preparation absorption of medicament into blood stream irritant effect properties which affect choice of an ointment base are: stability penetrability solvent property irritant effects ease of application and removal methods of preparation of ointments: fusion: in this method the ingredients are melted together in descending order of their melting points and stirred to ensure homogeneity. trituration: in this finely subdivided insoluble medicaments are evenly distributed by grinding with a small amount of the base followed by dilution with gradually increasing amounts of the base. paste paste combines three agents - oil, water, and powder. it is an ointment in which a powder is suspended. powder powder is either the pure drug by itself (talcum powder), or is made of the drug mixed in a carrier such as corn starch or corn cob powder (zeosorb af - miconazole powder). can be used as an inhaled topical (cocaine powder used in nasal surgery). shake lotion a shake lotion is a mixture that separates into two or three parts over time. frequently, an oil mixed with a water-based solution needs to be shaken into suspension before use and includes the instructions: "shake well before use". solid medication may be placed in a solid form. examples are deodorant, antiperspirants, astringents, and hemostatic agents. some solids melt when they reach body temperature (e.g. rectal suppositories). sponge certain contraceptive methods rely on sponge as a carrier of a liquid medicine. lemon juice embedded in a sponge has been used as a primitive contraception in some cultures. tape cordran tape is an example of a topical steroid applied under occlusion by tape. this greatly increases the potency and absorption of the topical steroid and is used to treat inflammatory skin diseases. tincture a tincture is a skin preparation that has a high percentage of alcohol. it would normally be used as a drug vehicle if drying of the area is desired. topical solution topical solutions can be marketed as drops, rinses, or sprays, are generally of low viscosity, and often use alcohol or water in the base. these are usually a powder dissolved in alcohol, water, and sometimes oil; although a solution that uses alcohol as a base ingredient, as in topical steroids, can cause drying of the skin. there is significant variability among brands, and some solutions may cause irritation, depending on the preservative(s) and fragrances used in the base. some examples of topical solutions are given below: aluminium acetate topical solution: this is colorless, with a faint acetous odour and sweetish taste. it is applied topically as an astringent after dilution with 10-40 parts of water. this is used in many types of dermatologic creams, lotions, and pastes. commercial premeasured and packed tablets and powders are available for this preparation. povidone iodine topical solution: this is a chemical complex of iodine with polyvinylpyrrolidone. the agent is a polymer with an average molecular weight of 40,000. the povidone iodine contains 10% available iodine, slowly released when applied to skin. this preparation is employed topically as a surgical scrub and non irritating antiseptic solution; its effectiveness is directly attributed to the presence and release of iodine from the complex. commercial product: betadine solution. transdermal patch transdermal patches can be a very precise time released method of delivering a drug. cutting a patch in half might affect the dose delivered. the release of the active component from a transdermal delivery system (patch) may be controlled by diffusion through the adhesive which covers the whole patch, by diffusion through a membrane which may only have adhesive on the patch rim or drug release may be controlled by release from a polymer matrix. cutting a patch might cause rapid dehydration of the base of the medicine and affect the rate of diffusion. vapor some medications are applied as an ointment or gel, and reach the mucous membrane via vaporization. examples are nasal topical decongestants and smelling salt. topical drug classification system (tcs) graphical abstract of topical drug classification system topical drug classification system (tcs) is proposed by the fda. it is designed from the biopharmaceutics classification system (bcs) for oral immediate release solid drug products which is very successful for decades. there are 3 aspects to assess and 4 classes in total. the 3 aspects include qualitative (q1), quantitative (q2) and similarity of in vitro release (ivr) rate (q3). advantages of topical drug delivery systems in the early 1970s, the alza corporation, through their founder alejandro zaffaroni, filed the first us patents describing transdermal delivery systems for scopolamine, nitroglycerin and nicotine. people found that applying medicines on the body surfaces is beneficial in many aspects. skin medicines can give faster onset and local effect on our body as the surface cream can bypass first pass metabolism such as hepatic and intestinal metabolism. apart from the absorption, dermal drugs effectively prevent oral delivery limitations such as nausea and vomiting and poor appliances due to unpalatable tastes of the drugs . topical application is an easy way for patients to tackle skin infections in a painless and non-invasive way. from a patient perspective, applying drugs on skin also provides stable dosage in blood so as to give the optimal bioavailability and therapeutic effects. in case of overdose or unwanted side effects, patients can take off or wash out the medicines quickly to eliminate toxicity by simply removing the patch to stop the delivery of drugs. disadvantages of topical drug delivery systems the site of putting the patches for topical drugs may get irritated and have rashes and feel itchy. hence, some topical drugs including nicotine patches for smoking cessation are advised to change places for each application to avoid continuous irritation of the skin. also, since the drug needs to penetrate the skin, some drugs may not be able to pass through the skin. some drugs are then “wasted” and the bioavailability of the drug will decrease. challenges for designing topical dosage form skin penetration is the main challenge for any topical dosage form. the drug needs to penetrate the skin in order to get into the body to apply its function. the drug follows the fick’s first law of diffusion. one of the most common versions of fick’s first law of diffusion is: j = − d d c d x where j is the diffusion flux. d is the diffusion coefficient. dc/dx is the concentration gradient. for d is described by the stokes–einstein equation. the equation is: d = r t 6 π η r n a where r is the gas constant. t is the temperature. η is the viscosity. r is the radius of the solute. n a is the avogadro constant. assuming concentration gradient is constant for all newly applied topical drugs and the temperature is constant (normal body temperature: 37 °c), the viscosity and radius of the drug determine the flux of diffusion. the higher the viscosity or the larger the radius of the drug is, the lower the diffusion flux of the drug is. new developments there are many factors for drug developers to consider in developing new topical formulations. the first one is the effect of the vehicle. the medium to carry the topical drugs can affect the penetration of the drug active ingredient and efficacy. for example, this carrier can have a cooling, drying, emollient or protective action to suit the required conditions of the application site such as applying gel or lotion for hairy areas. meanwhile, scientists need to match the type of preparation with the type of lesions. for example, they need to avoid oily ointments for acute weepy dermatitis. chemists also need to consider the irritation or any sensitization potential to ensure that the topical application can be stable during storage and transport to maintain its efficacy. another potential material is nanofiber-based dispersion to improve the adhesion of active ingredients on the skin. in order to enhance drug penetration into the skin, scientists have several ways to achieve their purposes by using chemical, biochemical, physical and super saturation enhancement. advanced emulgel technology is a breakthrough of painkilling topical drugs. it helps the gel to enter deeply down the skin layer to strengthen delivery of diclofenac to the point of pain so as to achieve better therapeutic effects by modifying the above properties.