A heat engine working in ISOTHERMAL conditions:

Artificial muscle basic-motion https://www.youtube.com/watch?v=Oggr2Io0fGQ

There is no information how the work-producing force is activated, but here we do have such information:

"pH-Responsive Hydrogel Composite Artificial Muscle. Here we see a pH-responsive polyacrylic acid hydrogel contained within an unbound carbon fibre braid. The artificial muscle (McKibben style) actuates when placed in a solution with high pH, generating contraction free strains of ~30%." https://www.youtube.com/watch?v=JGn2a21FvLM

pH-sensitive polymers are potential perpetual-motion machines of the second kind. That is, by regularly changing the pH of the system, the experimentalist is able to extract unlimited (limited only by the deterioration of the system) amount of work from them, and this work is done at the expense of ambient heat:

"When the pH is lowered (that is, on raising the chemical potential, μ, of the protons present) at the isothermal condition of 37°C, these matrices can exert forces, f, sufficient to lift weights that are a thousand times their dry weight." http://www.google.com/patents/US5520672

http://www.gsjournal.net/old/valev/val3.gif

A. KATCHALSKY, POLYELECTROLYTES AND THEIR BIOLOGICAL INTERACTIONS, p. 15, Figure 4: "Polyacid gel in sodium hydroxide solution: expanded. Polyacid gel in acid solution: contracted; weight is lifted." https://www.ncbi.nlm.nih..gov/pmc/ar...00645-0017.pdf

Consider Figure 4 in Katchalsky's article. The following four-step isothermal cycle, if carried out quasi-statically (reversibly), clearly violates the second law of thermodynamics:

1. The polymer is initially stretched. The experimentalist adds hydrogen ions (H+) to the system. The force of contraction increases.
2. The polymers contracts and lifts a weight.
3. The experimentalist removes the same amount of H+ from the system. The force of contraction decreases.
4. The experimentalist stretches the polymer and restores the initial state of the system.

The net work extracted from the cycle is positive unless the following is the case:

The experimentalist, as he decreases and then increases the pH of the system (steps 1 and 3), does (loses; wastes) more work than the work he gains from weight-lifting.

However electrochemists know that, if both adding hydrogen ions to the system and then removing them are performed quasi-statically, the net work involved is virtually zero (the experimentalist gains work if the hydrogen ions are transported from a high to a low concentration and then loses the same amount of work in the backward transport). That is, the net work involved in steps 1 and 3 is zero, and the net work extracted from steps 2 and 4 is positive, in violation of the second law of thermodynamics.

Pentcho Valev