Jonas Stålhand

In vivo estimation of the contribution of elastin and collagen to the mechanical properties in the human abdominal aorta: effect of age and sex

The mechanical properties of the aorta affect cardiac function and are related to cardiovascular morbidity/mortality. This study was designed to evaluate the isotropic (mainly elastin, elastin(iso)) and anisotropic (mainly collagen, collagen(ani)) material parameters within the human aorta in vivo. Thirty healthy men and women in three different age categories (23-30, 41-54, and 67-72 yr) were included. A novel mechanical model was used to identify the mechanical properties and the strain field with aid of simultaneously recorded pressure and radius in the abdominal aorta. The magnitudes of the material parameters relating to both the stiffness of elastin(iso) and collagen(ani) were in agreement with earlier in vitro studies. The load-bearing fraction attributed to collagen(ani) oscillated from 10 to 30% between diastolic and systolic pressures during the cardiac cycle. With age, stiffness of elastin(iso) increased in men, despite the decrease in elastin content that has been found due to elastolysis. Furthermore, an increase in stiffness of collagen(ani) at high physiological pressure was found. This might be due to increased glycation, as well as changed isoforms of collagen in the aortic wall with age. A marked sex difference was observed, with a much less age-related effect, both on elastin(iso) and collagen(ani) stiffness in women. Possible factors of importance could be the effect of sex hormones, as well as differing collagen isoforms, between the sexes.

A mechanochemical 3D continuum model for smooth muscle contraction under finite strains

This paper presents a modelling framework in which the mechanochemical properties of smooth muscle cells may be studied. The activation of smooth muscles is considered in a three-dimensional continuum model which is key to realistically capture the function of hollow organs such as blood vessels. On the basis of a general thermodynamical framework the mechanical and chemical phases are specialized in order to quantify the coupled mechanochemical process. A free-energy function is proposed as the sum of a mechanical energy stored in the passive tissue, a coupling between the mechanical and chemical kinetics and an energy related purely to the chemical kinetics and the calcium ion concentration. For the chemical phase it is shown that the cross-bridge model of Hai and Murphy [1988. Am. J. Physiol. Cell Physiol. 254, C99-C106] is included in the developed evolution law as a special case. In order to show the specific features and the potential of the proposed continuum model a uniaxial extension test of a tissue strip is analysed in detail and the related kinematics and stress-stretch relations are derived. Parameter studies point to coupling phenomena; in particular the tissue response is analysed in terms of the calcium ion level. The model for smooth muscle contraction may significantly contribute to current modelling efforts of smooth muscle tissue responses.

Knitting and weaving artificial muscles

Textile artificial muscles were developed using weaving to increase the force and knitting to amplify the strain. A need exists for artificial muscles that are silent, soft, and compliant, with performance characteristics similar to those of skeletal muscle, enabling natural interaction of assistive devices with humans. By combining one of humankind’s oldest technologies, textile processing, with electroactive polymers, we demonstrate here the feasibility of wearable, soft artificial muscles made by weaving and knitting, with tunable force and strain. These textile actuators were produced from cellulose yarns assembled into fabrics and coated with conducting polymers using a metal-free deposition. To increase the output force, we assembled yarns in parallel by weaving. The force scaled linearly with the number of yarns in the woven fabric. To amplify the strain, we knitted a stretchable fabric, exhibiting a 53-fold increase in strain. In addition, the textile construction added mechanical stability to the actuators. Textile processing permits scalable and rational production of wearable artificial muscles, and enables novel ways to design assistive devices.

A continuum model for excitation–contraction of smooth muscle under finite deformations

The main focus in most of the continuum based muscle models is the mechanics of muscle contraction while other physiological processes governing muscle contraction, e.g., cell membrane excitation and activation, are ignored. These latter processes are essential to initiate contraction and to determine the amount of generated force, and by excluding them, the developed model cannot replicate the true behavior of the muscle in question. The aim of this study is to establish a thermodynamically and physiologically consistent framework which allows us to model smooth muscle contraction by including cell membrane excitability and kinetics of myosin phosphorylation, along with dynamics of smooth muscle contraction. The model accounts for these processes through a set of coupled dissipative constitutive equations derived by applying first principles. To show the performance of the derived model, it is evaluated for two different cases: a chemo-mechanical study of pig taenia coli cells where the excitation process is excluded, and an electro-chemo-mechanical study of rat myometrium. The results show that the model is able to replicate important aspects of the smooth muscle excitation-contraction process.

