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Ventile, incompressible cushions
Applied Ergonomics 1972, 3.2, 101-105
Ventile, incompressible cushions C.B. BoRon Human Engineering Division, Royal Aircraft Establishment, Farnborough, Hampshire
The idea and initial trials of dry fluid cushioning are described. Better body support, comfort and ventility (ventilation permeability), are among the advantages expected to result from this development. Some design details and possible applications are discussed.
"llus article describes some experiments with dry fluid cushions. The work was intended to develop a less uncomfortable cushion for use on ejector seats without increasing the risk of spinal injur3". It seems that the objective can be achieved. However, during the course of the work other more general uses for dry fluid cushions became apparent and it is with these that this report is mainly concerned.
2. Water and most other liquids are so mobile that the3,' make a very unstable sitting platform. 3. Water has a high heat capacity and in the 'greenhouse' of an aircraft cockpit standing m tropical sun, can reach 160°F or more. Conversely, stand,ng out in the cold it can freeze. In neither conditmn is it a good seat cushion. 4. Water cushions are h e a D .
Background and purpose The basic purpose of cushiomng is to interpose between flesh and a more or less unyielding support, a material which can vary m thickness and so reduce unevenness of pressure. The usual cushion materials are springy and compress more under the user's bony prominences than elsewhere. In such materials umt pressure increases with compression, therefore pre.vsures are higher under high points of the anatomy. It also follows that the thicker the pad the more even the pressures as the difference in the amount of compression over the cushion's surface Is reduced. "D,lere arc apphcations for whlich it may be undes,rable Ol everl dangerous to use thick, soft cushions, eg an aHcraft ejector seat where such a cushum would much increase tile hkehhood of spinal injury. This is because as the seat accelerates up its rails the man's inertia causes the cushion to compress. The man's veloctt,,' therefore tmttally increases less rapidly than that of tile seat. Since the man must stay w,th tile seat he is subjected to a peak of acceleration when the cush,on is fmall.v flattened which matches his vehlcity with that ol the seat. With a thick compressible cushion that peak is ah'nost certain to cause spinal injury. A liquid filled bag Is virtually incompre.,Lsible and is excellent for equalising contact pressures as evadenced by tile use of water cushion:> and wateI beds. llowever, there ale certain disadvantages to ware, and other liquids, as a CUShlOlll I. ~,Vater reqmres an ,mperrneable case, which Inhibit.,, ventilation and tile escape ot ~,atel vapout (sweat).
l-,lom the shortcomings of water listed above it is simple to specify requirements of an ideal cushion filler. via:
A dry, incompressible fluid which can be contained within a woven cover through which water vapour may readily permeate. The particles composing the Tluid' should slide over each other such that although they flow freely they have a higher internal friction then water. The material, as used, should be light m weight.
Possible solutions Ilavmg decided brtxadly what was needed of a ct,shton filler it seamed that pellets or balls 2 or 3 mm in diameter, and of a material that slides easily against itself might be appropriate. At that time one could buy preos,on made plastic splleres, such as were used for valves, but nothing sufficientl3' cheap. Although IA in diameter H T I - balls would ahnost certainly make an excellent dr) fluid, the cost of a cu.',hion full at £0-09 each was quite unacceptable. It was therefore decided that the feasibility of the idea could perhaps be checked using natural .seeds as filler particles, and wtnte millet was selected to start with. A cushion was made of thin can,,a.,, material m a shape to fit on top t)t a fibre glass personal survival pack such as lorms the sitting surface of some e iectol seats. 'l'he top surface of such packs is formed to a c c o m m o d a t e tile 'standard' a,rcrew posterior, rathel like a 'W, ndsoi' ohall seal. ltowever, aircrew are not at present fitted with standald b o t t o m s and so ~)lne (orIll of pad IS lequued to
Applied Ergonomics
June 1972
101
reduce discomfort. The millet filled cushion proved quite effective for this purpose. The cushion was constructed with ~ in side wags and the top and bottom surfaces were stayed at I in intervals by tie threads ¾ in long to control the disposition of the filler.
Cush~o~~
There followed an exploration of the effect of filler particles size and shape and for this purpose natural seeds were again convenient, cheap and easily available. The smallest seeds tried were rape, approx 1% mm dia., the largest tick beans, approx 8 mm dia., and the least sphencal were sunflower seeds. These were all tried in standard cushions which were used on the normal, thinly padded, tubular framed, official office chairs. The standard cushion is 16 in square by 2 in thick and has five rows of five ½ in tapes, each 2 in long, staying the two faces (similar to Fig I ). There are advantages in having the filler beads large. They are more easily contained in open fabric, permit better ventilation and less beads are needed for a given volume. The latter may be important in reducing labour costs with man-made fillers. Of the natural seeds tick beans .seemed most comfortable to most people for seat cushions.
