Heart valve and arterial tissue engineering

Engineers in Medicine: Foster Innovation by Traversing Boundaries

Engineers play a critical role in the advancement of biomedical science and the development of diagnostic and therapeutic technologies for human well-being. The complexity of medical problems requires the synthesis of diverse knowledge systems and clinical experiences to develop solutions. Therefore, engineers in the healthcare and biomedical industries are interdisciplinary by nature to innovate technical tools in sophisticated clinical settings. In academia, engineering is usually divided into disciplines with dominant characteristics. Since biomedical engineering has been established as an independent curriculum, the term "biomedical engineers" often refers to the population from a specific discipline. In fact, engineers who contribute to medical and healthcare innovations cover a broad range of engineering majors, including electrical engineering, mechanical engineering, chemical engineering, industrial engineering, and computer sciences. This paper provides a comprehensive review of the contributions of different engineering professions to the development of innovative biomedical solutions. We use the term "engineers in medicine" to refer to all talents who integrate the body of engineering knowledge and biological sciences to advance healthcare systems.

CURRENT DEVELOPMENTS AND FUTURE CHALLENGES FOR THE CREATION OF AORTIC HEART VALVE

The concept of tissue-engineered heart valves offers an alternative to current heart valve replacements that is capable of addressing shortcomings such as life-long administration of anticoagulants, inadequate durability, and inability to grow. Since tissue engineering is a multifaceted area, studies conducted have focused on different aspects such as hemodynamics, cellular interactions and mechanisms, scaffold designs, and mechanical characteristics in the form of both in vitro and in vivo investigations. This review concentrates on the advancements of scaffold materials and manufacturing processes, and on cell–scaffold interactions. Aside from the commonly used materials, polyglycolic acid and polylactic acid, novel polymers such as hydrogels and trimethylene carbonate-based polymers are being developed to simulate the natural mechanical characteristics of heart valves. Electrospinning has been examined as a new manufacturing technique that has the potential to facilitate tissue formation via increased surface area. The type of cells utilized for seeding onto the scaffolds is another factor to take into consideration; currently, stem cells are of great interest because of their potential to differentiate into various types of cells. Although extensive studies have been conducted, the creation of a fully functional heart valve that is clinically applicable still requires further investigation due to the complexity and intricacies of the heart valve.

Polymer scaffolds for small-diameter vascular tissue engineering

To better engineer small-diameter blood vessels, a few types of novel scaffolds were fabricated from biodegradable poly(L-lactic acid) (PLLA) by means of thermally induced phase separation (TIPS) techniques. By utilizing the differences in thermal conductivities of the mold materials, the scaffolds with oriented gradient microtubular structures in axial or radial direction were created using benzene as the solvent. The porosity, tubular size, and the orientation direction of the microtubules can be controlled by polymer concentration, TIPS temperature, and materials of different thermal conductivities. The gradient microtubular structure was intended to facilitate cell seeding and mass transfer for cell growth and function. We also developed nanofibrous scaffolds with oriented and interconnected micro-tubular pore network by a one-step TIPS method using benzene/tetrahydrofuran mixture as the solvent without using porogen materials. The structural features of such scaffolds can be conveniently adjusted by varying the solvent ratio, phase separation temperature and polymer concentration to mimic the nanofibrous feature of extracellular matrix. These scaffolds were fabricated for the tissue engineering of small-diameter blood vessels by utilizing their advantageous structural features to facilitate blood vessel regeneration.

Cell Sourcing and Culture Conditions for Fibrin-Based Valve Constructs

Cell sourcing for tissue-engineered heart valves remains a critical issue. In this work, human dermal fibroblasts (HDF) or porcine valve interstitial cells (PVIC) were entrapped in adherent fibrin disk constructs and harvested at 3 and 5 weeks. We compared the fibrin remodeling abilities of each cell type in Dulbecco's Modified Eagle's Medium (DMEM) and DMEM/F12 supplemented with transforming growth factor beta (TGF), and the response of PVIC to DMEM/F12 supplemented with fibroblast growth factor (FGF), a combination of FGF and TGF, and TGF with varying ascorbic acid (AA) concentrations. Culture media were supplemented with serum, insulin, AA, a fibrinolysis inhibitor, and antibiotics. DMEM maximized collagen and elastin deposition by HDF, while DMEM/F12 with FGF yielded the highest fibrin remodeling response by PVIC. HDF degraded fibrin slower than PVIC, and PVIC constructs had higher cellularity than HDF constructs in DMEM and DMEM/F12 at 3 weeks. FGF addition increased collagen content, collagen deposited per cell, and collagen as percentage of total protein compared to medium supplemented with TGF or TGF and FGF. AA addition increased collagen deposition by PVIC, but there was no dose dependence between 50 and 150 microg/mL AA. These results collectively show that PVIC are able to remodel fibrin faster and exhibit greater mechanical stiffening compared to HDF. Conditions for increased collagen deposition are also identified toward the engineering of valve constructs.

