Brick-and-mortar model

The stratum corneum consists of separated, nonviable, cornified, almost nonpermeable corneocytes embedded into a continuous lipid bilayer made of various classes of lipids, for example, ceramides, cholesterol, cholesterol esters, free fatty acids, and triglycerides [6], Structurally, this epidermis layer is best described by the so-called brick-and-mortar model [7], The stratum corneum is crucial for the barrier function of the skin, controlling percutaneous absorption of dermally applied substances and regulating fluid homeostasis. The thickness of the stratum corneum is usually 10-25 /an, with exceptions at the soles of the feet and the palms, and swells several-fold when hydrated. All components of the stratum corneum originate from the basal layer of the epidermis, the stratum germinativum. 

Heisig M, Lieckfeldt R, Wittum G, Mazurkevich G, Lee G (1996) Non steady-state descriptions of drug permeation through stratum comeum. I. The biphasic brick-and-mortar model. Pharm Res 13 421 —426. 

Figure 9.21 Bricks and mortar model of ifie stratum corneum, illustrating possible pathways of drug permeation through intact stratum corneum (transcellular and tortuous intercellular pathways) and the lamellar structure of intercellular lipids.

The Stratum Corneum (Horny Layer). Typically, the stratum corneum comprises only 10 to 15 cell layers and is around 10 p.m thick when dry, although it may swell to several times this thickness when wet (see section 4.1). As with the viable epidermis, the stratum corneum is thickest on the palms and soles and is thinnest on the lips. This thin membrane, consisting of dead, anucleate, keratinised cells embedded in a lipid matrix allows for survival of terrestrial animals without desiccation. The stratum corneum serves to regulate water loss from the body while preventing the entry of harmful materials including microorganisms. The stratum corneum has been represented as a brick and mortar model (Michaels... 

Terminally differentiated keratinocytes of the stratum corneum are known as comeocytes and are largely devoid of normal cellular functions, being predominantly composed of protein (keratin). The ultrastructure of the stratum corneum is described by the brick and mortar model (Elias, 1983 Figure 3). The functional implication of this architecture is that some skin penetrants must diffuse via a long and tortuous route between adjacent comeocytes, thus reducing their rate of absorption. This is known as the intercellular route. In contrast, some chemicals may diffuse equally through both comeocytes and the lipid mortar, resulting in a transcellular route. 

Figure 3. Schematic representation of the stratum corneum ( Brick and Mortar model ) and corresponding routes of transcellular and intercellular penetration...

Products in decline hang on for dear life. Unless they move backward to another phase rejuvenating their product or supply chain, they will not survive. Crown Books, an early discounter, lost market share to Barnes Noble and Borders who sold the same books as they did, but with more extended product amenities such as coffee and reading areas. In another turn, Internet marketer Amazon.com challenged Crown s brick and mortar model. 

It must be emphasised that the brick-and-mortar model is not a structural model but a conceptual one, since it does not take lipid structure into account as a means for explaining the properties of the human skin barrier. This lack of a structure-function concept in skin-barrier research may be one reason for the fact that numerous studies on the penetration of substances... 

The permeability skin barrier, a highly specialized structure responsible for retaining skin moisture, is localized mainly at the stratum corneum (Rudikoff 1998) however, its formation begins deeper in the epidermis and its constituents are progressively modified during the process of keratinization until they reach their highest efficiency in the five layers of the stratum compactum (Rawlings et al. 1994). The modified keratinocytes - the corneocytes - and the intercellular complex lipid matrix in which they are embedded form this specialized structure, which Elias compared to a bricks and mortar model, in which the corneocytes are the bricks and the lipid matrix the mortar (Elias 1983). 

In an early, quite elaborate model for the diffusion through the stratum corneum, Michaels et al. derived an equation for diffusion through a two-dimensional brick-and-mortar structure [50], In this model, stratum corneum permeability for a given compound depended only on two parameters one was the product of the partition coefficient between the protein and the donor phase /fprot/donor and the diffusion coefficient in the protein phase >Prot the other was the product of the partition coefficient between the lipid and protein phases Aip/prot and the ratio of the diffusion coefficients in the two phases... 

