We could not find many studies regarding the topical use of L-Cysteine. In addition, there is a lack of information regarding how it could potentially be integrated into a topical product. Often, when ingredients are integrated into topical preparations, they need to be solubilized, emulsified or otherwise chemically altered in able to be effectively absorbed. Again, information on how this could successfully be done in a home mixture product is lacking.
Most often, L-cysteine is available in a slightly different, more commonly used form known as N-acetyl cysteine. N-acetyl cysteine has been shown to be a potent antioxidant and increases glutathione levels in the body. Glutathione is a very common component in many beauty products due to the its antioxidant properties so we can certainly understand trying in integrate L-cysteine (or N-acetyl cysteine) into a beauty regimen.
There does appear to be well known issues producing topical cysteine products in an acceptable form. N-acetyl cysteine apparently breaks down readily when prepared topically and can give off unpleasant odors. We unfortunately could not find information regarding how you could potentially integrate pure L-cysteine powder.
It may be more beneficial, and safer, to buy a commercially produced topical N-acetyl cysteine product and add that to your mixture. That way you know it will be in a form that is intended for topical use. Additionally, you can find many topical glutathione products on the market.