6.4.6　Stimulus responsiveness

Both the tumor and the skin microenvironment are weakly acidic.The pH value of the tumor microenvironment is around 6.5，and that of the skin microenvironment is between 4 and 7.4.Therefore，pH-responsive polymer nano-drug carriers can be used to promote the release of drugs in targeted areas or transdermal delivery efficiency.Buffer solutions with different pH values are required to simulate the behavior of nano-drug carriers in tumor or skin microenvironment.Besides，the buffer solutions also need to have certain ionic strength and good solubility.Phosphate buffer solution，citrate buffer solution，and acetate buffer solution are the commonly used acidic buffer solutions.The pH responsibility of nano-drug carriers under weakly acidic conditions can be obtained by combining of morphology，size，and drug release.(https://www.daowen.com)