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Initial trials it seemed appropriate to try to discover how uncomfortable bead cushions were compared with normal cushions. To this end a number of cushions were made half of which were filled with tick beans and the remainder with expanded polystyTene granules approx 4 mm dia.and weighing about 3 Ib/ft 3. The latter was the first try at a light weight synthetic filler particle. These cushions were issued in pairs to n;iive subjects, some of whom were paraplegics, who were asked to try them for three months and then to complete a questionnaire. Somewhat to our surprise a majority of users preferred bead cushions to their usual cushion - typically a foam pad about 3 in thick - and of these only one preferred the polystyrene filler (of 13 subjects 8 preferred tick beans, 3 normal cushion, I polystyrene and lundecided). Several other cushions were loaned to individuals and some interesting reports resulted. One man reported that using a standard cushion filled with pearl barley seemed to confer freedom from pressure sores, and a woman badly crippled by arthritis reported that her polystyrene filled cushion made a much more comfortable pillow than her usual foam or feather ones. These two reports started thought towards the possible use of bead cushions specifically as aids to reducing discomfort for the sick and elderly. At about this time we learned of a Danish source of small hollow polypropylene bags at a price which enabled a mattress to be tried. This was made suitable for a single bed and measured 6ft 3in x 3ft x 4in. A " f r a m e " of polyurethane foam 3in wide and 4in deep was placed in the two sides and bottom and at the head the foam was 9in wide where the pillow would lay. This framing was done mainly to reduce the volume of balls needed to fill the mattress but partly also to give it a good rectangular form. We bought two sizes of ball: I0 mm and 20 mm diameter. The mattress case was ftlled with 10 mm diameter balls and laid directly on the floor at the home of a male subject who has a badly ankylosed spine. He enjoyed good comfortable sleep and remarked on the unusually good support the mattress afforded to the hollows, eg small of 102
Applied Ergonomics
June 1972
~,1'/2{38] Fig 1 Dimensions of the ventile, incompressible cushion.
the back and behind the knees. Three other subjects, one female w~th a twisted spine, one male with an old slipped disc and one wqth a "normal" spine gave the same favourable opinion of the floor based bead mattress. Tests with the .same mattress supposted on a firm divan base and used by the same subjects yielded similar results. A minor disadvantage of the bead mattress is that on making the bed it is usually necessary to redispose the filler as with the old fashioned (obsolete?) feather bed. The same 4 in thick mattress was tried filled with the 20 mm dia bags. This was still acceptable as a sleeping pad but sometimes the hip sank far enough in to leave a single layer of bags which impinged on it and did not move. It was concluded that 20 mm was probably near the upper limit of size for mattress filler and that if it were to be used on an unyielding base the mattress would need to be deeper (4 in thick is only 5 times the diameter of the 20 mm balls).
Filler t y p e and characteristics From the foregoing some general conclusions were drawn as to filler particle size, shape and density.Obviously these parameters and the preferred type of filler will be affected by the purpose for which the cushion is being designed, and the following may be suggested at present. For plain sitting on a chair or invalid chair, tick beans seem about optimum in size. Paw polythene granules are quite a good, cheap synthetic material prowded weight is unimportant. Polystrene expanded to about 3 to b lb/ft ~ in granules about ¥16 in diameter, is acceptable for seat cushions where weight matters and crushability does not. (Expanding
to lower densmes renders the beads crushable in normal use and may be liable to cause a hot seat due to improved retention of body heat), l'his filler is good for pillows, especially for people with limited neck movement (arthritis etc), as there is no progressive compression during sleep. The shape of filler particles is relatively unimportant so long as they consist entirely of convex curvatures. Rice, peas and sunflower seeds are all usable natural products. I.-xpanded polystyrene spheroids, raw polythene or polypropylene granules are also usable provided all edges are rounded. Natural seeds are convenient, readily available and cheap to use for the evaluation of particle size and shape: but they have obvious limitations for long term use, especially in the presence of moisture.
