TL;DR:
Soft robots are built from flexible materials such as silicone and move by deforming, often by inflating chambers with air.
Pneumatic force follows
F = P × A: gauge pressure times area. One side that stretches more than the other makes the finger bend.They are gentle on delicate objects, but pressure still has to be limited and vented, because burst chambers and trapped air are real hazards.
A metal gripper with a hard fingertip can crush a strawberry or a ripe tomato unless you program the force carefully. A gripper made of soft silicone fingers wraps around the fruit and spreads the force over a larger area. That idea, letting the body of the robot do some of the work, is the heart of soft robotics.
What soft robots are
A soft robot is built largely from materials that bend and stretch, such as silicone rubber, fabric, and thin plastic films. Instead of rigid links joined by motors, it moves by changing the shape of its body. Because the body gives way when it hits something, the robot adapts to different object shapes without needing exact control.
Pneumatic actuators
The most common soft actuator is pneumatic: it uses air pressure. A finger is moulded with hollow chambers inside. When air is pumped in, the chambers expand. If one side of the finger is made stiffer or less stretchy, such as with a fabric layer or a thicker wall, the more flexible side stretches more and the finger curls toward the stiff side.
The basic physics is simple. A gas at gauge pressure P (pressure above the surrounding air) pushes on a surface of area A with a force F = P × A. Gauge pressure is what a typical pressure gauge reads. Absolute pressure is gauge pressure plus atmospheric pressure, about 101 kPa at sea level.
Worked example: an air-powered soft gripper
Take a two-finger gripper. Each finger has an inner chamber whose inside wall area is 6 cm², which is 6 × 10⁻⁴ m². A small pump supplies a gauge pressure of 20 kPa, which is 20,000 Pa.
Total force of the air on the chamber walls:
F = P × A = 20,000 × 0.0006 = 12 N.That 12 N is spread over the chamber, pushing in all directions. Only part of it becomes bending force at the fingertip, because the finger's own stiffness takes some, and the geometry turns it into a curl. Say, for this example, a grip force of 2 N per finger results. Measure yours with a small scale or force sensor.
Check against a load: a 0.15 kg fruit weighs
0.15 × 9.8 = 1.47 N. Two fingers with friction coefficientμ = 0.6can hold up to2 × 0.6 × 2 N = 2.4 N, which exceeds 1.47 N by a margin of2.4 / 1.47 ≈ 1.6. That holds the fruit.Raise the pressure to 40 kPa and the chamber force doubles to 24 N. The grip rises too, but so does the strain on the silicone. There is a limit set by the material and the manufacturer's rated pressure, not by what the pump can do.
Notice how the grip force is much smaller than the raw P × A figure. Soft actuators are inefficient in force terms, and usually slow. What they offer in return is compliance: if the object is 1 cm wider than expected, the fingers just curl a little less, with no sensor or control needed.
How much air is needed? If the finger's chambers must grow by 5 mL to close, a pump that delivers 50 mL per second fills them in 5 / 50 = 0.1 s, in theory. Hoses, valves and the stretch of the silicone slow that down in practice.
Safety with delicate objects, and with air
Soft grippers are good for fruit, eggs, glassware and people-adjacent tasks since there are no hard edges, and the squeezing force is limited by the pressure you choose. To keep things safe for the object, fix the maximum pressure with a regulator or a relief valve and test on a dummy before the real item.
Compressed air and burst chambers: Stay within the pressure rating of the silicone, tubing and fittings. Use a pressure regulator and a relief valve, wear safety glasses when testing, and never inflate a chamber you have cut or cracked. Do not use a container of compressed gas. Vent the system before disconnecting tubing, and keep your face away from a chamber while it is being pressurised. Small hobby pumps and syringes are the right scale to start with.
Where it goes wrong
Leaks. A pin-prick slowly loses pressure, and the gripper relaxes mid-lift.
Fatigue. Silicone chambers can tear after many cycles, especially at stress points near seams.
Hard to model. Soft materials deform in complicated ways, so a simulation takes much more care than for rigid links.
Slow and imprecise. Air must flow through tubes, so response is slow, and position is hard to know without extra sensors.
Low force. Soft fingers are weak for heavy loads.
Temperature. Air and silicone behave differently when hot or cold.
How real robots use it
Soft grippers are used for picking food and other irregular, delicate items, and in research on wearable devices and robots that move through tight spaces. Many designs add small sensors in the skin, which connects to how robotic hands sense touch. Because soft machines are hard to simulate, they also sharpen the ideas in sim-to-real. Not all soft robots swim in air: some are driven by fluids, cables or shape-memory materials, and some are made to work in water, as in underwater navigation.
Try it yourself
Take a party balloon and wrap one side with a strip of tape that does not stretch. Inflate gently by mouth or with a hand pump. The balloon curls toward the taped side. Try different tape widths and see how the bend changes. This is the stiff-layer idea in its simplest form. Do not over-inflate, and keep the balloon away from your face.
Frequently asked questions
Are soft robots always safer around people?
They remove hard edges, which helps, but pressure, speed and what the gripper holds still matter. Safety comes from the whole system.
Why not just use a rigid gripper with force control?
You can, and many do. Soft fingers add passive compliance, so they cope with variation in object shape without precise sensing.
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