A diagram showing different skin layers Human skin can be divided, from outside to inside, into the stratum corneum, viable epidermis, dermis, and underlying hypodermis. Stratum corneum The stratum corneum is<unk> ly 10–20 μm thick and consists of extracellular lipid-surrounded corneocytes. The lipophilic environment of the stratum corneum can serve as a reservoir for certain highly lipophilic molecules. Keratin found in the stratum corneum may provide great affinity with certain drugs. Both of these interactions can play a role in drug accumulation in the stratum corneum and the local drug action on skin disease. Viable epidermis Below the stratum corneum is the viable epidermis. The viable epidermis is usually 50–100 μm thick. It includes immunologically sensitive cells (e.g. Langerhans cells), and metabolically active cells (e.g. keratinocytes, melanocytes, merkel cells). Melanocytes are involved in melanoma pathogenesis. As a result, drugs treating melanoma need to be administered to the lower epidermis. Dermis The dermis lies next to the epidermis. It is a 1–2 mm layer mainly composed of fibroblasts and immune cells (e.g. dermal dendritic cells, macrophages, T cells, mast cells) in a collagen and elastic fiber extracellular matrix. These immune cells play important roles in parasitic infections, psoriasis induction, tumor progression, dermal inflammation, angiogenesis, wound healing, tissue remodeling, skin sensitization, and tolerance. Therefore, the regional accumulation of drugs in the dermis is necessary for the prevention and treatment of these local skin diseases. The hair follicle is an invagination of epidermis cells deep into the dermis. The follicular route is critical in the<unk>  delivery of particle-based formulations and hydrophilic, high-molecular-weight drugs. The follicular route provides benefits such as deeper penetration, prolonged residence duration, faster entry into the skin, and site-specific targeting. Hypodermis Beneath these layers lies the hypodermis, which is composed of adipose tissue, fascia, as well as larger lymphatic and blood vessels. For joint and muscle disease treatment,<unk> ly applied drugs such as non-steroidal anti-inflammatory drugs (NSAIDs) can penetrate the subcutaneous tissue or even deeper tissues in the hypodermis. Drugs are primarily delivered into the underlying muscle by direct penetration without prior entry into the systemic circulation to prevent unintended side effects. Routes for<unk>  drug administration In general, there are three possible routes for drug administration into or across the skin. The first one is the transcellular route, where the drugs are required to pass through both lipid matrix and dead corneocytes of the stratum corneum. The second one is the intercellular route, where the drugs only have to pass through lipid domains between corneocytes. The third one is the trans-appendageal route, where the drugs are transported by hair follicles, sebaceous glands, or sweat glands. The most common route for drug delivery into the skin is the intercellular route. Multiple steps are involved. # Dissolution and release from the formulation # Partitioning into the stratum corneum # Diffusion through the stratum corneum # Partitioning into the aqueous viable epidermis # Diffusion into the dermis
Topical cream formulation
Topical drug delivery system
Topical medication
Polyherbal formulation
Barrier cream
Injectable filler
Topical tobacco paste
Aqueous cream