* Large Molecular Size: Collagen and elastin molecules are relatively large and complex. The skin's primary function is to act as a barrier, and its outermost layer, the stratum corneum, is particularly good at preventing large molecules from passing through. These large molecules simply can't squeeze through the tight spaces between skin cells.
* Skin Structure and Composition: The skin's layers are designed to protect the body from external threats. The stratum corneum, composed of dead skin cells and lipids, is very effective at preventing absorption of many substances. While some small molecules can penetrate, collagen and elastin are simply too large to pass through this barrier effectively.
* Lack of Active Transport Mechanisms: The skin doesn't have active transport mechanisms to pull these specific proteins through. Active transport requires energy and specific carrier molecules, and there's no evidence that the skin has such mechanisms for collagen or elastin applied topically.
* Breakdown on the Skin Surface: Even if a small amount could penetrate, the proteins are susceptible to degradation by enzymes present on the skin's surface.
* No Delivery System: Skin creams often lack the necessary delivery systems to get these molecules past the stratum corneum. Advanced techniques like liposomes or nanoparticles *can* help deliver some ingredients, but most over-the-counter collagen creams don't use them effectively.
What They Can Do (and why companies use them):
* Surface Hydration: Collagen and elastin *can* act as humectants on the skin's surface. They can bind to water and help to hydrate the outer layer of the skin, leading to a temporary plumping effect and a smoother appearance.
* Film-Forming: They can also form a thin film on the skin's surface, which can provide a temporary tightening effect and a smoother feel.
* Marketing Appeal: "Collagen" and "elastin" are buzzwords that consumers associate with youthful skin. Including them in a product, even if they don't penetrate deeply, can make the product more appealing to consumers.
In short: While collagen and elastin in creams can provide some superficial benefits like hydration and a temporary smoothing effect, they are unlikely to penetrate the skin deeply and directly replenish the skin's own collagen and elastin stores. Claims suggesting otherwise are often misleading. If you want to boost collagen production in your skin, look for ingredients like retinoids, vitamin C, and peptides, which have more evidence of stimulating collagen synthesis within the skin. Or consider in-office procedures like microneedling or laser treatments.