A continuum model for skeletal muscle contraction at homogeneous finite deformations

Abstract The contractile force in skeletal muscle models is commonly postulated to be the isometric force multiplied by a set of experimentally motivated functions which account for the muscle’s active properties. Although both flexible and simple, this approach does not automatically guarantee a thermodynamically consistent behavior. In contrast, the continuum mechanical model proposed herein is derived from fundamental principles in mechanics and guarantees a dissipative behavior. Further, the contractile force is associated with a friction clutch which provides a simple and well-defined macroscopic model for cycling cross-bridges. To show the performance of the model, it is specialized to standard experiments for rabbit tibialis anterior muscle. The results show that the model is able to capture important characteristics of skeletal muscle.

Parameter Identification in Arteries Using Constraints

To characterize the behaviour of arteries in a reliable way, it is necessary to know both the material parameters and the residual stress remaining in arteries after all external loads have been removed. Parameter identification techniques are often applied to obtain material parameters, while the residual stress is, generally, estimated by dissecting an arterial segment and using the opening-angle method. In a recent study by Stalhand et al. (2004), a method that avoids dissecting arterial segments was proposed. The method is tailored for using clinical in vivo data, and estimates the residual strain field simultaneously with the material parameters in a parameter identification process. The method is limited to simple box constraints on the parameters, although more advanced constraints are possible. In this paper, we suggest a way to introduce an in vitro observed arterial behaviour as a constraint to the minimization. In addition, we also introduce some motivated constraints that increase the efficiency of the minimization. Finally, to illustrate the potential of the method, we given an example where it is applied to an in vivo pressureradius measurement for a human aorta.

Simulating uterine contraction by using an electro-chemo-mechanical model

Contractions of uterine smooth muscle cells consist of a chain of physiological processes. These contractions provide the required force to expel the fetus from the uterus. The inclusion of these physiological processes is, therefore, imperative when studying uterine contractions. In this study, an electro-chemo-mechanical model to replicate the excitation, activation, and contraction of uterine smooth muscle cells is developed. The presented modeling strategy enables efficient integration of knowledge about physiological processes at the cellular level to the organ level. The model is implemented in a three-dimensional finite element setting to simulate uterus contraction during labor in response to electrical discharges generated by pacemaker cells and propagated within the myometrium via gap junctions. Important clinical factors, such as uterine electrical activity and intrauterine pressure, are predicted using this simulation. The predictions are in agreement with clinically measured data reported in the literature. A parameter study is also carried out to investigate the impact of physiologically related parameters on the uterine contractility.

Modeling of Stretch-Activated Sarcolemmal Channels in Smooth Muscle Cells

This paper concerns the electrochemical modeling of vascular smooth muscle cells. The aim is to describe the influence of cell stretch on the intracelluar calcium concentration. A state-of-the-art cell model is extended by a stretch-activated sarcolemmal channel. Two representations for this channel are developed which describe several electrical and chemical cell membrane properties. The unknown parameters of the new channels are identified by comparing the simulation results with experimental data. The extended cell model shows a typical transient behavior for stretch stimulation. As the most important characteristic it reflects the generation of action potentials induced by cell elongation.

Length adaptation of smooth muscle contractile filaments in response to sustained activation.

Airway and bladder smooth muscles are known to undergo length adaptation under sustained contraction. This adaptation process entails a remodelling of the intracellular actin and myosin filaments which shifts the peak of the active force-length curve towards the current length. Smooth muscles are therefore able to generate the maximum force over a wide range of lengths. In contrast, length adaptation of vascular smooth muscle has attracted very little attention and only a handful of studies have been reported. Although their results are conflicting on the existence of a length adaptation process in vascular smooth muscle, it seems that, at least, peripheral arteries and arterioles undergo such adaptation. This is of interest since peripheral vessels are responsible for pressure regulation, and a length adaptation will affect the function of the cardiovascular system. It has, e.g., been suggested that the inward remodelling of resistance vessels associated with hypertension disorders may be related to smooth muscle adaptation. In this study we develop a continuum mechanical model for vascular smooth muscle length adaptation by assuming that the muscle cells remodel the actomyosin network such that the peak of the active stress-stretch curve is shifted towards the operating point. The model is specialised to hamster cheek pouch arterioles and the simulated response to stepwise length changes under contraction. The results show that the model is able to recover the salient features of length adaptation reported in the literature.

Modeling of Smooth Muscle Activation

Smooth muscle contraction is governed by a complex chain of events including both mechanical and electrochemical stimuli such as stretch and calcium ion concentration. A homogeneous model for smooth muscle contraction is derived in this paper by using a continuum thermodynamical framework. The model is based on an additive decomposition of the deformation, and balance laws for the mechanical and electrochemical scales are obtained using the principle of virtual power. Constitutive equations are derived by applying the dissipation inequality, and a first-order kinetic model for the chemical state of myosin and standard linear or nonlinear mechanical models for the tissue are introduced. The constitutive equations also provide couplings between the scales. The model includes experimentally observed features like stretch dependent active force generation and hyperbolic relation between shortening velocity and afterload. The model is applied to an experimentally relevant example to illustrate its potential.