The polymerase chain reaction (PCR) exponentially amplifies a DNA template and produces large numbers of specific DNA fragments. Because of its high sensitivity and efficiency, it becomes a common technique that is widely used in molecular biological. The addition of nano-gold particles into the template solution was found to greatly enhance the efficacy of PCR [1, 2]. Although the thermal effect has been suggested, the underlying mechanisms are still not clear, e.g. how the temperature field affects the replication of DNA at the molecular level. A typical PCR process consists of repeated cycling of three major steps: (1) fast heating to a temperature around 94°C for separating of the two strands DNA templates into single strand (Denaturation); (2) lowering the temperature to about 54°C for the primers to find the complementary part and anneal to the single stranded templates (Annealing); (3) increase of the temperature again to around 72°C for copying of the single templates with the action of the polymerase enzyme (Extension). The thermal history of the PCR determines the activity of the polymerase and rate of each reaction taken place during the process. And the motion of the primers, the DNA templates and the bases owing to both the Brownian effects and concentration gradients can also influence the specificity and reaction rates. The addition of nano-gold particles is expected to greatly alter both the heat and mass transfer efficiency. Thus, the micro-heat and mass transfer analysis of the nano-gold added PCR process has been performed. The temporal and spatial temperature distribution, and the reactants and products concentration have been numerically simulated. The influence of added nano-gold particles on the polymerase reaction rate, the efficiency and specificity of the PCR has evaluated. The possible thermal wave and resonance of the two-phase fluid at the micro-scale level has also been investigated, as well as the enhanced mass transfer of the templates and the reaction rate. The results show that both the thermal effects and physical properties of the nano-particles have contributed to the increase of the efficiency and specificity in the PCR process.

It has been shown that the contact motion path in artificial knee bearings is implant design dependent [1]. It is also known that multidirectional motion of artificial joints leads to higher wear rates compared to the application of unidirectional motion [2,3]. This phenomenon has been linked to “strain-hardening” of ultra-high molecular weight polyethylene (PE): the molecular chains in the surface layer align along the direction of shear. Since more energy is required to rupture the fibrils in their longitudinal direction than perpendicularly, less wear is generated if the direction of movement follows the molecular chain orientation [3]. Computer models could potentially become an important tool in wear prediction if this relationship could be quantified. A variable, defined as ‘cross-shear’ A/(A+B), with A being the fraction of motion along the molecular orientation and B being the fraction of motion perpendicular to the molecular orientation, takes this into account (Fig.1); however, pin-on-disc studies trying to validate this model found agreement for small cross-shear values only [4].

The meniscus plays an important role in many biomechanical functions of the knee, including shock absorption, load distribution, and joint lubrication. The ability to perform these functions is determined by the interactions of the major tissue constituents (water, proteoglycan, and collagen), although these interactions have not been as thoroughly studied in fibrocartilage as those in other soft tissues. In articular cartilage, electrochemical interactions between negatively charged glycosaminoglycan (GAG) side chains of proteoglycans and the interstitial fluid generate an osmotic swelling pressure that contributes to the compressive stiffness [1], and proteoglycan degradation dramatically decreases compressive and shear moduli [2]. Although the concentration of proteoglycan in the meniscus is substantially lower (<1%) than that in articular cartilage, aggrecan degradation also greatly decreases the compressive and shear moduli of meniscal fibrocartilage [3]. The proteoglycan distribution in meniscal fibrocartilage is macroscopically heterogeneous [4] and is concentrated in the secondary matrix surrounding the circumferential collagen bundles, and the extent to which osmotic interactions explain the influence on fibrocartilage material properties is unknown. Altering the osmotic strength of the bathing solution supplies a means to control osmotic interactions between the GAGs and the environment without degrading the tissue matrix. The objective of this study was to examine the effects of an altered osmotic environment on the dynamic shear modulus of meniscal tissue.

