For a couple months now I've been talking about the crystal deodorants and how they still do contain aluminum even though most of them claim to be "aluminum-free." I wrote about how crystal deodorants are made up of potassium alum, the nickname for potassium aluminum sulfate. Once the potassium alum is wetted, as one would do using a crystal deodorant, it dissolves into ionic aluminum. Ionic aluminum is the smallest form of aluminum possible. My argument is that if larger molecules such as aluminum chlorohydrate pose a health risk by absorption, aluminum ions would as well. Although I found a number of empirical studies that would suggest ionic aluminum is absorbed through the skin, I had yet to find a direct study stating that aluminum ions are absorbed through the skin. Today I am glad to say, I found the research.
A French study published in the Journal of Biomedical Materials Research explored the effects of aluminum ions on collagen. As stated in the study's introduction, "Heavy metal ions are capable of inducing crosslinking between peptide chains of collagen. The metal ions improve the capacity of collagen to resist denaturization as well as the attack by enzymes, bacteria, and chemical agents." [p 1339] In other words, metal ions have been found to stabilize collagen. This is a process that is used in the leather-making industry (known as "tanning" hides) and in some medical devices/products.
Chromium salts were already well-known to be a good collagen stabilizing agent at the time of the study. The purpose of the study was to find if aluminum ions, as found in aluminum salts, had the same collagen stabilizing effects, and could be used in the same way industrially to tan hides and to create collagen-based medical materials.
The researchers treated collagen with aluminum ions and found that it became significantly dehydrated. They concluded that "the substitution of water molecules by aluminum ions on intramolecular hydrophilic sites is suggested to be responsible for this evolution." In other words, the aluminum ions substituted the water molecules in the collagen, thus drying it out. This may explain why alum has a drying effect on skin-- as it robs the collagen of its moisture, aluminum ions replacing water molecules.
The researchers studied the effects of aluminum ions on both extracted collagen and skin tissue. The aluminum ions were found to affect the collagen in the skin samples. Thus, we can ascertain that the aluminum ions were absorbed into the skin at some level. The deodorant companies' claims that their aluminum molecules are "too large to be absorbed through the skin" are thus disproven.
Aluminum ions, as found in crystal deodorants, are absorbed through skin, and do have a biological reaction therein.