Brick and mortar structure

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... 

Fig. 2. Nacre of red abalone shell Halitotis refescens), imaged here by transmission electron microscopy (left), has a bricks-and-mortar structure. The bricks are CaCOs (aragonite) platelets, and the mortar is a composite of macromolecules, including structural proteins and polysaccharides, that form a thin film around the platelets. The three-dimensional structure is depicted on the right [23]...

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. 

Due to the brick-and-mortar structure of the stratum corneum, the skin is a difficult layer to permeate across for most active pharmaceutical ingredients. Because of this diffusional barrier, new strategies have been developed to allow compounds to better penetrate the stratum corneum [28], These strategies can be defined as either chemical or physical approaches to disrupting the barrier function of the skin. 

The structure of the stratum corneum has been illustrated as a "bricks and mortar" structure, which is similar to a wall. The "bricks" are mainly corneocytes of hydrated keratin and the... 

The stratum corneum is the outer most layer of nonviable epidermis. It has a thickness of about 10 to 12 pm. The stratum corneum consists of 15 to 25 layers of flattened, stacked, hexagonal, and cornified cells known as corneocytes. Each cell is approximately 40 pm in diameter and 0.5 pm in thickness [Bouwstra, 1997]. The thickness of stratum corneum varies with the site of human body. The body extremities such as palms and soles have a thicker stratum corneum [Walters and Roberts, 2002]. The stratum corneum is characterized by an array of keratin-rich corneocytes surrounded by lipid lamella made of cholesterol, free fatty acids, and ceramides [Bouwstra, 1997]. The corneocytes are arranged in brick and mortar structure. Such structural arrangement creates a tortuous intercellular diffusion pathway for water or any other molecules that transverse the stratum corneum. The hydrophobic lipids that surround these diffusion paths or water pores are organized in tight lamellar structure. The summative effects translate to the formation of a tight permeation barrier [Menon, 2002]. 

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). 

FINITE ELEMENT ANALYSIS OF CRACK-PATH SELECTION IN A BRICK AND MORTAR STRUCTURE... 

We employ the finite element method and interface fracture mechanics concepts to parametrieally examine interfacial crack path selection in a unit brick-and-mortar structure. The analysis methodology, to be described below briefly, is based on the analyses of cracks kinking into and out of a bi-material interface as discussed by He and Hutchinson [5], He et al [6,7], and Suo and Hutchinson [8]. 

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. 

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. 

Highly structured, 3-D nanoparticle-polymer nanocomposites possess unique magnetic, electronic, and optical properties that differ from individual entities, providing new systems for the creation of nanodevices and biosensors (Murray et al. 2000 Shipway et al. 2000). The choice of assembly interactions is a key issue in order to obtain complete control over the thermodynamics of the assembled system. The introduction of reversible hydrogen bonding and flexible linear polymers into the bricks and mortar concept gave rise to system formation in near-equilibrium conditions, providing well-defined stmctures. 

The previous chapter described the types of secondary and tertiary structures that are the bricks-and-mortar of protein architecture. By arranging these fundamental structural elements in different combinations, widely diverse proteins can be constructed that are capable of various specialized functions. This chapter examines the relationship between structure and function for the clinically important globular hemeproteins. Fibrous structural proteins are discussed in Chapter 4. 

The barrier properties of human skin have long been an area of multidisciplinary research. Skin is one of the most difficult biological barriers to penetrate and traverse, primarily due to the presence of the stratum corneum. The stratum cor-neum is composed of comeocytes laid in a brick-and-mortar arrangement with layers of lipid. The corneocytes are partially dehydrated, anuclear, metabolically active cells completely filled with bundles of keratin with a thick and insoluble envelope replacing the cell membrane [29]. The primary lipids in the stratum corneum are ceramides, free sterols, free fatty acids and triglycerides [30], which form lamellar lipid sheets between the corneocytes. These unique structural features of the stratum comeum provide an excellent barrier to the penetration of most molecules, particularly large, hydrophilic molecules such as ASOs. 

Brick and mortar materials should only be used alone where the fact that gas and liquid can penetrate, though slowly, through them is not important, but where their considerable compressive strength (load bearing ability), combined with their resistance to chemical attack can be useful. Examples of these types of structures are self-supporting chimney liners (some of them 800+feet tall), foundations set in acid contaminated soil, and supports for chemical equipment subject to splash or spill. 

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.

Human skin has a multifunctional role, primary among which is its role as a barrier against both the egress of endogenous substances such as water and the ingress of xenobiotic material (chemicals and drugs). This barrier function of the skin is reflected by its multilayered structure (Fig. 5.1). The top or uppermost layer of the skin known as the stratum comeum (SC) represents the end product of the differentiation process initially started in the basal layer of the epidermis with the formation of keratinocytes by mitotic division. The SC, therefore, is composed of dead cells (comeocytes) interdispersed within a lipid rich matrix. It is the brick and mortar architecture and lipophilic nature of the SC, which primarily accounts for the barrier properties of the skin [1,2]. The SC is also known to exhibit selective permeability and allows only relatively lipophilic compounds to diffuse into the lower layers. As a result of the dead nature of the SC, solute transport across this layer is primarily by passive diffusion [3] in accordance with Pick s Law [4] and no active transport processes have been identified. 

The deterioration of brick masonry is a complex problem, in which two variable ingredients, brick and mortar, make up the whole. As each component can have a large variation in composition and structure, when both are combined in a wall, the number of variations that result is given by all the possible combinations of the two components. 

The section on masonry deterioration focuses on limestone, coquina, sandstone, marble, concrete, brick, and mortar as related to acid deposition effects on structures such as buildings and on cultural resources such as monuments. 

Brick linings, like other masonry structures, have low flexural and tensile strength but high compressive strength. Brick linings, especially on vertical walls, must, therefore, be designed so that the bricks and mortar are always under a compressive load-... 

The organizational structure of the skin layers is diagrammatically represented in Figure 2. The relative thickness of the epidermis and dermis is shown. If the skin layers are drawn in a diagrammatic way, the orderly layers of cells forming the viable (live) epidermis are seen. The dead, dry, scaling cells of the stratum corneum have a structure that is likened to bricks and mortar. ... 

The skin is a complex organ that is the major interface between hiunans and their environment. The skin s primary function is to restrict passive water loss from the body. It is the outermost 10 to 15 paa thick layer of the skin, the stratum corneum (SC), which provides this barriCT function. The SC is composed of dead keratinized cells (comeocytes) embedded in a matrix of hpids, a structure that has been represented as a brick-and-mortar wall (Schaefer and Redelmeier, 1996). However, the SC is not totally impermeable, of course, and chemicals with appropriate physicochemical properties can be absorbed in amounts sufficient to provoke both local and systranic pharmacological and toxic effects (Guy, 1996). 

As if site selection is not difficult enough, then comes the actual construction of the facility. On a blueprint, the requirements have Uttle more than conceptual meaning once brought into reality with bricks and mortar, the structure will either prove immensely effective and efficient or be a capitally intensive reminder of an organization not fully comprehending its strategy. 

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