Barrier characteristics

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane... 

Demethylation of the tricyclic antihistamine 9, with cyanogen bromide gives the secondary amine 10 acylation of that intermediate with ethyl chloroformate affords the nonsedating H-1 antihistaminic agent loratidine (11) [3], It is of interest that this compound does not contain the zwitterionic funcrion which is thought to prevent passage through the blood-brain barrier, characteristic of this class of compounds. 

Improved polyurethane can he produced hy copolymerization. Block copolymers of polyurethanes connected with segments of isobutylenes exhibit high-temperature properties, hydrolytic stability, and barrier characteristics. The hard segments of polyurethane block polymers consist of 4RNHCOO)-n, where R usually contains an aromatic moiety. 

Polyurethane networks based on triisocyante and diisocyanate connected by segments consisting of polyisobutylene are rubbery and exhibit high temperature properties, hydrolyic stability, and barrier characteristics. ... 

Honeywell has also been active in developing a combined active-passive oxygen barrier system for polyamide-6 materials [201]. Passive barrier characteristics are provided by nanoclay particles incorporated via melt processing techniques, while active contribution comes from an oxygen-scavenging ingredient (undisclosed). Oxygen transmission results reveal substantial... 

Stewart PA, Wiley MJ. Developing nervous tissue induces formation of blood-brain barrier characteristics in invading endothelial cells a study using quail-chick transplantation chimeras. Dev Biol 1981 84 183-192. 

Krum JM, Kenyon KL, Rosenstein JM. Expression of blood-brain barrier characteristics following neuronal loss and astroglial damage after administration of anti-thy-1 immunotoxin. Exp Neurol 1997 146 33-45. 

Organotypic corneal constructs do resemble the in vivo cornea in many aspects, but they have the drawback of a complex isolation/setup procedure and longer cultivation periods. Additionally, in contrast to the well-established corneal cell lines that have been used for several years, these constmcts are relatively new and further validation of the barrier characteristics and transporter expression/function are needed. Nonetheless, we surmise that both in vitro models are promising tools for evaluating transcorneal drug delivery. 

The cerebral endothelial cells of the blood-brain barrier originate from the middle germinal sheet of the embryo, the mesoderm [17]. Concomitant with migration and proliferation of capillary endothelial cells during formation of the cerebral vascular network occurs the imprinting of the cells. Thereby, induction by the cellular surrounding plays an important role [18-21], The relevance of the cellular environment for the development of the barrier function of cerebral microvessels was first demonstrated by Stewart and Wiley [22], who transplanted embryonic brain tissue of a quail into embryonic gut tissue of chicken and vice versa. The cerebral transplant was vascularized by intestinal vessels, in which properties of the blood-brain barrier had been induced. In transplanted brain vessels, however, no characteristics of a barrier could be demonstrated, due to the lack of a neuronal environment. These results indicated that the cerebral microvessels are of extraneuronal origin, with properties that are induced by the cellular environment. In addition, brain tissue has the capability to induce blood-brain barrier characteristics also in noncerebral vascular tissue [23],... 

Schreiner V, Gooris GS, Pfeiffer S, Lanzendorfer G, Wenck H, Diembeck W, Proksch E, Bouwstra J (2000) Barrier characteristics of different human skin... 

For the correct interpretation of brain uptake studies in general and for the pharmacokinetic validation of a given delivery strategy, it is necessary to be familiar with characteristics and limitations of the applied technique [41]. 7k vivo methods remain the gold standard, as there are still no cell culture models available that fully represent the barrier characteristics. 

At the inner border of ONH, the ILM becomes continuous with the basement membrane of fibrous astrocytes lining the internal surface of the ONH [21]. However, the lateral borders between the ONH and the adjacent choroid and retina are not well defined. Furthermore, it was reported [49] that micro vessels in the prelaminar region of the ONH lack classical blood-brain barrier characteristics and display nonspecific permeability, possibly mediated by vesicular transport. Thus, there is a theoretical possibility that topically applied drugs can penetrate indirectly through the retrobulbar space and then, through the ONH, reach the posterior choroid and retina. It was reported that following retrobulbar administration of fluorescein, the dye rapidly accumulated in the ONH and penetrated later to the vitreous [50],... 

