Gel patch made by mixing the raw drug with a suitable hydrophilic matrix and laying it on the backing material. Gel patch has the advantages of high water content, good air permeability, large drug loading capacity, high absorption efficiency, no odor, and small skin irritation. It is more easily accepted by patients and clinicians, and has become one of the hot directions in the development of percutaneous drug delivery systems.

Gel paste usually uses polymer material as the skeleton material, and then adds crosslinking agent, moisturizing agent, filler and transdermal enhancer to form a pseudoplastic fluid with a certain viscosity. During use, the drug components will be released from the skeleton material and reach the skin surface, and then enter the blood circulation through the epidermis to play a role.

Therefore, the release rate and transdermal absorption rate of drug components of gel plaster will directly affect its clinical efficacy, and is an important quality index of gel plaster.

Franz vertical diffusion cell method is generally used for in vitro release test (IVRT) and in vitro permeation test (ivpt) of gel paste. This article will share the in vitro release test case of a gel paste, hoping to bring you help and inspiration.

Experimental Instrument: RT800 Automatic Sampling Transdermal Diffusion System

Apparatus: Raytor improved Franz vertical diffusion cell

Temperature: 32 ± 0.5 ℃

Medium: technical confidentiality

Rotational Speed：300 RPM

Medium Volume: 40 mL

Sampling / Replenishment Volume: 1 ml

Gel Paste Diameter: 16 mm

Screening Filter Membrane

In vitro release test of gel paste, appropriate inert and commercial artificial membrane will be selected. The permeation rate of the sample to be tested in different filter membranes may be different. When developing the method, the influence of the filter membrane on the release rate of the sample should be fully investigated.

The following figure shows the in vitro release test results of gel paste samples under three filter membranes during the filter membrane screening process. After comprehensive consideration of other factors in the process of method development, it is decided to use filter membrane A as the test filter membrane.

Experimental Result

Through the early development of the method, the key parameters such as sample loading volume, filter membrane, medium, medium volume and rotation speed have been determined. In the subsequent stage, the accuracy, repeatability and discrimination of the test method were verified.

According to the established method, the gel paste samples were tested for in vitro release. Then, according to USP <1724>, calculate the cumulative amount released per square centimeter area at each sampling time point:

The test results are shown in the following figure. The abscissa of the cumulative drug release curve is the square root of time.

The release of gel paste samples generally follows Higuch formula, that is, the cumulative drug release amount is proportional to the square root of time. The cumulative drug release of six test samples at each sampling time point was linearly regressed with the square root of the sampling time to obtain the regression equation and correlation coefficient, and the slope value was taken as the drug release rate constant.

Results Discussion

The results show that the Franz vertical diffusion cell method has high precision and good reproducibility. It can be applied to the in vitro release test of gel paste and provide valuable in vitro release measurement data for the formulation development of cream products.

Thanks to the high-precision automatic design of RT800 automatic sampling transdermal diffusion system, the error introduced by the experimental system or manual operation is effectively reduced, making the repeatability of the test results more ideal.