Pad thickness This should be such that in the thinnest portion of the pad there are several layers of beads so that movement away from the pressure points may occur. If the base on which the cushion rests is itself contoured correctly to the anatomy, the cushion thickness may be reduced from that required on a plane hard substrate. It follows from the above that if a pad must be thin, the filler particles must be small. A disadvantage of small particles is an increase in the resistance to ventilation flow through the pad. For office chairs a cushion l b in square and 2 in thick with a filling of tick beans, maple peas or raw polythene granules seems quite acceptable. Appropriate adjustment of the chair seat height will of course be necessary.
Any filler should have a hard, smooth surface with as low a coefficient of friction as possible. PTFE coating is hkely to give the optimum surface.
Cushion construction The earl)" cushions or pillows were constructed as rectangular prisms: that is, ha~ang a vertical wall of fabric round the four sides of a height determining the pad thicknesss. On a square grid pattern the upper and lower
Synthetics for some uses should be of a nonflammable variety. For hospital use the cushions etc should be sterihsable.
faces were stayed by ties the same length as the wall height. This is most conveniently done with thin tape
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Fig 2 Construction details of theventile, incompressible pillows.
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F i g 3 Current design of aventile, incompressible mattress.
Applied Ergonomic~
June 1972
103
to ~ in wide sewn to the cushion fabric. On our "standard" 16 in square x 2 in thick cushions there are five rows of five ties with equal spacing between ties and ties and walls. A later version (Fig I ) shows a simpler construction omitting the separate wall but otherwise similar. Tie spacing must be proportional to cushion thickness. One to two times cushion thickness is a good starting dimension. Also, if ties get closer, webbing width must be reduced to avoid stifling particle flow. In very thin pads such as may be used on contoured ejector seat packs -- say ~6 in thick -. the ties are best made from strong twine. The spacing should not exceed 4 in even in very thick pads. The ties control the volumes of the pad and stop too much of the filler gathering in one place. A zip fastener is used in one side to enable changes in filler type and/or quantity to be made in experimental cushions.
Filling ratio This must be determined for each application. A good starting point is to put in sufficient just to fill the cushion with no bulging between ties. There must be space for filler movement but top and bottom fabrics must not meet in use. Comfort is very dependent on the filling ratio and some experimentation is essential. Once the ratio for a given filler/cushion is determined, other covers of the same volume and shape may safely receive a similar quantity (by volume) of the same Idler.
Discussion The simple idea of sitting on a bag of beans has considerable potential in a wide variety of situations. Designing the pad is not so simple as there are so many variables. Natural seeds, while useful and cheap for experimental work, have disadvantages in their liability to deteriorate or germinate and in their comparatively heavy weight. Synthetic f'dlers are undoubtedly needed for widespread use of the idea. However, there is a 'chicken and egg" situation: potentially such fillers could be made very cheaply if the demand is large, but the demand will not bc large until cheap fillers are available. It is suggested that at current prices a bead mattress would be justified for those liable to bed sores. One cannot put a price on the pain of a deep pressure ulcer, but one can say that 3 months in hospital is typical to effect a cure and such a stay would typically cost about £700. In geriatric hospitals much disturbance to patients and valuable nursing time might be saved. There is certainly no single ideal size and shape of filler particle but for use in cushions, pillows and mattresses a spheroid about 5 to 10 mm diameter would be acceptable. Some departure from the spherical shape is permissible, but both shape and size should be reasonably consistent in any one pad. The construction should be aerated for lightness; either a single bubble of a rigid foam core could be used. The outer skin should be hard, smooth and slippery and may be of the same or different plastic as the core. The plastic(s) employed in the construction should permit sterilization for hospital use and should be flame retardant. It is likely that a
104
Applied Ergonomics
June 1972
film of PTFE will provide the best sliding surface for pellets, but other means of lubrication, either contained in the plastic or applied, should not be ignored. Expanded polystyrene is good for pillows and heel cushions, but tinder heavier loadings the roughness of the surface seems to prevent the desired easy movement of the filler pellets. If a means could be found of skinning over the pellets, say by dropping them through a hot air flow, expanded polystyrene would be usable for many applications where its low softening temperature (about 70°C) is not a problem. It has the great advantage that there are many firms currently expanding vast quantities of polystyrene granules as a step in the manufacture of ceiling tiles and the like. There is still much exploring to be done in the design of padding or cushioning using dry fluid fillers. That there is benefit to be obtained from a combination of sprung and bead cushioning is shown by some enthusiastic reports from people who have taken a bead cushion with them on lengthy motor tours. For some apphcatmns it may be advantageous It) make a seat with the seat pan, back and two sides formed in one piece of. say, plastic or metal sheet and to form the top. or sitting surface, and the front from cloth. Some ties will probably be needed between top cover and seat pan but such a design may give simple construction with optimum performance. The foam frame, described earlier, provides :l somewhat smlilar effect for loose cushions at the expense ot considerably increased construction time and difficulty. h ~,eetns that the important properties of a bead pad are Its ventility, and its reduced fluidity compared with most true hquids. This reduced fluidttity is expected to ensure that unit pressures never go so high :is to cause sores but that the fluid stiction is enough to permit movement or wriggling to achieve a change of pressure profile and thus afford a change of sensation.
