Pressure wounds can have terrible even deadly consequences. If you have a high spinal cord injury, it's very easy to get them, and very very hard to heal. You don't want them!
Things to do when sitting in a wheelchair all day:
- Get the best possible cushion, and care for it. It's not a matter of how soft it is; what counts is the shape, how well the pressure is spread, reduced at the sitbones. Also important is the fabric around the cushion; there should be no tension on it! And for some cushions it's very important that it has support not just at the bottom, but also on all sides, to prevent tension on the surface of the cushion.
- Make sure all wrinkles are removed from your clothes, especially at the sitbones.
- Make sure there is no tension in the pants's fabric, especially at the sitbones. Tension can be worse than wrinkles!
- Tilt the chair backwards some 15 to 20 degrees. That will bring some of your weight to your back, away from your sitbones. Also will it make you sit more stable, reducing the tendency to slide forwards. A sliding kind of tension can be really bad for your skin.
- If the skin at your sitbones is red, with a sharp edge in color; stay in bed! Don't risk it, not even if you had a vacation planned.
- Driving outside can be bumpy, but I learned this is a good thing. Not too bumpy of course, but a bit of motion all the time is good for the flesh.
- EMS, Electrical Muscle Stimulation, doing that at least 4 times a week helps, it makes your muscles work, and that's definitely a good thing! Be sure to get the wide electrodes (5x10cm). There are machines that aren't expensive anymore, and pretty good quality, like this one: Sanitas SEM 40 Digital EMS/TENS. Aim for the "Gluteus Maximus"

PRESSURE RELIEVING THEORY
Pressure wounds (search for it if you dare) are really nasty and can kill you. So take good care you don't get them. Pressure wounds are wounds caused by pressure (surprise!). If you sit in a wheelchair, the pressure you have to deal with is equal to your weight. That's why I'd like to be a small person and live on the moon.
The most pressure is on the flesh between your sit bones and the cushion. Taking some pressure away there is possible, but it can't disapear, so other areas have to take over that pressure. Goal of a cushion that prevents pressure wounds, is to spread the pressure equally over a large surface. "Pressure relieving".

A fetus (foetus) inside the placenta is perfectly pressure suported, but putting me in that position would be very much Matrix like (The cocoons in that movie).

The ideal pressure relieving situation would be floating on mercury liquid metal. But that is no option, because it's too heavy, and poisonous! Also is it cold and very expensive.

The ideal material isn't especially soft, the most important for it is to have the right shape. It may be as hard as stone, al long as the cushion has the right shape. But a body is dynamic, so stone is not a good option after all. The ideal cushion has to be able to adapt to a changing shape.

Air is: light, non-toxic, isolating, and very cheap. But air weighs very little, so it doesn't lift you up like mercury does. Putting air inside an airtight bag stops the air from flowing away. A couple of problems though:
- If it gets leak, your protection is fully gone.
- The bag's material may not be flexible, because the air would then just bubble out like chewing gum. When you put your weight on an airbag, the surface tension will become very high, almost as hard as stone and NOT being flexible.
- Sitting on one airbag is very unstable, like sitting on a ball.
- An airtight bag doesn't breathe, can be sweaty.

All together air seems no option, but there are some good solutions:
- Putting the airbag in an open box, will stop the bag's skin from popping out sideways.
- Sitting on two or four airbags instead of one, will make it very stable.
- Filling these two or four bag's with tens of smaller air bags/elements, will protect you, even when a couple of elements get leak.
- Placing the bones you sit on inbetween airbags will reduce the surface tension effect to a minimum. (this only works with the model 4)
- The splits in the cushion, allows a small bit of air to circulale. Make many small holes in the box too, to improve ventilation.

This would be the ideal air element: a very flexibel but not stretching ball, filled with air, with no extra air pressure. Very smooth on the outside, and not easy to turn leak. Soap bubbbles act the way these air elements should. I don't know of a product like this, and imagine it can be pretty hard to make. The size should be some 4 cm diagonal. Silicon breasts filled with air maybe? (probably too big and expensive, and not air-filled). If any of you know of a product like this on the market, please let me know.

This illustration above is the solution that VICAIR invented, and patented. They call their air elements "SmartCells". Made out of a tube, with two seals. The SmartCells have a very smooth/slippery surface, and they do not leak air easily, so that makes them very secure to use (except for the light blue SmartCells from 2010, the newer ones are much better). Compared to the ideal airball element, the SmartCells are much less expensive to make, but have 4 sharp corners that can sometimes sting through the cushion. Not a good idea to press corners in a sensitive skin!
I do sleep on a VICAIR matras, because it's so secure and silent, but I regularly have pain from the points/bumps. What helps is placing 3D fabrics on top, and I have good experience with placing a 7 cm Tempur "NASA foam" on top.

This is the solution I found for my LaesieCushion: very flexibel though non stretching bags. These bags don't need a very smooth surface, because there's no need for the air elements to move. As they are only filled to about 20%, the air inside can move around freely. I sealed these bags myself, as I couldn't find ready ones for sale.
I have a lot more comfort sitting on them instead of the VICAIR elements, but my air elements do leak a bit air, so I have to quick-check the cushion every day.