Tissue engineering of biological cardiovascular system surrogates

Artificial Cardiac Muscle with or without the Use of Scaffolds

During the past several decades, major advances and improvements now promote better treatment options for cardiovascular diseases. However, these diseases still remain the single leading cause of death worldwide. The rapid development of cardiac tissue engineering has provided the opportunity to potentially restore the contractile function and retain the pumping feature of injured hearts. This conception of cardiac tissue engineering can enable researchers to produce autologous and functional biomaterials which represents a promising technique to benefit patients with cardiovascular diseases. Such an approach will ultimately reshape existing heart transplantation protocols. Notable efforts are accelerating the development of cardiac tissue engineering, particularly to create larger tissue with enhanced functionality. Decellularized scaffolds, polymer synthetics fibrous matrix, and natural materials are used to build robust cardiac tissue scaffolds to imitate the morphological and physiological patterns of natural tissue. This ultimately helps cells to implant properly to obtain endogenous biological capacity. However, newer designs such as the hydrogel scaffold-free matrix can increase the applicability of artificial tissue to engineering strategies. In this review, we summarize all the methods to produce artificial cardiac tissue using scaffold and scaffold-free technology, their advantages and disadvantages, and their relevance to clinical practice.

The cell-stretcher: A novel device for the mechanical stimulation of cell populations

Mechanical stimulation appears to be a critical modulator for many aspects of biology, both of living tissue and cells. The cell-stretcher, a novel device for the mechanical uniaxial stimulation of populations of cells, is described. The system is based on a variable stroke cam-lever-tappet mechanism which allows the delivery of cyclic stimuli with frequencies of up to 10 Hz and deformation between 1% and 20%. The kinematics is presented and a simulation of the dynamics of the system is shown, in order to compute the contact forces in the mechanism. The cells, following cultivation and preparation, are plated on an ad hoc polydimethylsiloxane membrane which is then loaded on the clamps of the cell-stretcher via force-adjustable magnetic couplings. In order to show the viability of the experimentation and biocompatibility of the cell-stretcher, a set of two in vitro tests were performed. Human epithelial carcinoma cell line A431 and Adult Mouse Ventricular Fibroblasts (AMVFs) from a dual reporter mouse were subject to 0.5 Hz, 24 h cyclic stretching at 15% strain, and to 48 h stimulation at 0.5 Hz and 15% strain, respectively. Visual analysis was performed on A431, showing definite morphological changes in the form of cellular extroflections in the direction of stimulation compared to an unstimulated control. A cytometric analysis was performed on the AMVF population. Results show a post-stimulation live-dead ratio deviance of less than 6% compared to control, which proves that the environment created by the cell-stretcher is suitable for in vitro experimentation.

Biomimetic 3D Tissue Models for Advanced High-Throughput Drug Screening

Most current drug screening assays used to identify new drug candidates are 2D cell-based systems, even though such in vitro assays do not adequately re-create the in vivo complexity of 3D tissues. Inadequate representation of the human tissue environment during a preclinical test can result in inaccurate predictions of compound effects on overall tissue functionality. Screening for compound efficacy by focusing on a single pathway or protein target, coupled with difficulties in maintaining long-term 2D monolayers, can serve to exacerbate these issues when using such simplistic model systems for physiological drug screening applications. Numerous studies have shown that cell responses to drugs in 3D culture are improved from those in 2D, with respect to modeling in vivo tissue functionality, which highlights the advantages of using 3D-based models for preclinical drug screens. In this review, we discuss the development of microengineered 3D tissue models that accurately mimic the physiological properties of native tissue samples and highlight the advantages of using such 3D microtissue models over conventional cell-based assays for future drug screening applications. We also discuss biomimetic 3D environments, based on engineered tissues as potential preclinical models for the development of more predictive drug screening assays for specific disease models.