Suggestions and conclusions h ts proposed that a cushion fiUed with incompressible beads would make a comfortable, safe ejection seat cushion. Similarly it is suggested that ttns Idea wall yield than cushions with reduced risk of pressure sores as well as reduced discomfort to those who are torced to sit for long periods, eg paraplegics. Filled with appropriate crushable tiller particles a bead cushion might provide useful energy "absorption" m crash situations (eg gliders and helicopters)without discomfort m normal use. lJghtwelght beads could be used m pads on stretchers for Ion G term occupancy, as m casualty evacuation by air etc. Pillows and mattresses can be made especaally I~)~ infants. Suitably chosen fabrics and fillers should much reduce the possibility of smothering. It should also bc unnecessary to use rubber sheets, as svnthetic filler ~s readily washabletsterilisable and dries quickly. Thus it ,s possible to avoid the child having to lie in a pool of urine. Current designs are shown in Figs 2 .rod 3. The properties of specaal benelit to Infants apply equally to the old and to sufferers from epilepsy etc.
Thin tapering pillows have been used successfully in hospitals to avoid sore heels. The pillow is placed inside the bed, under the legs from just below the knee. Expanded polystyrene is a suitable filler for this purpose.
required and there are no mechanisms to fail or to produce noise. The only fault likely to occur is a leak'which would cause no damage and could be repaired with a few stitches.
For those with limited neck movement a bead pillow is useful as it is free from progressive flattening, lightweight filler is required as the head is not hea~, enough to displace natural Reds or solid plastic pellets.
The freedom from rubbing, due to movement permitted by compressible padding, and the permeability to air and water vapour of bead filled pads should make them useful as liners for prosthetic devices. Similarly, they could provide suitable support for burns victims, possibly with sterile air flowing up through the pad.
Bead mattresses gwe good spinal support and may therefore be advantageous in orthopeadic wards. 'The incompressibility of a bead mattres,s, in conjunction with a non-yielding base, would give a firm support for a patient who needs cardiac massage. Tests s() far made indicate that the mattress filled with 10 mm dia hollow polypropylene balls may be a substitute for water or ripple beds in the avoidance or cure of bed sores. Two such mattresses have been in use for .several months in the Regional Bedsore Unit of Dr Russell Grant, Consuhant in Physical Medicine and Rheumatology, at The Royal Hampshire County Hospital, Winchester. Patients who were admitted with bad pressure ulcers have been cured of these on specially developed water beds and then translerred to the bead mattress. None has developed pressure ulcers on the bead mattress. More recently, patients with partly cured pressure ulcers have been transferred from the water bed to the bead mattress and the cure has continued apparently unhindered. As a ripple or water bed replacement a bead mattress would have .several advantages. The case, in a suitable synthetic material, would have a long life and the filler beads should last indefinitely. No power supply is
Mattresses, pillows and cushions for boats, wath hollow or expanded synthetic pellets of suitable plastics, could be non-flammable, rot free and provide considerable flotation in entergency. In certain vibrating situations a bead cushion may reduce the amplitude of movement between man and machine. In recent trials on a five axis simulator reproducing hovercraft motion, the time a sight was held aligned on a target was approxamately ten times longer with a bead cushion than with a foam pad (Lovesey, 1971 ). Generally, it is suggested that this idea may potentially be applied wherever a ventile, incompressible, easily sterilisable, cheap, lightweight, non-flammable pad is needed.
References Lovesey, E.J. 1971 Some effects of multi-axis vibration upon weapon aiming through a binocular sight. Unpublished RAE Technical Memorandum. 0
('.B. Bohon 1972
Applied Ergonomics
June 1972
105
Ventile, incompressible cushions C.B. BoRon Human Engineering Division, Royal Aircraft Establishment, Farnborough, Hampshire
The idea and initial trials of dry fluid cushioning are described. Better body support, comfort and ventility (ventilation permeability), are among the advantages expected to result from this development. Some design details and possible applications are discussed.
