and the deformed cushion surface

Figure 10 compares the pressure distribution of a subject sitting on a flat foam cushion (Figure 10 (a)) and that of the same subject sitting on a custom contoured cushion fabricated by the process described in the previous section (Figure 10 (b)), and Table 1 shows the comparison of the test data. The contact area in the buttock-cushion interface rises by 12.8% in the custom-contour cushion, and the peak pressure and average pressure decreases by 22.8% and 11.2%, respectively. Moreover, the standard deviation of the pressure data on the 42×48 sensing points decreases by 16.4%, which indicates a more evenly distributed pressure over the buttock-cushion interface.

 

Figure 10. Pressure distributions on flat and contoured foam cuhion

Table 1. Comparison of contoured and flat foam cushions

 

Contact area

Peak pressure

Ave. pressure

STD

Flat cushion

1639.22cm2

0.1484kg/cm2

0.0481kg/cm2

0.0256kg/cm2

Custom-contoured cushion

1849.80cm2

0.1146kg/cm2

0.0427kg/cm2

0.0214kg/cm2

Results

+12.8%

-22.8%

-11.2%

-16.4%

4.     Conclusions

This paper presents a complete rapid prototyping system to generate custom contoured foam cushion using pressure distribution at the buttock-cushion interface. The shape and pressure-relief performance of the custom contoured foam cushion is validated. This system is completely automated and provides a quick and economical way of custom fabricate custom contoured foam cushion.

Acknowledgement

This research is supported by the National Science Council, Taiwan ROC, grant no. 93-2622-E-155-001-CC3, and by SEDA Co. Ltd. This support is gratefully acknowledged.

Reference

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