The endothelial cell (EC) cytoskeleton mediates several biological functions such as adhesion, migration, phagocytosis, cell division, and mechanosensitivity. These functions are carried out in part through dynamic cytoskeletal polymerization, modulation of crosslinking, and development of tension between intracellular organelles and the extracellular matrix via focal adhesion plaques. One important component of the cytoskeleton is actin which polymerizes into filaments and is thought to be prestressed by virtue of crosslinking proteins such as α-actinin, filamin and myosin II molecular motors. Additionally, actomyosin interaction has been hypothesized to act as a stress dissipation mechanism by virtue of dynamic crossbridging which facilitates actin diffusion through the polymer network of the cytoplasm (Humphrey et al., 2002).

Bone fracture has imposed a significant burden on the health of society. The “bone quality” is used to refer to factors affecting bone fracture risk [1]. Energy dissipation till fracture, known as toughness, is a major measure of bone quality [2]. However, underlying mechanisms of energy dissipation in bone is still not clear. It has been well documented that the post-yield behavior of bone determines the major part of the toughness of bone [3, 4]. Therefore, it is important to study post-yield behaviors of human bone, especially the different pathways for energy dissipation, in order to better understand how age-related change affects bone quality. Bone behaves differently under different loading modes [5]. Different from loading in tension, after reaching the maximum stress, cortical bone in compression can continue to bear load till large deformation without brittle failure and dramatic reduction in elastic modulus [5, 6]. However, few studies of progression of post-yield behaviors of cortical bone in compression were reported in the literature.

Independent of the disease type, high hemodynamic energy dissipation in the circulation always impacts the pumping ventricle that, often always resulting in chronic heart failure as a secondary disease. For aortic stenosis, which is an arterial disease, an index based on total pressure (static plus dynamic) loss, which is a pseudo indicator of energy dissipation based on Bernoulli’s Equation, is widely used, while raw energy loss has been proposed to assess the severity of the Fontan connections clinically. Both diseases have separate frameworks for hemodynamic assessment of the respective dissipative lesions.

The architecture of the vascular wall is highly intricate and requires unique biomechanical properties in order to function properly. Native artery is composed of a mix of collagens, elastin, endothelial cells (ECs), smooth muscle cells (SMC), fibroblasts, and proteoglycans arranged into three distinct layers: the intima, media, and adventitia. Throughout artery, collagen and elastin play an important role, providing a mechanical backbone, preventing vessel rupture, and promoting recovery while undergoing pulsatile deformations [1]. The low-strain mechanical response of artery to blood flow is dominated by the elastic behavior, of elastin, which prevents pulsatile energy from being dissipated as heat [2]. A higher amount of energy loss indicates a decrease in recoverability, which could lead to eventual disruption of blood flow. An effective way to quantify recoverability is through hysteresis and compliance measurement. The hypothesis of this study was that the fabrication of a multi-layered electrospun tissue engineering scaffold composed of polycaprolactone (PCL), elastin, and collagen would demonstrate dynamic mechanical properties indicative of a highly elastic material, similar to the three distinct layers of native arterial tissue, while remaining conducive to tissue regeneration. PCL was chosen, in this case, to provide mechanical integrity and elasticity, while elastin and collagen would provide further elasticity and bioactivity [3,4].

CFD modeling of the anatomical pathways (TCPC) created by the palliative single-ventricle heart defect surgeries is an area studied by several research groups [1–5]. To the best of our knowledge except Walker et al. (in idealized experimental models) [6], all studies focused on the prediction of hydrodynamic power losses or dissipation function and ignored venous hepatic flow distribution to the left and right lungs. Hepatic flow coming from the inferior vena cava (IVC) transports essential growth factors that are required for normal lung growth. Inadequate IVC blood distribution also cause protein loosing enteropathy [7], a serious complex pathology leading to total failure of the surgically created single-ventricle circulation system. Early TCPC configurations distributing the hepatic flow unidirectionaly to one of the lungs [8] were promptly discontinued from the operating rooms and replaced with “+” shaped connections for these reasons, Fig 1.

Wave dissipation in arteries was intensively studied previously in the frequency domain, which focused on the waves with “pushing effect”. Meanwhile, we noted that waves with “suction effect” are observed in pulmonary veins [1], and in coronary arteries [2]. In spite of this, the dissipation of “suction waves” has not been studied before.

