Q:

Artificial muscles and their efficiency

Most of the artificial muscles are based on either pneumatic power or nylon fibers being heated so they contract. These methods consume more energy than what can be utilized.
Likewise, in early days when the light bulbs that produced photons by heating the filament (which also requires considerable energy), they were replaced by energy efficient light-emitting diodes.
Are there any promising materials that can do the same for artificial muscles by converting electric energy directly to actuation instead of into heat and actuation being the side product of the heat itself?

A:

This question is more about engineering than materials modeling, but I will try to supply a partial answer and let others decide if it isn't.  You may get a better answer at Physics.SE.
My first thought would be that hydraulic power is a straightforward improvement over pneumatic since there is less energy wasted due to the compression of air.  But if we want to move to a more materials based way of thinking you can also look at piezoelectric actuators.
A piezoelectric actuator is a linear (typically) motor that works by applying a voltage to a material that can contract or expand in response to the voltage.  This is something that I imagine could actually be modeled to some degree using computational methods such as DFT.  These types of motors are currently used for some applications already, but not in the same manner as muscle fiber.  They are used for really fine control of small movements (in the nm range).
Since these materials actually work in a similar manner to a muscle, I think these are the "LED"s of a muscle fiber.  A quick google search gives a variety of efficiencies for electric to mechanical work but it seems that it is somewhere around 25%.  Surely this is higher than needing to constantly heat a nylon fiber for example, but I am not sure.  This also may have a lot of room for improvement, but I am not an expert in the topic.

A:

Photochemical "Muscles"
All motors require energy, and you mentioned that heat maybe problematic because of heat loss. So you might be interested in photochemistry. These reactions can be made to be highly photon (i.e. energy) efficient and directional.

Feringa Motors
A promising technology for artificial muscles are those based off the light-driven  molecular motors such as the Feringa rotary motors. For example, the Feringa motor can be utilized to produce "Macroscopic contraction of a gel induced by the integrated motion of light-driven molecular motors" [1]. This is almost exactly how a muscle fiber works. Essentially, there are three components: a) the motor which turns unidirectionally with light and heat, b) two polymer strands which are connected to the opposite ends of the motor, and c) the modulator which binds together the end of the polymer. Therefore, when the motor spins it causes the polymer strands to wrap around each other and to contract. Furthermore, when you want to uncontract/uncoil the strands you open the modulator with a different wavelength of light.

Optomechanical devices
Another great example are "optomechanical devices" which can produce macroscopic motion. My favorite is the "Photoinduced peeling of crystals"[2]. See this
Photoinduced peeling a molecular crystal on Chemistry World
Another  author to look at for optomechanics is the Professor Garcia-Garibay, he has been interested in crystalline artificial muscles. For example, in reference [3] is a picture similar to this so it deserves a citation.

Rotaxane Shuttles
Finally, rotaxane is a mechanically interlocked molecular architecture that can be used to produce macroscopic motion similar to a muscle. These typically operate by changing the protonation states. But this action can be performed by either chemical or photochemical inputs (e.g. a photoacid!)
The operation of a molecular shuttle on Wikipedia
References:

Ke, C. A Light-Powered Clockwork. Nat. Nanotechnol. 2017, 12 (6), 504–506. https://doi.org/10.1038/nnano.2017.44.
Tong, F.; Al-Haidar, M.; Zhu, L.; Al-Kaysi, R. O.; Bardeen, C. J. Photoinduced Peeling of Molecular Crystals. Chem. Commun. 2019, 55 (26), 3709–3712. https://doi.org/10.1039/c8cc10051a.
Vogelsberg, C. S.; Garcia-Garibay, M. A. Crystalline Molecular Machines: Function, Phase Order, Dimensionality, and Composition. Chem. Soc. Rev. 2012, 41 (5), 1892–1910. https://doi.org/10.1039/c1cs15197e.

A:

A research group lead by Prof. Ray Baughman, from the University of Texas at Dallas, uses a yarn of carbon nanotubes. From the abstract of one of its works1:

We show that an electrolyte-filled twist-spun carbon nanotube yarn,
much thinner than a human hair, functions as a torsional artificial
muscle in a simple three-electrode electrochemical system, providing a
reversible 15,000° rotation and 590 revolutions per minute. A
hydrostatic actuation mechanism, as seen in muscular hydrostats in
nature, explains the simultaneous occurrence of lengthwise
contractiona nd torsional rotation during the yarn volume increase
caused by electrochemical double-layercharge injection. The use of a
torsional yarn muscle as a mixer for a fluidic chip is demonstrated.

A similar idea is used by the Intelligent Polymer Research Institute in Australia (link).
The image below is from a mini-review authored by Prof. Baughman2, showing SEM images of (a) a CNT sheet being drawn from a CNT forest (which is on the left in this image), (b) Fermat-type twist insertion during spinning a yarn from a CNT forest, (c) a single-ply, twisted, non-coiled CNT yarn, and (d) a single-ply, coiled yarn

Javad Foroughi, et al. Torsional Carbon NanotubeArtificial Muscles, Science, 334, 494, 2011 (DOI: 10.1126/science.1211220, ResearchGate).
Jae Ah Lee, Ray H Baughman & Seon Jeong Kim, High performance electrochemical and electrothermal artificial muscles from twist-spun carbon nanotube yarn. Nano Convergence, 2, 8, 2015 (DOI: 10.1186/s40580-014-0036-0 Open Acess!)

