Hydrophilic pastes

After a short reminder of the published results, we extend these in vitro experiments in vivo, and we investigate in a more detailed fashion the interaction of semisolid pastes with the skin. Emphasis was put on hydrophilic pastes, however, whenever necessary and for comparison purposes, results obtained with lipophilic pastes will be shown as well. 

From this, it is obvious that it is not possible to use any hydrophilic paste in any given dermatologic situation. Apart from water (exudate) absorption, which may be a significant (or desirable) component of a paste s action on the skin, occlusion is another factor of importance in situations where skin protection is required. 

FIGURE 22.4 Occlusive properties of different pastes on stripped skin in vivo (percent decrease in transepi-dermal water loss) of n = 6 healthy volunteers. HP3 hydrophilic paste LP1 and LP2 lipophilic pastes. For the sake of clarity, means are shown +HSD (HP3) or —HSD (white petrolatum) only. 

FIGURE 22.7 Measurement of skin surface water loss (Evaporimeter EP-1, Servomed, Stockholm) during application and after removal of two hydrophilic pastes for 30 min on previously hydrated skin of n = 6 healthy volunteers. Means =t HSD for the sake of clarity. Closed squares control, hydrated skin triangles HP2 open circles HP3 statistically significant differences with the control group. M indicates homogenous subsets. 

In vivo, hydrophilic pastes showed different interactions with the skin. Some pastes clearly hydrated the skin, others could indeed remove water from a preliminary hydrated horny layer. Elements contributing to these properties may be the presence of humectants such as glycerol, contributing to a long-lasting presence of water on the skin in the first case, or the acceleration of skin surface water loss, contributing to an accelerated removal of water from a hydrated horny layer in the second case. However, this represents, in our opinion, at most one of the elements contributing to the measured events and may simply be due to a different water content of the pastes. 

We conclude that pastes cannot be pooled in a single group and be generally characterized as drying and exudate binding. Lipophilic pastes did not bind any water at all and were highly occlusive. Thus, they are likely to hydrate the skin through an impairment of the transepidermal water loss. They should be preferably used for skin protection. Hydrophilic pastes, on the other hand, hydrated the skin or maintained an elevated hydration state if they contained humectants. Only an hydrophilic paste without any additional component was able to reduce a hydrated state and led to measurably decreased skin hydration values. 

Chapter 22 Hydrophilic Pastes 279 Bernard Gabard and Christian Surber... 

Aqueous pastes, also called hydrophilic pastes, consist of a hydrophilic base with 40-60 % solid substance. This type of paste may consist of water only, made viscous by a viscosity enhancer (see Table 12.41) or by the addition of a hydrophilic cream or emulsion. They are supposed to have a good absorptive capacity and are therefore used in the treatment of wetting skin disorders [59b]. 

Almost all urethane materials are synthesized without the use of solvents or water as diluents or earners and are referred to as being 100% solids. This is true of all foams and elastomers. There are many products, however, which do utilize solvents or water, and these are known as solvent-borne and waterborne systems, respectively. In the past, many coatings, adhesives, and binders were formulated using a solvent to reduce viscosity and/or ease application. However, the use of volatile solvents has been dramatically curtailed in favor of more environmentally friendly water (see Section 4.1.3), and now there are many aqueous coatings, adhesives, and associated raw materials. Hydrophilic raw materials capable of being dispersed in water are called water reducible (or water dispersible), meaning they are sufficiently hydrophilic so as to be readily emulsified in water to form stable colloidal dispersions. 

However, the choice of a class of polymer for use in a given drug delivery system is often made for reasons unrelated to its swelling properties the polymer might be chosen on the basis of cost, availability, supplier, biocompatibility, past use history, etc. Thus the hydrophilicity and % will be fixed, and only the crosslink density and the ionic component can be readily adjusted to provide the swell-... 

Historically, organic environmental pollutants were hydrophobic, often persistent, neutral compounds. As a consequence, these substances were readily sorbed by particles and soluble in lipids. In modern times, efforts have been made to make xenobiotics more hydrophilic - often by including ionisable substituents. Presumably, these functional groups would render the compound less bioaccumulative. In particular, many pesticides and pharmaceuticals contain acidic or basic functions. However, studies on the fate and effect of organic environmental pollutants focus mainly on the neutral species [1], In the past, uptake into cells and sorption to biological membranes were often assumed to be only dependent on the neutral species. More recent studies that are reviewed in this chapter show that the ionic organic species play a role both for toxic effects and sorption of compounds to membranes. 

Multipurpose tinting pastes usually contain pigment and hydrophilic solvents, sometimes some amount of water, and suitable wetting agents, which define the equilibrium between hydrophilic and lipophilic character. Conditions to be met in each case depend on the type of paste and on the method by which it is produced and also on the intended use of the coating or paint. 

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