"llus article describes some experiments with dry fluid cushions. The work was intended to develop a less uncomfortable cushion for use on ejector seats without increasing the risk of spinal injur3". It seems that the objective can be achieved. However, during the course of the work other more general uses for dry fluid cushions became apparent and it is with these that this report is mainly concerned.
2. Water and most other liquids are so mobile that the3,' make a very unstable sitting platform. 3. Water has a high heat capacity and in the 'greenhouse' of an aircraft cockpit standing m tropical sun, can reach 160°F or more. Conversely, stand,ng out in the cold it can freeze. In neither conditmn is it a good seat cushion. 4. Water cushions are h e a D .
Background and purpose The basic purpose of cushiomng is to interpose between flesh and a more or less unyielding support, a material which can vary m thickness and so reduce unevenness of pressure. The usual cushion materials are springy and compress more under the user's bony prominences than elsewhere. In such materials umt pressure increases with compression, therefore pre.vsures are higher under high points of the anatomy. It also follows that the thicker the pad the more even the pressures as the difference in the amount of compression over the cushion's surface Is reduced. "D,lere arc apphcations for whlich it may be undes,rable Ol everl dangerous to use thick, soft cushions, eg an aHcraft ejector seat where such a cushum would much increase tile hkehhood of spinal injury. This is because as the seat accelerates up its rails the man's inertia causes the cushion to compress. The man's veloctt,,' therefore tmttally increases less rapidly than that of tile seat. Since the man must stay w,th tile seat he is subjected to a peak of acceleration when the cush,on is fmall.v flattened which matches his vehlcity with that ol the seat. With a thick compressible cushion that peak is ah'nost certain to cause spinal injury. A liquid filled bag Is virtually incompre.,Lsible and is excellent for equalising contact pressures as evadenced by tile use of water cushion:> and wateI beds. llowever, there ale certain disadvantages to ware, and other liquids, as a CUShlOlll I. ~,Vater reqmres an ,mperrneable case, which Inhibit.,, ventilation and tile escape ot ~,atel vapout (sweat).
l-,lom the shortcomings of water listed above it is simple to specify requirements of an ideal cushion filler. via:
A dry, incompressible fluid which can be contained within a woven cover through which water vapour may readily permeate. The particles composing the Tluid' should slide over each other such that although they flow freely they have a higher internal friction then water. The material, as used, should be light m weight.
Possible solutions Ilavmg decided brtxadly what was needed of a ct,shton filler it seamed that pellets or balls 2 or 3 mm in diameter, and of a material that slides easily against itself might be appropriate. At that time one could buy preos,on made plastic splleres, such as were used for valves, but nothing sufficientl3' cheap. Although IA in diameter H T I - balls would ahnost certainly make an excellent dr) fluid, the cost of a cu.',hion full at £0-09 each was quite unacceptable. It was therefore decided that the feasibility of the idea could perhaps be checked using natural .seeds as filler particles, and wtnte millet was selected to start with. A cushion was made of thin can,,a.,, material m a shape to fit on top t)t a fibre glass personal survival pack such as lorms the sitting surface of some e iectol seats. 'l'he top surface of such packs is formed to a c c o m m o d a t e tile 'standard' a,rcrew posterior, rathel like a 'W, ndsoi' ohall seal. ltowever, aircrew are not at present fitted with standald b o t t o m s and so ~)lne (orIll of pad IS lequued to
Applied Ergonomics
June 1972
101
reduce discomfort. The millet filled cushion proved quite effective for this purpose. The cushion was constructed with ~ in side wags and the top and bottom surfaces were stayed at I in intervals by tie threads ¾ in long to control the disposition of the filler.
Cush~o~~
There followed an exploration of the effect of filler particles size and shape and for this purpose natural seeds were again convenient, cheap and easily available. The smallest seeds tried were rape, approx 1% mm dia., the largest tick beans, approx 8 mm dia., and the least sphencal were sunflower seeds. These were all tried in standard cushions which were used on the normal, thinly padded, tubular framed, official office chairs. The standard cushion is 16 in square by 2 in thick and has five rows of five ½ in tapes, each 2 in long, staying the two faces (similar to Fig I ). There are advantages in having the filler beads large. They are more easily contained in open fabric, permit better ventilation and less beads are needed for a given volume. The latter may be important in reducing labour costs with man-made fillers. Of the natural seeds tick beans .seemed most comfortable to most people for seat cushions.
