Is there a way to harden your nails and/or fingers, for example for use in an eye strike?
Since eyes are soft tissue, fingers will have no problem penetrating them. If you miss and hit bone, you will not cause serious injury to the person no matter how tough your fingers are. If you miss, the attempt will probably enrage the attacker so much that you will have a tougher time defeating him than you would have had without the attempt. When it comes to attacks to the head, the safest bet is always elbow attacks. Elbows can inflict lots of damage without being damaged themselves. If it becomes necessary and the opportunity arises, thumb eye gouges are always an option.
As far as strengthening the fingers, the traditional method is to drive the fingers repeatedly into a bucket of dried beans, gradually working your way up to using a bucket of sand. Another way is to drive the fingers repeatedly, but softly, into a makawara board; after a few months, you will be able to drive them in with great force.
There are problems with any type of body part forging. First, toughening hampers the body part’s normal operation. You may be able to punch through a brick wall, but you will have to spend the rest of your life using a pencil to peck at computer keys since your fingers will be too stiff to use them. Second, since most people will never incur a self-defense situation in the lives, toughening is a waste of time since the problems it causes far outweigh any potential benefits. Lastly, toughening is not necessary. People normal hands and no martial arts training are fully capable of killing with their bare hands. Mas Oyama, the great Kyokushin karate master, who toured the world breaking wood and stone in demonstrations, and who often attempted to knockout a bull with a punch, had hands that looked similar to paws. In his later years, he expressed regret for the damage he had done to his hands.