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Figure 12.10 In vitro glyceryl trinitrate (GTN) release (24 h) into 80 cm3 isotonic NaCI versus release area of Deponit TTS.

Reproduced from reference 8 with permission.

Figure 12.10 In vitro glyceryl trinitrate (GTN) release (24 h) into 80 cm3 isotonic NaCI versus release area of Deponit TTS.

Reproduced from reference 8 with permission.

expected. Release increases as temperature increases; between 32 and 37°C an increase in release rate of between 1 and 2 ^g cm 2 h"1 is detected. The BP specifies a distribution (release) test for transdermal patches based on the paddle apparatus for tablet and capsules.

12.4 Rheological characteristics of products

The rheological behaviour of liquids and semisolids can be described as discussed in sections 7.3.10 and 7.4.4 of Chapter 7. Viscosity monitoring can be used as a quality control procedure, but some very practical rheological tests may be carried out. Some are discussed here.

The injectability of nonaqueous injections, which are often viscous and thus difficult to inject, can be assessed by a test for syringeabil-ity. Sesame oil has a viscosity of 56 cP, but added drugs and adjuvants can increase the viscosity of such vehicles.

The desired physical properties of various topical preparations are listed in Table 12.1. The terms 'soft and unctuous' and 'hard and stiff' are difficult to quantify, but it is useful at least to consider the variety of descriptions that may be applied in a consistency profile of an external pharmaceutical product. Scheme 12.2 lists these comprehensively, dividing them into primary, secondary and tertiary characteristics. The tertiary properties are probably the most elusive.

Table 12.1 Properties of some topical preparations0

Class Nature Required physical properties

Table 12.1 Properties of some topical preparations0

Class Nature Required physical properties

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