Protein adsorption onto polyester surfaces: Is there a need for surface activation?

Surface hydrolysis of polyester scaffolds is a convenient technique suggested to promote protein adsorption for improving cell attachment. We have, therefore, investigated the effect of hydrolysis of polyester surfaces for protein adsorption to clarify the conditions needed. Three polyesters, poly(ethylene terephthalate) (PET), poly(lactic acid) (PLA), and poly(glycolic acid) (PGA), were selected. Adsorption was investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and quartz crystal microbalance (QCM). Hydrolyzed PET adsorbed significantly more proteins than nonhydrolyzed. Degradable polymers adsorbed at higher rates when the polymers were hydrolyzed prior to adsorption, but the same amount as nonhydrolyzed, suggesting spontaneous hydrolysis during the adsorption. XPS shows that hydrolysis prior to absorption for PET results in a surface nitrogen composition of approximately 14%, similar to pure protein (16%). Nonhydrolyzed PET surfaces showed only approximately 7% nitrogen, indicating protein layers thinner than approximately 10 nm. Adsorption to PLA and PGA shows nitrogen contents of 14-15% in both cases. SEM revealed striking differences in morphology of the protein coating. Hydrolyzed or spontaneously hydrolyzable surfaces display a pronounced fibrous structure while nonhydrolyzed surfaces give smooth structures. In combination, the results show that surface hydrolysis increase adsorption rate, but not the amount of proteins on polyesters that degrades in vivo. Surface treatment of nondegradable polyester increases the total amount of proteins and induces the formation of fibrous protein structures. Post hydrolysis treatment by acetic acid, replacing the counter-ion to a proton, further enhances protein attachment. Finally, cell attachment experiments verifies that protein adsorption increase the cell attachment to polyester surfaces.

Radial artery as an autologous cell source for valvular tissue engineering efforts

To create a viable tissue-engineered aortic valve, it is important to identify suitable autologous cell sources that may be seeded onto a biocompatible scaffold. This study focused on the radial artery (RA) as one possible source, investigated optimal culture conditions, and determined the usefulness of small intestinal submucosa (SIS) as a scaffold for tissue-engineering. Porcine RA cells were cultured on either two-dimensional (2D) 100-mm dishes or three-dimensional (3D) 1-cm(2) SIS sheets, producing cell-scaffold composites (CSCs). Both 2D and 3D cultures were maintained in either Medium 199 (M199) or endothelial growth media (EGM) to determine optimal growth conditions. Cellular phenotype and matrix metalloproteinase (MMP) profiles were determined by immunoblotting of cell lysates and zymography of conditioned media, respectively. Cellular invasion was analyzed immunohistochemically on CSC tissue sections. We show that the RA contains phenotypes consistent with those found in the normal aortic valve. EGM, compared with M199, promotes the invasion and remodeling of SIS by RA cells, which is crucial in the process of replacing the foreign tissue scaffold prior to implantation. To our knowledge, this is the first study to show that the RA is a suitable source for the generation of a tissue-engineered valve.

Heart Valve Tissue Engineering

Tissue-engineered heart valves have been proposed by physicians and scientists alike to be the ultimate solution for treating valvular heart disease. Rather than replacing a diseased or defective native valve with a mechanical or animal tissue-derived artificial valve, a tissue-engineered valve would be a living organ, able to respond to growth and physiological forces in the same way that the native aortic valve does. Two main approaches have been attempted over the past 10 to 15 years: regeneration and repopulation. Regeneration involves the implantation of a resorbable matrix that is expected to remodel in vivo and yield a functional valve composed of the cells and connective tissue proteins of the patient. Repopulation involves implanting a whole porcine aortic valve that has been previously cleaned of all pig cells, leaving an intact, mechanically sound connective tissue matrix. The cells of the patients are expected to repopulate and revitalize the acellular matrix, creating living tissue that already has the complex microstructure necessary for proper function and durability. Regrettably, neither of the 2 approaches has fared well in animal experiments, and the only clinical experience with tissue-engineered valves resulted in a number of early failures and patient death. This article reviews the technological details of the 2 main approaches, their rationale, their strengths and weaknesses, and the likely mechanisms for their failure. Alternative approaches to valvular tissue engineering, as well as the role of industry in shaping this field in the future, are also reviewed.