1
i2{s~]
/-
%
M81457~ ~
~81457] ...,--
....
4-b
[ 1 0 2 - 1 S 2 ] z l p , ~ V
Initial trials it seemed appropriate to try to discover how uncomfortable bead cushions were compared with normal cushions. To this end a number of cushions were made half of which were filled with tick beans and the remainder with expanded polystyTene granules approx 4 mm dia.and weighing about 3 Ib/ft 3. The latter was the first try at a light weight synthetic filler particle. These cushions were issued in pairs to n;iive subjects, some of whom were paraplegics, who were asked to try them for three months and then to complete a questionnaire. Somewhat to our surprise a majority of users preferred bead cushions to their usual cushion - typically a foam pad about 3 in thick - and of these only one preferred the polystyrene filler (of 13 subjects 8 preferred tick beans, 3 normal cushion, I polystyrene and lundecided). Several other cushions were loaned to individuals and some interesting reports resulted. One man reported that using a standard cushion filled with pearl barley seemed to confer freedom from pressure sores, and a woman badly crippled by arthritis reported that her polystyrene filled cushion made a much more comfortable pillow than her usual foam or feather ones. These two reports started thought towards the possible use of bead cushions specifically as aids to reducing discomfort for the sick and elderly. At about this time we learned of a Danish source of small hollow polypropylene bags at a price which enabled a mattress to be tried. This was made suitable for a single bed and measured 6ft 3in x 3ft x 4in. A " f r a m e " of polyurethane foam 3in wide and 4in deep was placed in the two sides and bottom and at the head the foam was 9in wide where the pillow would lay. This framing was done mainly to reduce the volume of balls needed to fill the mattress but partly also to give it a good rectangular form. We bought two sizes of ball: I0 mm and 20 mm diameter. The mattress case was ftlled with 10 mm diameter balls and laid directly on the floor at the home of a male subject who has a badly ankylosed spine. He enjoyed good comfortable sleep and remarked on the unusually good support the mattress afforded to the hollows, eg small of 102
Applied Ergonomics
June 1972
~,1'/2{38] Fig 1 Dimensions of the ventile, incompressible cushion.
the back and behind the knees. Three other subjects, one female w~th a twisted spine, one male with an old slipped disc and one wqth a "normal" spine gave the same favourable opinion of the floor based bead mattress. Tests with the .same mattress supposted on a firm divan base and used by the same subjects yielded similar results. A minor disadvantage of the bead mattress is that on making the bed it is usually necessary to redispose the filler as with the old fashioned (obsolete?) feather bed. The same 4 in thick mattress was tried filled with the 20 mm dia bags. This was still acceptable as a sleeping pad but sometimes the hip sank far enough in to leave a single layer of bags which impinged on it and did not move. It was concluded that 20 mm was probably near the upper limit of size for mattress filler and that if it were to be used on an unyielding base the mattress would need to be deeper (4 in thick is only 5 times the diameter of the 20 mm balls).
Filler t y p e and characteristics From the foregoing some general conclusions were drawn as to filler particle size, shape and density.Obviously these parameters and the preferred type of filler will be affected by the purpose for which the cushion is being designed, and the following may be suggested at present. For plain sitting on a chair or invalid chair, tick beans seem about optimum in size. Paw polythene granules are quite a good, cheap synthetic material prowded weight is unimportant. Polystrene expanded to about 3 to b lb/ft ~ in granules about ¥16 in diameter, is acceptable for seat cushions where weight matters and crushability does not. (Expanding
to lower densmes renders the beads crushable in normal use and may be liable to cause a hot seat due to improved retention of body heat), l'his filler is good for pillows, especially for people with limited neck movement (arthritis etc), as there is no progressive compression during sleep. The shape of filler particles is relatively unimportant so long as they consist entirely of convex curvatures. Rice, peas and sunflower seeds are all usable natural products. I.-xpanded polystyrene spheroids, raw polythene or polypropylene granules are also usable provided all edges are rounded. Natural seeds are convenient, readily available and cheap to use for the evaluation of particle size and shape: but they have obvious limitations for long term use, especially in the presence of moisture.
