<PERSON> appeared in seven of <PERSON>'s novels (and one short story), but <PERSON> did not provide too many details on Daneel's skin.
What was Daneel's skin made of?
<PERSON> made it clear from the beginning, i.e. in “The Caves of Steel,” that Daneel's skin is artificial:
<PERSON> let his arm rest easily in the plain-clothes man’s grip. <PERSON> stared at it, pinching the skin along the median line. Was there a faint seam?
It was logical, of course, that there should be. A robot, covered with synthetic skin, and deliberately made to look human, could not be repaired in the ordinary fashion.
This was confirmed 30 years later in “The Robots of Dawn”:
“Do you have real skin on your face, <PERSON>? It’s an extraordinary imitation if it isn’t.” She reached toward him and her hand rested delicately on his cheek. “It even feels right.” “Nevertheless, madam, it is not real skin. It does not heal of its own accord—if cut. On the other hand, a tear can easily be welded closed or a patch can even be replaced.”
So we only know that Daneel's skin is made of an unknown synthetic material that can be welded.
And how was his skin manufactured?
<PERSON>'s works do not contain any details on the manufacturing of Daneel's skin. 652
However, Daneel is not the only humanoid robot that <PERSON> had devised. There are, for example, <PERSON> (“Satisfaction Guaranteed”), <PERSON> (“The Bicentennial Man”) and <PERSON>. Regarding the latter, <PERSON> of U.S. Robots is asked in “Evidence” if it were possible to create a humanoid robot:
<PERSON> harrumphed and considered, “It’s been done experimentally by U.S. Robots,” he said reluctantly, “without the addition of a positronic brain, of course. By using human ova and hormone control, one can grow human flesh and skin over a skeleton of porous silicone plastics that would defy external examination. The eyes, the hair, the skin would be really human, not humanoid. And if you put a positronic brain, and such other gadgets as you might desire inside, you have a humanoid robot.”
Obviously this method is not compatible with the information from the robot novels, but at least here <PERSON> actually described the manufacturing process for the skin of a humanoid robot. 652