The purpose of this study was to understand how the side group dictates thermo-mechanical properties of shape-memory acrylate networks, specifically strain to failure and toughness. A useful parameter in assessing shape memory polymers is the strain to failure because it is critical to know how much recovery strain the material can experience. To understand how the structure is related to mechanical properties, such as strain to failure, materials of differing chain stiffness ratio, C ∞ , were compared at varying percentages of crosslinker. While the chemical and thermal properties of acrylate networks have been discussed in much detail, methods of toughening networks by the precise choice of certain acrylates have not been thoroughly examined. In order for these networks to be of practical use as biomedical devices, such as minimally invasive shape memory polymer stents, detailed structure-property relationships must be established.

Dynamic knee simulators use muscle analogs to incite motion at the knee joint [1–5]. There is a great variation between simulators in choice of powering of muscle analog systems, what movements are fixed, controlled, or unconstrained, as well as whether or not the system is open or closed kinetic chain. The objective of this work is to develop a portable dynamic knee simulator that allows simulation of knee joint motions when it is maximally flexed during a large range of activities involving the lower limb in both a vertical (e.g. deep squat) as well an horizontal position (e.g. rowing).

Collagen is one of the most abundant proteins in the human body. Due to its biological significance and omnipresence, it has been intensely studied for several decades both at the molecular and at the tissue level. A single collagen molecule is a rope like super helix of three α-chains. Collagen molecules self assemble in an orderly fashion to form different suprastructures such as fibrils and networks, providing mechanical strength to the tissue. Collagen density and suprastructural organization determines the local mechanical tension in a particular region of the tissue. Cells sense the tension and in turn trigger a controlled process of growth and remodeling of tissues through secretion or degradation of extracellular components including collagen. Tissue homeostasis is a complex process and adding to such complexity, a single collagen molecule is unstable at body temperature [1], but its stability increases after self-assembly and cross-linking.

Kinesin is a processive molecular motor found in various cells including neurons, that transports membrane-bound vesicles and organelles along the microtubule. Kinesin typically consists of three distinct domains: two large globular heads that attach to the microtubule, a central coiled region, and a light-chain that attaches to the cellular cargo. The metabolic energy that drives kinesins is provided in the form of ATP. The energy released by ATP hydrolysis is converted into direct movement after kinesin binds strongly to the microtubule. Two mechanisms were proposed to explain the movement of kinesin along microtubules: the “hand over hand” model in which the two heads alternate in the role of leading and the “inchworm” model in which one head always leads.

Collagen degradation is a mechanism for normal musculoskeletal development and extracellular matrix (ECM) maintenance, and in response to trauma, disease and inflammation. Matrix metalloproteinases (MMP-1, 8, and 13, the collagenases) are the primary enzymes that act to degrade collagen. These MMPs gain access to the collagen triple helix by binding to the enzyme’s attachment domain along the α-chains, followed by separation (unwinding) of the α-chains to expose the 3/4–1/4 cleavage site, and then cleavage of the α-chain by the enzyme’s catalytic domain [3, 5].

Alternative splicing within proteins is common but not well understood in its influence on protein structure and stability. Filamins are ubiquitous actin-crosslinking proteins with two dozen Immunolgobulin (Ig) repeats and one alternatively-spliced ‘hinge’ that has been hypothesized to add flexibility. The hinge is also predicted to perturb folding. The molecular mechanics of filamins are probed here by AFM-forced extension, with a particular focus on the ∼30 aa hinge between repeats R15 and R16. After re-examining full-length filamin to clarify the single molecule limit for AFM experiments on long chains, short concatemers of (R15-R16) m and (R15- hinge -R16) m were studied by both AFM and solution structural methods. AFM shows that the hinged isoform extends and unfolds at smaller forces (60 pN) than the hinge-less form (80 pN), implying that the alternative splicing introduces a random coil that softens both adjacent domains. Circular Dichroism confirms that the hinge is a random coil, and thermal unfolding in solution suggests a weak destabilization by the hinge. Together with the rate-dependence of forced extension in AFM, the results reveal added resilience as the unfolding transition shifts to longer lengths upon insertion of the alternatively spliced hinge.