The cushioning and conduit functions of large arteries allow the body to efficiently circulate blood and perfuse organs. These functions can be characterized by quantifying the viscoelastic properties of arteries. In this study, we investigated the effects of smooth muscle cell (SMC) tone and pressurization frequency on vascular viscoelasticity using an isolated, perfused vessel test. We tested mouse carotid arteries in control, dilated (via sodium nitroprusside) and constricted (via the thromboxane receptor analogue U46619) states at 0.1, 1, 3, and 5 Hz with a pulse pressure from 90 to 120 mmHg. Dose response experiments were first performed in order to determine the optimal vasoconstrictor concentration to be used in frequency response experiments. Our results showed that energy dissipation was significantly higher and elasticity was significantly lower for vasoconstricted arteries. We also found that frequency significantly changed both energy dissipation and elasticity as the frequency was increased from 0.1 to 5 Hz. These results provide insights into the changes in vascular viscoelasticity caused by SMC tone and pressurization frequency, which have implications for vascular function in vivo.

Left ventricular flows in the human heart are very complex due to the intricate geometry, the pulsatile character of the flow, the interaction of the jets with the LV flexible walls, and the unsteady motion of the leaflets, which generate intrinsically complicated flow structures. Furthermore, although mechanical heart valves (MHV) have received universal recognition, their performance is still unmatched to that of the natural valves. The flow past MHVs induces a combination of flow characteristics, which are clearly dependent on the specific valve design and valve orientation. Traditionally, valve performance and hemodynamics have been quantified using effective orifice size and pressure gradients. However, quality and direction of flow, often overlooked, are also important aspects of valvular function and relate to valve design and orientation. One of the key characteristics in LV flows is the shear layers shedding past heart valves which generate vortex ring formation resulting into energy entrapment, thus reducing the efficiency of the heart.

Further investigations of the mechanical properties of whale blubber will benefit its morphology and those who use it. Located below the dermas, blubber is an insular tissue constructed of a lipid matrix cross-weaved with strong, structural collagen and elastic fiber bundles. The blubber transitions into the superficial fascia layer, a loose connective tissue, which sheaths the muscle surrounding the whale. [1] Blubber should behave viscoelastically because it is a soft tissue. [2] The complex shear modulus G* = G′+iG″ is a viscoelastic property commonly used in defining soft tissues. It is comprised of both an elastic energy storage term (G′) and a viscous energy dissipation term (G″). Apart from adding to the morphology of whale blubber, these properties can currently be used for the improvement of certain whale tracking tag designs. The tags that would gain from these measurements deploy remotely and anchor subdermally in the body of the whale, near the dorsal fin. Once attached, they transmit a radio signal to a monitoring satellite. Knowing the migratory and behavioral patterns of whales allows for the adjustment of human activities to help in the recovery of endangered species.

Single Ventricle Heart Defects (SVHD) are present in 2 per 1000 live births in the US. SVHD are characterized by cyanotic mixing between the de-oxygenated blood from the systemic circulation return and the oxygenated blood from the pulmonary arteries. In the current practice, surgical interventions on SVHD patients commonly result in the total cavopulmonary connection (TCPC) [1]. In this configuration the systemic venous returns (inferior vena cava, IVC, and superior vena cava, SVC) are directly routed to the right and left pulmonary arteries (RPA and LPA), bypassing the right heart. The resulting anatomy has complex and unsteady hemodynamics characterized by flow mixing and flow separation. Pulsation of the inlet venous flow during a cardiac cycle and wall motion may result in complex and unsteady flow patterns in the TCPC. Although vessel wall motion and different degrees of pulsatility have been observed in vivo , non-pulsatile (time-averaged) flow boundary conditions and rigid walls have traditionally been assumed in estimating the TCPC hemodynamic parameters (such as energy loss). Recent studies have shown that these assumptions may result in significant inaccuracies in modeling TCPC hemodynamics [2, 3].

Purpose : Aortic valve disease (AVD) in the form of stenosis, insufficiency, or congenital defect will disrupt normal function beyond the valve itself. This includes an increase in cardiac afterload and a drastic alteration in post-valvular 3D blood flow patterns 1, 2 . The current AHA/ACC standard-of-care guidelines, however, assess disease severity based on simplified measurements local to the valve, such as: peak velocity, effective orifice area, regurgitation, aortic diameter and transvalvular pressure gradient 3 . Paradoxically, it is known that similarly classified AVD patients under these guideline metrics can exhibit radically divergent outcomes — implying an incomplete characterization of the disease 4 . For this reason, functional assessment and risk-stratification may benefit from a robust methodology capable of quantifying the energetic load placed on the left ventricle (LV) due to the presence of AVD. The measurement of viscous energy loss, a parameter which is directly responsible for increased cardiac afterload and is independent of pressure recovery effects, is a promising candidate to quantify LV loading. With this in mind, the 4D flow technique (time-resolved 3D phase-contrast MRI with all principal velocity directions encoded) provides the necessary information to calculate this parameter. Therefore, we present a theoretical basis for the use of 4D flow MRI to characterize in-vivo energy loss and apply the technique in a pilot study of patients with aortic valve stenosis (n = 13) or aortic dilation (n = 17) as compared to normal controls (n = 12).

