Author：Che-Chang Yang (2006-06-21)；recommend：Yeh-Liang
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
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
(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.
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.  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.  indicated that cushion with contour
generated by this method reveals similar pressure-relieving characteristics
compared with other types of cushions. Brienza et al.  also concluded that custom contour can be generated
using interface pressure measurements without the need of a contour gauge.
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
Generating the custom-contour foam
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
Figure 1. An example of pressure distribution at
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
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
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.
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
Figure 5. Contour generated after surface
Figure 6. Custom-contour cushion fabricated using
a CNC machine
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
Figure 7. Pressure distribution of the sphere
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
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
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.
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.
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