Decorin (DCN), a class I member of the small leucine-rich proteoglycan (SLRP) family, is composed of a protein core of approximately 40kDa [1, 2] substituted with a single glycosaminoglycan (GAG) chain of chondroiton/dermatan sulfate on the N-terminal site [3]. DCN has been reported to interact with collagen [4,5] via its core protein, influence collagen fibrillogenesis [6], and inhibit the growth rates of various cell types when added exogenously to cell cultures [5,6]. There has recently been growing interest and studies in DCN related research using the knockout (KO) mice model which provides an excellent example of inherited disorders that stem from deficiencies in decorin expression [7]. Skin and tendon tissues from DCN KO mice have been characterized as being extremely fragile with significantly reduced strength and stiffness [8, 9]. The DCN KO tissues also show potential functional biglycan compensation [9] and at the microscopic level collagen fibrils with highly irregular diameters, abnormal lateral fusion, and loose packing [6] in contrast to wild type (WT) mice. Despite the intensive investigation of the DCN KO mice, the complexity of the animal model makes it difficult to assess the actual influence of decorin. In an attempt to take a more simplistic approach 2D cell phenotypic characterization studies were performed in addition to studying cell growth, contraction, and matrix organization in 3-D models to show the very distinct biochemical responses to type I collagen when compared to WT control cells.

The mechanical properties and structure of connective tissues such as the cornea and articular cartilage are derived from the functions and properties of their extracellular matrix, which is a polyelectrolyte gel composed of collagenous fibers embedded in an aqueous matrix. The collagen fibrils in the extracellular matrix of the corneal stroma are arranged in a regular lattice structure, which is necessary for corneal transparency and transmitting the incident light to the back of the eye. This regular pseudo hexagonal arrangement is attributed to the interaction of collagen fibrils with the proteoglycans; these regularities are lost in proteoglycan knock-out mice [1]. Proteoglycans are heavily glycosylated glycoproteins consisting of a core protein to which glycosaminoglycan chains are covalently attached. The main proteoglycan in the corneal stroma is decorin. Decorin is the simplest small leucine-rich proteoglycan with only a single glycosaminoglycan side chain. It has a horse shape core protein and binds collagen fibrils at regular sites. Under normal physiological conditions, these linear carbohydrate polymers are ionized and carry negative charges due to the presence of negatively charged carboxylate and sulfate groups. The presence of these fixed charges creates an imbalance of charge density within the stroma and its surrounding aqueous domain. Therefore, the tissue has a tendency to swell when immersed in a bathing solution. In order to create mathematical models for the corneal mechanics, a proper experimental characterization of the swelling properties of the tissue is necessary.

Shear induced hemolysis (SIH) refers to the rupture of red blood cells in shear flows resulting in the release of hemoglobin into plasma. Hemolysis in turn triggers a chain of bio-chemical events involving all the components of blood leading to the formation of thrombosis (blood clots). While SIH is negligible in a healthy circulatory system, it is widespread in life saving blood contacting devices such as heart valves, ventricular assist devices, and blood pumps. Thus an accurate physical understanding of the process of hemolysis is critical towards developing better predictive models of SIH, particularly in the context of engineering future blood contacting devices that are significantly more blood cell friendly.

Irregular sickle red blood cells (RBCs) can contribute to the pathogenesis of vasoocclusion and other complications of sickle cell disease (SCD) via abnormal adherence to the vascular endothelium. It has previously been demonstrated that epinephrine enhances SCD RBC adhesion by activating the BCAM/Lu and ICAM-4 surface receptors [1–2]. Epinephrine acts on the RBC β2-adrenergic receptor, thereby activating Gas proteins that stimulate adenylyl cyclase (AC). This enzyme catalyzes the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP), leading to protein kinase A (PKA) activation, an intermediate step in the upregulation of BCAM/Lu and ICAM-4 mediated adhesion. The interaction of BCAM/Lu with the α5 chain of laminin may contribute to vaso-occlusive events in SCD due to overexpression of BCAM on SCD RBCs.