ADDITION OF CONDUCTIVE ELEMENTS TO POLYMERIC SCAFFOLDS FOR MUSCLE TISSUE ENGINEERING

Cardiac and skeletal muscles are two tissues that would benefit from an electrically conductive scaffold to regenerate lost or lower functioning areas. By augmenting polymeric scaffolds with conductive elements, the contractile process for both muscles could increase. In this review, the components reviewed include polyaniline (PANi), gold (Au) nanoparticles, and carbon nanotubes (CNT). PANi has been combined with several polymers and increased the conductivity of the scaffolds. It is biocompatible, but increases mechanical properties and decreases scaffold elongation. Tissue engineering using nanoparticles is an emerging area and considerable research focuses on determining possible toxicity due to nanoparticle concentration. Contradicting data exists for both Au nanoparticles and CNT. Smaller Au nanoparticles damage cardiac tissue in vivo while larger ones do not. By comparison, in vitro data shows no harmful results for skeletal muscle cells. Data for CNT is just as diverse as the amount, orientation and further purification or functionalization could all play a role in determining biocompatibility. Future research should focus on establishing the conductivity level needed for each muscle tissue to ascertain the amount of conductive element needed so the most suitable one can be utilized.

Tissue engineering and regenerative medicine: history, progress, and challenges

The past three decades have seen the emergence of an endeavor called tissue engineering and regenerative medicine in which scientists, engineers, and physicians apply tools from a variety of fields to construct biological substitutes that can mimic tissues for diagnostic and research purposes and can replace (or help regenerate) diseased and injured tissues. A significant portion of this effort has been translated to actual therapies, especially in the areas of skin replacement and, to a lesser extent, cartilage repair. A good amount of thoughtful work has also yielded prototypes of other tissue substitutes such as nerve conduits, blood vessels, liver, and even heart. Forward movement to clinical product, however, has been slow. Another offshoot of these efforts has been the incorporation of some new exciting technologies (e.g., microfabrication, 3D printing) that may enable future breakthroughs. In this review we highlight the modest beginnings of the field and then describe three application examples that are in various stages of development, ranging from relatively mature (skin) to ongoing proof-of-concept (cartilage) to early stage (liver). We then discuss some of the major issues that limit the development of complex tissues, some of which are fundamentals-based, whereas others stem from the needs of the end users.

Culturing and differentiating human mesenchymal stem cells for biocompatible scaffolds in regenerative medicine

Mesenchymal stem cells from a variety of sites are a natural resource that using appropriate skills can be cultured in the laboratory, in scaffolds, to provide differentiated-cell replacement tissues, for clinical application. To perform such work with human cells, strict ethical integrity must be observed at all stages. Adipocytes, osteocytes and chrondrocytes are amongst the most desirable end-point cells. Hydrolytic degradable scaffolds allow implanted cells to synthesise their own extracellular matrix in situ after implantation, degeneration of the foreign scaffold to temporally match creation of the new innate one. For preliminary in vitro stem cell differentiation protocols, initial investigation is commonly performed with stem cells in commercially available porous collagen sponges or cell-free small intestinal submucosa. Differentiation of stem cells to a specific phenotype is achieved by culturing them in apposite culture media under precise conditions. Once the cells have differentiated, they are checked and characterised in a wide variety of systems. This chapter describes differentiation media for adipocytes, osteocytes, chondrocytes, myocytes and neural precursors and methods of observing their characteristics by microscopy using phase contrast microscopy, standard light microscopy and electron microscopy with tinctorial, immunocytochemical and electron dense stains, respectively. Cell sorting techniques are not dealt with here. Immunocytochemistry/microscopy staining for specific differentiated-cell antigens is an invaluable procedure, and the range of commercially available antibodies is wide. Precautions need to be considered for using actively proliferating cells in vivo, so that implanted cells remain controlled by the body's molecular signals and avoid development of malignancy.

Experimental measurement of dynamic fluid shear stress on the ventricular surface of the aortic valve leaflet