Pad thickness This should be such that in the thinnest portion of the pad there are several layers of beads so that movement away from the pressure points may occur. If the base on which the cushion rests is itself contoured correctly to the anatomy, the cushion thickness may be reduced from that required on a plane hard substrate. It follows from the above that if a pad must be thin, the filler particles must be small. A disadvantage of small particles is an increase in the resistance to ventilation flow through the pad. For office chairs a cushion l b in square and 2 in thick with a filling of tick beans, maple peas or raw polythene granules seems quite acceptable. Appropriate adjustment of the chair seat height will of course be necessary.
Any filler should have a hard, smooth surface with as low a coefficient of friction as possible. PTFE coating is hkely to give the optimum surface.
Cushion construction The earl)" cushions or pillows were constructed as rectangular prisms: that is, ha~ang a vertical wall of fabric round the four sides of a height determining the pad thicknesss. On a square grid pattern the upper and lower
Synthetics for some uses should be of a nonflammable variety. For hospital use the cushions etc should be sterihsable.
faces were stayed by ties the same length as the wall height. This is most conveniently done with thin tape
t
,
P
4[102;;
\
\
~ . "~-,~
2./21641
\ \
.
\
•
19
rows
of
9
4 x 3/4[ ~ 2 • 19] topes
•
". ; •
'
•
Fig 2 Construction details of theventile, incompressible pillows.
•
.
•
I
. . . .
', ' . . ,
'= '
,
"t
,
•
• . . ,
,
.
F i g 3 Current design of aventile, incompressible mattress.
Applied Ergonomic~
June 1972
103
to ~ in wide sewn to the cushion fabric. On our "standard" 16 in square x 2 in thick cushions there are five rows of five ties with equal spacing between ties and ties and walls. A later version (Fig I ) shows a simpler construction omitting the separate wall but otherwise similar. Tie spacing must be proportional to cushion thickness. One to two times cushion thickness is a good starting dimension. Also, if ties get closer, webbing width must be reduced to avoid stifling particle flow. In very thin pads such as may be used on contoured ejector seat packs -- say ~6 in thick -. the ties are best made from strong twine. The spacing should not exceed 4 in even in very thick pads. The ties control the volumes of the pad and stop too much of the filler gathering in one place. A zip fastener is used in one side to enable changes in filler type and/or quantity to be made in experimental cushions.
Filling ratio This must be determined for each application. A good starting point is to put in sufficient just to fill the cushion with no bulging between ties. There must be space for filler movement but top and bottom fabrics must not meet in use. Comfort is very dependent on the filling ratio and some experimentation is essential. Once the ratio for a given filler/cushion is determined, other covers of the same volume and shape may safely receive a similar quantity (by volume) of the same Idler.
Discussion The simple idea of sitting on a bag of beans has considerable potential in a wide variety of situations. Designing the pad is not so simple as there are so many variables. Natural seeds, while useful and cheap for experimental work, have disadvantages in their liability to deteriorate or germinate and in their comparatively heavy weight. Synthetic f'dlers are undoubtedly needed for widespread use of the idea. However, there is a 'chicken and egg" situation: potentially such fillers could be made very cheaply if the demand is large, but the demand will not bc large until cheap fillers are available. It is suggested that at current prices a bead mattress would be justified for those liable to bed sores. One cannot put a price on the pain of a deep pressure ulcer, but one can say that 3 months in hospital is typical to effect a cure and such a stay would typically cost about £700. In geriatric hospitals much disturbance to patients and valuable nursing time might be saved. There is certainly no single ideal size and shape of filler particle but for use in cushions, pillows and mattresses a spheroid about 5 to 10 mm diameter would be acceptable. Some departure from the spherical shape is permissible, but both shape and size should be reasonably consistent in any one pad. The construction should be aerated for lightness; either a single bubble of a rigid foam core could be used. The outer skin should be hard, smooth and slippery and may be of the same or different plastic as the core. The plastic(s) employed in the construction should permit sterilization for hospital use and should be flame retardant. It is likely that a
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June 1972
film of PTFE will provide the best sliding surface for pellets, but other means of lubrication, either contained in the plastic or applied, should not be ignored. Expanded polystyrene is good for pillows and heel cushions, but tinder heavier loadings the roughness of the surface seems to prevent the desired easy movement of the filler pellets. If a means could be found of skinning over the pellets, say by dropping them through a hot air flow, expanded polystyrene would be usable for many applications where its low softening temperature (about 70°C) is not a problem. It has the great advantage that there are many firms currently expanding vast quantities of polystyrene granules as a step in the manufacture of ceiling tiles and the like. There is still much exploring to be done in the design of padding or cushioning using dry fluid fillers. That there is benefit to be obtained from a combination of sprung and bead cushioning is shown by some enthusiastic reports from people who have taken a bead cushion with them on lengthy motor tours. For some apphcatmns it may be advantageous It) make a seat with the seat pan, back and two sides formed in one piece of. say, plastic or metal sheet and to form the top. or sitting surface, and the front from cloth. Some ties will probably be needed between top cover and seat pan but such a design may give simple construction with optimum performance. The foam frame, described earlier, provides :l somewhat smlilar effect for loose cushions at the expense ot considerably increased construction time and difficulty. h ~,eetns that the important properties of a bead pad are Its ventility, and its reduced fluidity compared with most true hquids. This reduced fluidttity is expected to ensure that unit pressures never go so high :is to cause sores but that the fluid stiction is enough to permit movement or wriggling to achieve a change of pressure profile and thus afford a change of sensation.
