Designing living creatures is hard, so I came up with a generalized idea to speed up the process in the future:

Biological padding is a spongy structure made of spider silk and filled with either methane or ammonia. It's lightweight, offers protection against blunt force trauma and seemingly circumvents the negative consequences of the square-cube law. It supposed to be located between the hypodermis and the muscle layer:
Cross-section of skin

But is this possible? Is it safe to place ammonia or methane there, or it'd damage the silk or be extremely risky? Could it work as intended?

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    $\begingroup$ Spider silk works well under traction, not under compression. And why adding methane or ammonia when you can just use air? $\endgroup$ – L.Dutch - Reinstate Monica Aug 18 '18 at 18:46
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    $\begingroup$ @L.Dutch I just want it to be lighter. $\endgroup$ – Mephistopheles Aug 18 '18 at 19:05
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    $\begingroup$ Quick point: the square cube law is a mathematical construct. Choice of material only really alters when you start to notice it’s effects. $\endgroup$ – Joe Bloggs Aug 18 '18 at 22:12
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    $\begingroup$ What problem is your biological padding trying to solve? Do you have a bunch of creatures walking around like the robot from Big Hero 6? Does this deploy like an airbag? $\endgroup$ – Cort Ammon Aug 19 '18 at 3:39
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    $\begingroup$ You need a lot of gas to act as an effective defense against blunt force. If you want a reality-based, biologically-based "spongy structure", then... look to sponges!!! en.wikipedia.org/wiki/Sponge $\endgroup$ – RonJohn Aug 19 '18 at 10:17

The problem with using a gas - any gas - is that gases are compressible. So if someone punched, bit or stabbed your creature the gas would be squashed down in front of the blow, and not be as great a protection as you want it to be.

Also gases are really good at shifting elsewhere within a container. Think about holding a long, thin balloon by the middle. If you clench your fist, the air in the middle rushes away from your hand and makes the ends of the balloon bulge out. Anything inside the middle of the balloon would be crushed by your grip, and not protected by the gas at all.

Methane and ammonia have biological problems: there are a whole host of micro-organisms which would love to feed on those gases (methanotropes and ammonia oxidisers). You have just created a lovely environment for those to live in. Therefore you've given your creature's immune system a massive problem to keep the bacteria populations of those gas pockets under control.

Finally spider silk, as pointed out by L.Dutch in comments, is not that great at resisting compression. No-one has ever cut themselves on a strand of spider silk. Most things will punch or pierce their way straight through it.

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