A recent study of college and high school football players demonstrated that 5.1% sustained at least one concussion in a single season. Considering the number of individuals that participate in football in the United States, this percentage equates to a staggering number [1]. The information and attention dealing with concussions and traumatic brain injury (TBI) has greatly increased recently, and represents a need for more advanced helmets that can eliminate concussions as well as other forms of TBI. In order to obtain this goal, lessons can be learned from high speed impacts in nature, particularly the shock-mitigating effects of the bighorn sheep’s (or ram’s) horn and woodpecker’s hyoid bone. For instance, during fights between male bighorn sheep, the rams clash together at speeds up to 5.5 m/s, causing forces up to 3400 N [2]. Even while undergoing these tremendous forces, the animals are rarely injured, which leads to the notion that the horn geometry plays a role in mitigating the shock wave. The woodpecker’s hyoid bone extends around the skull in a spiral shape. It aids the woodpecker in extending its tongue and helps bypass vibrations generated from drumming, which protects the brain from shock [3]. Does the reoccurrence of this curious (tapered spiral) shape throughout nature have some significance in regards to energy dissipation and shock absorption abilities inherent to its geometry? Answering this was the primary goal of this study.

Flow problems in cardiovascular mechanics involve complex geometries and pulsatile flow that may give rise to instabilities, especially in pathological cases. High-order methods are particularly suitable for resolving such unsteady phenomena whereas low-order methods may exhibit excessive dissipation and hence suppress any such physical instabilities. This, for example, is the case for certain type of cerebral aneurysms, see [1], for which we have demonstrated that shear layer instabilities may be triggered even at physiological flow rates, giving rise to audible frequencies in the range of 10Hz to 50 Hz. Similar phenomena may be present in stenotic arteries, where a jet type flow may develop that is also susceptible to temporal instabilities, especially during the decelerating systole.

Mitral valve (MV) asymmetry in the left ventricle causes a large anterior vortex during left ventricle (LV) filling which is considered to store kinematic energy. This vortex persists in the end of diastole and may facilitate early closure of the mitral valve and transfer kinetic energy in the anterior vortex to help blood ejection through the aortic valve. Washout effect from the anterior vortex may prevent thrombosis in the left ventricle. Recently one of the repair techniques, the mitral valve edge-to-edge repair (ETER) procedure, has been used for the correction of mitral regurgitation. According to this procedure, MV’s competence is restored by suturing the edges of the leaflets, at the central, lateral or commissural position, depending on the mechanism of the regurgitation and the site of valvular lesions. Changes in the mitral valve geometry, due to ETER, alter LV fluid mechanics. Our group has investigated suture length effect on LV fluid mechanics. MV ETER increased energy loss and lowered LV filling efficiency. The larger orifice area after ETER might prevent significant energy loss and transmitral pressure drop during diastole. However, suture position effect is unknown. In this study, we aim to investigate the effect of different MV suture position on LV fluid mechanics, especially the vortex, LV pumping efficiency.

The recent development of energy based endpoints, to quantify the pathophysiology of congenital heart disease such as tetralogy of Fallot (TOF), requires accurate measurement of cardiac blood flow and pressure data. Consequently, invasive cardiac catheterization is required for those measurements. In this research we used 4D phase contrast magnetic resonance imaging (PC MRI) data to determine the pressure drop non-invasively. This enables us to obtain pressure-flow variation, which, in turn, allowed us to calculate energy loss along the branch pulmonary arteries (PA). Based on our result, we believe that the hemodynamic status of the PA of a subject can be non-invasively evaluated by both pressure drop and energy loss values along the PA.