Suggestions and conclusions h ts proposed that a cushion fiUed with incompressible beads would make a comfortable, safe ejection seat cushion. Similarly it is suggested that ttns Idea wall yield than cushions with reduced risk of pressure sores as well as reduced discomfort to those who are torced to sit for long periods, eg paraplegics. Filled with appropriate crushable tiller particles a bead cushion might provide useful energy "absorption" m crash situations (eg gliders and helicopters)without discomfort m normal use. lJghtwelght beads could be used m pads on stretchers for Ion G term occupancy, as m casualty evacuation by air etc. Pillows and mattresses can be made especaally I~)~ infants. Suitably chosen fabrics and fillers should much reduce the possibility of smothering. It should also bc unnecessary to use rubber sheets, as svnthetic filler ~s readily washabletsterilisable and dries quickly. Thus it ,s possible to avoid the child having to lie in a pool of urine. Current designs are shown in Figs 2 .rod 3. The properties of specaal benelit to Infants apply equally to the old and to sufferers from epilepsy etc.
Thin tapering pillows have been used successfully in hospitals to avoid sore heels. The pillow is placed inside the bed, under the legs from just below the knee. Expanded polystyrene is a suitable filler for this purpose.
required and there are no mechanisms to fail or to produce noise. The only fault likely to occur is a leak'which would cause no damage and could be repaired with a few stitches.
For those with limited neck movement a bead pillow is useful as it is free from progressive flattening, lightweight filler is required as the head is not hea~, enough to displace natural Reds or solid plastic pellets.
The freedom from rubbing, due to movement permitted by compressible padding, and the permeability to air and water vapour of bead filled pads should make them useful as liners for prosthetic devices. Similarly, they could provide suitable support for burns victims, possibly with sterile air flowing up through the pad.
Bead mattresses gwe good spinal support and may therefore be advantageous in orthopeadic wards. 'The incompressibility of a bead mattres,s, in conjunction with a non-yielding base, would give a firm support for a patient who needs cardiac massage. Tests s() far made indicate that the mattress filled with 10 mm dia hollow polypropylene balls may be a substitute for water or ripple beds in the avoidance or cure of bed sores. Two such mattresses have been in use for .several months in the Regional Bedsore Unit of Dr Russell Grant, Consuhant in Physical Medicine and Rheumatology, at The Royal Hampshire County Hospital, Winchester. Patients who were admitted with bad pressure ulcers have been cured of these on specially developed water beds and then translerred to the bead mattress. None has developed pressure ulcers on the bead mattress. More recently, patients with partly cured pressure ulcers have been transferred from the water bed to the bead mattress and the cure has continued apparently unhindered. As a ripple or water bed replacement a bead mattress would have .several advantages. The case, in a suitable synthetic material, would have a long life and the filler beads should last indefinitely. No power supply is
Mattresses, pillows and cushions for boats, wath hollow or expanded synthetic pellets of suitable plastics, could be non-flammable, rot free and provide considerable flotation in entergency. In certain vibrating situations a bead cushion may reduce the amplitude of movement between man and machine. In recent trials on a five axis simulator reproducing hovercraft motion, the time a sight was held aligned on a target was approxamately ten times longer with a bead cushion than with a foam pad (Lovesey, 1971 ). Generally, it is suggested that this idea may potentially be applied wherever a ventile, incompressible, easily sterilisable, cheap, lightweight, non-flammable pad is needed.
References Lovesey, E.J. 1971 Some effects of multi-axis vibration upon weapon aiming through a binocular sight. Unpublished RAE Technical Memorandum. 0
('.B. Bohon 1972
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