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. The exact causes and mechanisms of AV calcification are unclear, although previous studies suggest that mechanical forces play a role. It has been clinically demonstrated that calcification preferentially occurs on the aortic surface of the AV. This is hypothesized to be due to differences in the mechanical environments on the two sides of the valve. It is thus necessary to characterize fluid shear forces acting on both sides of the leaflet to test this hypothesis. The current study is one of two studies characterizing dynamic shear stress on both sides of the AV leaflets. In the current study, shear stresses on the ventricular surface of the AV leaflets were measured experimentally on two prosthetic AV models with transparent leaflets in an in vitro pulsatile flow loop using two-component Laser Doppler Velocimetry (LDV). Experimental measurements were utilized to validate a theoretical model of AV ventricular surface shear stress based on the Womersley profile in a straight tube, with corrections for the opening angle of the valve leaflets. This theoretical model was applied to in vivo data based on MRI-derived volumetric flow rates and valve dimension obtained from the literature. Experimental results showed that ventricular surface shear stress was dominated by the streamwise component. The systolic shear stress waveform resembled a half-sinusoid during systole and peaks at 64-71 dyn/cm(2), and reversed in direction at the end of systole for 15-25 ms, and reached a significant negative magnitude of 40-51 dyn/cm(2). Shear stresses from the theoretical model applied to in vivo data showed that shear stresses peaked at 77-92 dyn/cm(2) and reversed in direction for substantial period of time (108-110 ms) during late systole with peak negative shear stress of 35-38 dyn/cm(2).

Experimental measurement of dynamic fluid shear stress on the aortic surface of the aortic valve leaflet

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. Although exact causes and mechanisms of AV calcification are unclear, previous studies suggest that mechanical forces play a role. Since calcium deposits occur almost exclusively on the aortic surfaces of AV leaflets, it has been hypothesized that adverse patterns of fluid shear stress on the aortic surface of AV leaflets promote calcification. The current study characterizes AV leaflet aortic surface fluid shear stresses using Laser Doppler velocimetry and an in vitro pulsatile flow loop. The valve model used was a native porcine valve mounted on a suturing ring and preserved using 0.15% glutaraldehyde solution. This valve model was inserted in a mounting chamber with sinus geometries, which is made of clear acrylic to provide optical access for measurements. To understand the effects of hemodynamics on fluid shear stress, shear stress was measured across a range of conditions: varying stroke volumes at the same heart rate and varying heart rates at the same stroke volume. Systolic shear stress magnitude was found to be much higher than diastolic shear stress magnitude due to the stronger flow in the sinuses during systole, reaching up to 20 dyn/cm(2) at mid-systole. Upon increasing stroke volume, fluid shear stresses increased due to stronger sinus fluid motion. Upon increasing heart rate, fluid shear stresses decreased due to reduced systolic duration that restricted the formation of strong sinus flow. Significant changes in the shear stress waveform were observed at 90 beats/min, most likely due to altered leaflet dynamics at this higher heart rate. Overall, this study represents the most well-resolved shear stress measurements to date across a range of conditions on the aortic side of the AV. The data presented can be used for further investigation to understand AV biological response to shear stresses.

Cardiac fibroblasts: friend or foe?

Cardiac function is determined by the dynamic interaction of various cell types and the extracellular matrix that composes the heart. This interaction varies with the stage of development and the degree and duration of mechanical, chemical, and electrical signals between the various cell types and the ECM. Understanding how these complex signals interact at the molecular, cellular, and organ levels is critical to understanding the function of the heart under a variety of physiological and pathophysiological conditions. Quantitative approaches, both in vivo and in vitro, are essential to understand the dynamic interaction of mechanical, chemical, and electrical stimuli that govern cardiac function. The fibroblast can thus be a friend in normal function or a foe in pathophysiological conditions.

Heart valve and arterial tissue engineering

In the industrialized world, cardiovascular disease alone is responsible for almost half of all deaths. Many of the conditions can be treated successfully with surgery, often using transplantation techniques; however, autologous vessels or human-donated organs are in short supply. Tissue engineering aims to create specific, matching grafts by growing cells on appropriate matrices, but there are many steps between the research laboratory and the operating theatre. Neo-tissues must be effective, durable, non-thrombogenic and non-immunogenic. Scaffolds should be bio-compatible, porous (to allow cell/cell communication) and amenable to surgery. In the early days of cardiovascular tissue engineering, autologous or allogenic cells were grown on inert matrices, but patency and thrombogenicity of grafts were disappointing. The current ethos is toward appropriate cell types grown in (most often) a polymeric matrix that degrades at a rate compatible with the cells' production of their own extracellular matrical proteins, thus gradually replacing the graft with a living counterpart. The geometry is crucial. Computer models have been made of valves, and these are used as three-dimensional patterns for mass-production of implant scaffolds. Vessel walls have integral connective tissue architecture, and application of physiological level mechanical forces conditions bio-engineered components to align in precise orientation. This article reviews the concepts involved and successes achieved to date.