The mechanical behavior of the cornea is mainly governed by the microstructure and composition of the stroma. The stroma is a highly ordered extracellular matrix and constitutes about 90% of the corneal thickness. From the mechanics point of view, the corneal stroma can be considered as a polyelectrolyte gel which is composed of collagen fibrils embedded in an aqueous matrix. The collagen fibrils compose about 70% of cornea’s dry mass and are arranged in a regular lattice structure [2]. Previous studies have shown that while the collagen fibrils are primarily located parallel to the surface, they are not distributed uniformly in all directions and their preferred orientation is not same in different species. For example, collagen fibrils are almost equally distributed in the nasal-temporal and inferior-superior directions in healthy human corneas [4] and they are mainly aligned in the inferior-superior direction in bovine corneas[2]. The differences in the orientations of the collagen fibrils have seen to have important implications on the mechanical properties of the cornea. In addition to this observation, the relative distance between the collagen fibrils is expected to play a role in defining the mechanics of the tissue. It is well-documented that the proteoglycans bind collagen fibrils at regular sites and control their relative position. The main proteoglycan in the corneal stroma is decorin. Decorin is the simplest small leucine-rich proteoglycan with a single glycosaminoglycan side chain. Chondroitin sulfate, dermatan sulfate, and keratan sulfate are among the prevalent glycosaminoglycans found in the cornea. Under physiological conditions, these linear carbohydrate polymers are ionized and carry negative charges. Therefore, a hydrated gel is formed in the empty space between collagen fibrils by attracting water. It is known that the interaction of these negatively charged glycosaminoglycans with themselves and with the free ions contribute to the corneal swelling pressure and subsequently to its compressive stiffness. Nevertheless, their possible influence on the corneal tensile properties is yet to be determined. In this work, we experimentally characterized the tensile properties of the bovine corneal stroma in different bathing solutions. Furthermore, a quasi-linear viscoelastic (QLV) model was used to examine the effect of bathing fluids and corneal hydration on mechanical parameter of the cornea.

We have previously reported that stress fibers (SFs) in cells with normal contractility reorient perpendicular to the direction of cyclic stretch [1], while cells treated with inhibitors of myosin light-chain kinase (MLCK) or members of the Rho GTPase pathway oriented parallel to the direction of stretch [1, 2]. Our theoretical modeling predicts that myosin II acts as a tension sensor acting to maintain SF tension through sliding along actin filaments under low strain rates or promoting SF reorientation under high strain rates [3]. This model predicts that SFs on stiff elastic substrates are at an optimal level of tension and that cyclic stretching subjects these SFs to excessive loads in the direction of stretch, hence SFs preferentially orient away from the direction of stretch. On soft substrate, intracellular tension is expected to be much lower than on stiff substrates [4]. Consequently, we postulated that stretching cells on a soft substrate would induce cell and SF alignment parallel to the direction of stretch in a manner dependent on cell contractility.

The basement membrane is a specialized part of the extra-cellular matrix. It is usually characterized as a scaffold for epithelial cells but in some tissues it serves other, mechanical, roles [1]. The mechanical properties of the basement membrane are mainly determined by one of its main constituents — type IV collagen. Unlike the well-known fibrous type I collagen, collagen IV assembles into planar networks (Fig. 1) [2]. The α 1 (IV) and α 2 (IV) collagen IV chains assemble into the so-called major chain network, present in all basement membranes. The α 3 (IV), α 4 (IV), α 5 (IV) collagen IV chains form the minor chain network which is found only in the adult basement membranes of the kidney glomerular capillaries (GBM), ocular lens (LBM), cochlea, and the testes [3]. The minor chains have a higher number of cysteine residues, allowing them to form a higher number of lateral interactions. In the minor chain network, the greater potential to interact laterally manifests in the formation of super-coils, which are rarely observed in the major chain network [4]. Increasing the number of cross-links in a polymeric material is known to increase material stiffness; therefore, it is believed that the minor chain network confers basement membranes with additional strength and stability [5]. In the hereditary disease Alport syndrome, a mutation causes the absence of the minor chain network. The GBM and LBM of Alport patients appear weakened and unable to meet their mechanical demands, further supporting this theory [6]. The objective of this study was to evaluate the importance of cross-linking in the minor chains for the mechanical properties of type IV collagen networks, specifically in the GBM and LBM where the absence of the minor chains has an observed mechanical effect.