Neuhaus, J., W. Risau, and H. Wolburg. 1991. Induction of blood-brain barrier characteristics in bovine brain endothelial cells by rat astroglial cells in transfilter coculture. Ann N Y Acad Sci 633 578. 

Schreiner, V. et al., Barrier characteristics of different human skin types investigated with X-ray diffraction, lipid analysis, and electron microscopy imaging, J. Invest. Dermatol., 114, 654, 2000. 

The restorative properties of high-glycerin therapeutic moisturizers are hypothesized to be related to a glycerol reservoir within the SC. This provides a mechanism for enhancing barrier characteristics... 

PLA compares well with petrochemical-based plastics used for packaging. It is clear and naturally glossy like polystyrene, it is resistant to moisture and grease, it has flavour and odour barrier characteristics similar to polyethylene terephthalate (PET). The tensile strength and modulus of elasticity of PLA is also comparable to PET. 

It is generally known that only a very limited number of packaging materials such as glass or metal provide absolute protection properties concerning the penetration of chemical compounds from layers behind or from the environment. In the case of multi-layers with plastics materials as functional barriers there occurs, in most cases to a certain extent, an unavoidable mass transfer from the plastics layers into the product. This must be understood as a functional quantity which, however, must comply with food regulations. Therefore it is necessary firstly to understand functional barrier characteristics and mechanisms and, secondly, to define the functional barrier efficiency in relation to food safety and to establish appropriate test methods. This is especially important with those food packaging applications where recycled plastics are covered by plastics functional barriers. 

With an experimental protocol in place that facilitated studies aimed at characterizing the porous permeation pathway, a systematic study of polar compound permeation through HEM was undertaken (Peck et al., 1994). As has already been described, there is a void in the literature with respect to the passive permeation of polar solutes through skin. The initial purpose of the studies outlined in this section was to add to the polar solute permeation database. An effort was again made to determine the degree to which the barrier characteristics of skin with respect to polar compounds approach, or deviate from, those of an ideal porous membrane. 

Oxygen barrier data for pure H40 systems and H40 network systems measured at RT and 0 and 50%RH are summarized in Table 1 which also reports Tg s and densities in the dry state. Due to the high concentration of hydroxyl functional groups in the periphery, excellent gas barrier characteristics for pure H40 are predicted. As seen in Table 1, the pure H40 at 0%RH displayed considerably better oxygen barrier characteristics than PET and comparable to those for EVOH with 48% (mol/mol) of ethylene. EVOH copolymers with low and moderate ethylene content are considered benchmark materials for packaging applications. When exposed to ambient humidity (50% RH) the barrier properties were reduced, but still better than PET. Figure 6 (a), (b), and... 

Table I. Oxygen barrier characteristics of pure Boltorn H40 and H40/HDMI networks at 0% and 50% RH.

Thus, recognition of the characteristic internal stress buildup in a plasma polymer is important for estimating the upper limit of thickness of a plasma polymer for a practical application. Poor results with respect to such parameters as adhesion and barrier characteristics are often due to the application of too thick a plasma polymer layer. The tighter the network of plasma polymer, the higher is the internal stress. Consequently, the tighter the structure, the thinner is the maximal thickness... 

Barrier characteristics with respect to water and salt and other corrosive chemicals... 

The same plasma polymer deposited in a closed-system reactor has a graded elemental composition with a carbon-rich top surface, and the oligomer content is much lower [10], both of which increase the level of adhesion. The adhesion of the same water-borne primer is excellent and survives 8 h immersion in boiling water. When this surface is treated with O2 plasma, the adhesion does not survive 1 h of boiling, while the dry tape test still remains at the level of 5. The water-sensitivity of adhesion depends on the chemical nature of the top surface as depicted by XPS data shown in Figure 28.12. Water-insensitive tenacious adhesion, coupled with good transport barrier characteristics, provides excellent corrosion protection, as supported by experimental data [1-4], and constitutes the basic principle for the barrier-adhesion approach. 

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