6.5.9 The design principles of polymer nano-drug c...
The biggest problem of local topical drug administration is that the stratum corneum of the skin prevents penetration and absorption of the drugs，resulting in low drug availability.Especially in psoriasis，the skin structure changes caused by skin keratinization and epidermal hyperplasia further reduce the drug penetration ability.Polymer nano-drug carriers can be loaded with a large number of hydrophobic drugs，so as to improve the action concentration of hydrophobic drugs.At the same time，they can use their own properties to improve the skin penetration ability and skin residence time of drugs，and achieve controlled and targeted release of drugs，thus contributing to local drug therapy for psoriasis.
Nanomaterials can increase the penetration of drugs by releasing drugs inside the skin's affiliated structures such as wrinkles and hair follicles.They can also enter the skin through intracellular and intercellular pathways.In addition，the recently finding showed that hydrophilic transdermal pathway also plays an important role in the process of NPs entering the skin.Since nanomaterials can enter the skin through diffusion，increasing the concentration of nanomaterials on the skin surface is beneficial to increase their diffusion depth and drug concentration in the skin.There are certain size restrictions on the passage of nanomaterials in the skin.For example，the sizes of hair follicles，sweat glands and other skin accessory structures are several to dozens of microns，which easily allow nanomaterials to enter.However，the sizes of the intercellular space are only a few to dozens of nanometers.Therefore，the construction of small-sized polymer nano-drug carriers can improve the diffusion depth of nanomaterials in the skin.In addition to the size，the transdermal capability of the polymer nano-drug carriers is also related to its deformation capability and surface charge properties.The easily deformable polymer nano-drug carriers can enter the intercellular space by changing its own shape，thus improving the diffusion efficiency of the nanomaterials.The acidic environment of the skin makes the negatively charged nanomaterials to aggregate in the skin，resulting in increased size，thus reducing its ability to diffuse.Therefore，the polymer nano-drug carriers with neutral or positively charged surfaces have stronger skin penetration capability.(https://www.daowen.com)