In 1975, Michaels et al.33 presented a conceptual model of the arrangement of corneocytes and lipids in stratum corneum. They envisaged stratum corneum as a brick and mortar structure with the keratin filled corneocytes as bricks and the intercellular lipids as mortar. This model was further explored by Elias and co-worker.34-37 This model does not per se include a structure-function perspective on the barrier but has had a tremendous impact on the research on stratum corneum and its composition, function, and the regulation of homeostasis. 

Of course, true competitive advantage is usually the result of making coordinated innovations on multiple fronts. Through the combination of its iPod and iPhone products with its iTunes business model, Apple created about 70 billion in shareholder value in just three years. Amazon s business model innovation of cutting out the brick-and-mortar store is coupled with service innovations such as 1-Click, Recently Viewed Items, Customer Reviews, and Books You Might Also Like. This is a taste of how these companies have innovated across the board. 

We provide recommendations along two broad categories. Section 4.1 discusses some limitations in existing model representations of the various channel types, especially in capturing the differences between online and bricks-and-mortar channels. Some perspectives on the analytical appraisal of alternative channel strategies are offered in Section 4.2. Section 4.3 presents our closing remarks. 

A number of experiential distinctions are especially pronounced when comparing direct and bricks-and-mortar channels. These include visual and tactile interaction with the product, the method of obtaining product information (e.g., face to face, over a phone line, or through a computer screen), the delay between purchase and receipt, and even the crowds that often define bricks-and-mortar shopping (which can either be a positive or a negative depending on individual preferences). Naturally, these types of qualitative factors have been difficult to model in any detail. 

Alternative types of multi-channel strategies. The basic distinctions among the channel types covered are (1) intermediated vs. manufacturer-owned and (2) online vs. bricks-and-mortar. Multi-channel models typically represent some combination of the following possibilities ... 

Elias and his co-workers have popularized a model which views the stratum corneum as a brick-and-mortar structure (Elias and Menon 1991). The bricks are protein-rich corneocytes separated by lipid-rich intercellular domains consisting of stacks of bilaminar membranes. Excellent reviews of the complex structure of the stratum corneum and the dynamics of its behavior have been presented by Menon and Fartasch Menon and Ghadially 1997 Farasch et al. 1993). 

In this paper, we examine the pathological behavior of cracks propagating in model brick-and-mortar structures using the interface fracture mechanics methodology described while adopting a Kn = 0 criteria for the crack kinking out of an interface. 

We can also contrast Blue Nile, with its online retailing model for diamonds, with Zales, which sells diamond jewelry through retail outlets. Blue Nile has emphasized the variety of diamonds available from its website and the fact that its margins are significantly lower than its bricks-and-mortar competition. Customers, however, have to wait to get their jewelry and do not have any opportunity to touch and see it before purchase (Blue Nile does provide a 30-day return period, though). At Zales, in contrast, a customer can walk into the retail store, be helped by a salesperson, and leave immediately with a diamond ring. The amount of variety available at a Zales store, however, is limited. Whereas Blue Nile offers more than 90,000 stones on its site, a typical Zales store carries fewer than a thousand. 

Meanwhile, Redbox, a unit of Coinstar Inc., operated vending machines that rented DVDs for as little as 1 a night. Despite its best efforts. Blockbuster s brick-and-mortar stores could not match the low-cost operating models of Netflix and Redbox, leading to its bankruptcy (see financial results in Table 2-5). 

Regarding the modeling of the infill panels, the great number of influencing factors, such as dimension and anisotropy of the bricks, joint width and arrangement of bed and head joints, material properties of both brick and mortar, and quality of workmanship, make the simulation of plain brick masonry extremely difficult. The level of complexity of the analytical model depends on whether masoiuy is considered as one-, two-, or three-phase material, where the three phases are comprised of the units, mortar, and unit-mortar interface. 

In the FEM simulations, pinned support (Ux = 0, Uy = 0, and Uz = 0) was assumed at the base of the walls. The Plastic Damage model available in ABAQUS was also adopted in the simulations. The materials used in the walls were concrete (concrete brick tmits) and mortar (head/bed joints between bricks). Both materials have been considered as a continuum media and simulated using the continuum damage model. The only material properties available in (Li et al. 2005) were the concrete and mortar ultimate compression strengths. Other needed data was extracted using ACI formulas. Some of the parameters needed in the Plastic... 

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