Single Ventricle Heart Defects (SVHD) are present in 2 per 1000 live births in the US. SVHD are characterized by cyanotic mixing between the de-oxygenated blood from the systemic circulation return and the oxygenated blood from the pulmonary arteries. Palliative surgical repairs (Fontan procedure) are performed to bypass the right ventricle in these patients. In current practice, the surgical interventions commonly result in the total cavopulmonary connection (TCPC). In this configuration the systemic venous returns (inferior vena cava, IVC, and superior vena cava, SVC) are directly routed to the right and left pulmonary arteries (RPA and LPA), bypassing the right heart. The resulting anatomy has complex and unsteady hemodynamics characterized by flow mixing and flow separation. Pulsation of the inlet venous flow during a cardiac cycle results in complex and unsteady flow patterns in the TCPC. Although various degrees of pulsatility have been observed in vivo , non-pulsatile (time-averaged) flow boundary conditions have traditionally been assumed in modeling TCPC hemodynamics, and only recently have pulsatile conditions been incorporated without completely characterizing their effect or importance. In this study, 3D numerical simulations were performed to predict TCPC hemodynamics with both pulsatile and non-pulsatile boundary conditions and to investigate the accuracy of applying non-pulsatile boundary conditions. Flow structures, energy dissipation rate and pressure drop were compared under rest and estimated exercise conditions. The results show that TCPC hemodynamics can be strongly influenced by the presence of pulsatile flow. However, there exists a minimum pulsatility threshold, identified by defining a weighted pulsatility index (wPI), above which the influence is significant.

Last stage of the palliative surgical reconstruction (i.e. Fontan procedure) for the infants with functional single-ventricle is total cavopulmonary connection (TCPC), where the superior vena cavae (SVC) and inferior vena cavae (IVC) are routed directly into the pulmonary arteries. Limited pumping energy available due to the absence of right-ventricle and altered venous characteristics require optimized hemodynamics inside the TCPC pathway, which can be achieved by minimizing the power losses.

Hierarchical structures at multiple length scales are characteristic in a class of natural ( e.g. , bone) and synthetic ( e.g. , nano-composites) materials that are quasi-brittle in nature but allow for appreciable plastic deformation ( 1, 2 ). Since the bulk mechanical properties of the materials are heavily dependent on their nano/microscopic structures, micro/nano mechanics approaches are often required to study their behaviors. However, lack of an effective means to exemplify the post-yield and failure behavior directly at micro/nanometer scales has significantly precluded understanding the constitutive relationship of these materials. A compelling example is that such paucity has significantly hindered establishment of physically sound constitutive relationships for bone tissues. Recent progresses in nanotechnology have allowed for estimation of the stiffness and hardness of bone tissues at submicron and nano length scales ( 3–5 ). However, no methods are currently available to assess the post-yield and failure behavior of bone tissues at nano/microscopic levels. Our pilot study ( 6 ) has shown that nanoscratch tests could be used in assessing the in situ energy dissipation during the post-yield deformation of bone tissues. To this end, the objective of the present study is to establish a mechanistic model for the nanoscratch methodology based on a assumption that the in situ toughness of bone tissues or the capacity to dissipate energy until failure can be estimated based on the removal energy of the tissue consumed during a nanoscratch test .

Post-yield behavior is important for bone fragility since it accounts for the major part of energy dissipation of bone. Therefore, it is essential to study the post-yield behavior of bone to understand the different pathways for energy dissipation [1]. The post-yield behavior of bone may depend on the different loading modes. Previous studies have utilized a novel progressive loading scheme to study the post-yield behavior of cortical bone at tension [2] and compression [3]. However, few studies have reported post-yield behaviors of cortical bone in shear [4]. One of major challenges in shear tests of cortical bone is to achieve a uniform stress field over a test region. For example, the notches of the Iosipescu test may cause non-constant stress fields and locally high stresses when small amounts of bending are present [5]. The objective of this study was to develop the progressive loading scheme of shear in bone using an inclined double notch shear test, in which homogeneous shear stress fields were produced [5].

Collagen is the most abundant protein in the body. It plays critical roles in many supporting and connecting tissues such as tendon, ligament, bone, blood vessels, skin, etc. Collagen gels prepared from commercially available collagen solutions mimic the in vivo environment and have been widely used as three-dimensional (3-D) tissue scaffolds for cell culture. Collagen thin film is the dehydrated form of collagen gel. A number of studies have examined the cell-collagen thin film interactions (1, 2). As a biphasic material, collagen scaffolds contain a solid phase which represents by collagen fiber network and an interstitial fluid phase (3). This special structure makes collagen a viscoelastic material. Viscoelasticity is related to force or energy storage, transmission and dissipation in tissues and has a great influence on the growth and development of cells (4).