Different muscle groups function in a synchronized and coordinated manner to perform a given task, wherein the activity of one muscle group affects that of another (1, 3). Osteoarthritis (OA) is a common, chronic joint disease characterized by pain, disability and progressive loss of function. It is the most common musculoskeletal complaint worldwide and is associated with significant health and welfare costs (2, 7). The knee is the most frequently affected joint of the lower limb and prevalence of knee OA increases with age (6). Several studies have shown that muscle recruitment patterns and neuromuscular efficiency are different for patients with OA compared to normal controls during simple closed chain activities (4). However, no studies have investigated frontal plane control or the modulation of control due to, for example, a change in time or distance in OA (5).

The mechanical properties and structure of connective tissues such as the cornea and the cartilage are derived from the functions and properties of their extracellular matrix, a polyelectrolyte gel composed of collagenous fibers embedded in an aqueous matrix. The collagen fibers in the extracellular matrix of the corneal stroma are arranged in regular lattice structures, which is necessary for corneal transparency and transmitting the incident light to the back of the eye. This regular pseudo hexagonal arrangement is attributed to the interaction of collagen fibers with the proteoglycans as these regularities are lost in knock-out mice [i]. Proteoglycans (PGs) are heavily glycosylated glycoproteins. They consist of a core protein to which is glycosaminoglycan chains are covalently attached. The main PG in the corneal stroma is the proteoglycan decorin. Decorin is the simplest small leucine-rich PG and only has a single glycosaminoglycan side chain. It has a horse shape core protein and binds collagen fibrils at regular sites. Chondroitin sulfate (CS), dermatan sulfate (DS), keratan sulfate (KS) are among the prevalent glycosaminoglycans found in the cornea. Under physiological conditions, these linear carbohydrate polymers are ionized and carry negative charges due to the presence of negatively charged carboxylate and sulfate groups. Therefore, a hydrated gel is formed in the empty space between collagen fibrils by attracting water. The interaction of negatively charged glycosaminoglycans with themselves and their interaction with the free ions contribute to the corneal swelling pressure and subsequently to its compressive stiffness. From structural view point, the corneal stroma is a composite polyelectrolyte system in which the observed regular spacings of the collagens are suggested to exist because of the structural interaction of collagens, negatively charged glycosaminoglycans, and the free ions in the interfibrillar space.

The inhomogeneous distribution of crosslinks in polymer networks results in nonuniform swelling. Concomitant with this behavior is local variability in the elastic properties of synthetic and biopolymer gels. Articular cartilage exemplifies the compositional and structural complexities found in soft tissues. At the most basic level, cartilage extracellular matrix (ECM) is a relatively stiff network of collagen type II fibers with entangled hyaluronic acid chains and enmeshed aggrecan molecules. Despite significant differences in composition, synthetic and biological gels exhibit qualitatively similar responses (e.g., viscoelasticity and nonlinear stress-strain behavior at large deformation). Scaling theory [1] and experiments [2–3] have verified that the shear modulus ( G e ) of chemically identical, fully swollen gels differing only in the degree of crosslinking is proportional to the polymer concentration ( c e ): (1) G e = A c e n where A and n are constants. In a good solvent, n = 2.25 [1]. Recent studies have shown that the power law applies to collagen gels, with n ≈ 2.68 [4]. In the general case, (2) G = A c e n - m c m where G and c are the general shear modulus and polymer concentration, respectively, and m = 1/3 [5].

Endothelial cells (ECs) form the inner lining of the blood vasculature and are exposed to shear stress (τ), the tangential component of hemodynamic forces. ECs transduce τ into biochemical signals possibly via EC-membrane perturbations. We have previously used confocal-FRAP on the DiI-stained plasma membranes of confluent cultured bovine aortic ECs (BAECs) to show that τ induces a rapid, spatially heterogeneous, and time-dependent increase in the lateral diffusion of the fluorescent lipoid probe in the BAEC membrane [1]. We now present evidence at the single molecule level that shear stress differentially perturbs membrane domains that are defined by their selective staining by lipoid dyes (DiI) of differing alkyl chain lengths. This study is the first to directly measure perturbation by shear stress of endothelial cell membrane microdomains.