Factors Affecting TDDS can be categorized into drug-related, skin-related, formulation-related, and external factors.
1) Drug-Related Factors
Molecular Size: Smaller molecules penetrate more easily.
Lipophilicity: Moderately lipophilic drugs traverse lipid-rich stratum corneum efficiently.
Ionization: Non-ionized drugs have better skin penetration.
2) Skin-Related Factors
Skin Condition: Damaged or inflamed skin increases permeability.
Hydration: Hydrated skin enhances drug permeation.
Age: Infants and elderly have more permeable skin.
3) Formulation-Related Factors Affecting TDDS
Permeation Enhancers: Chemicals that disrupt the stratum corneum to increase permeability.
Vehicle Type: Ointments, creams, gels, or patches affect drug delivery differently.
Drug Concentration: Higher concentration gradients drive more drug into the skin.
4) Environmental and Physiological Factors
Temperature and Humidity: Higher levels improve permeability.
Occlusion: Increases skin hydration and enhances drug permeation.
5) Application Techniques
Devices: Methods like iontophoresis or microneedles improve penetration.