Freeze-Dried Heart Valves Hold Promise in Replacement Procedures

Tissue-engineered replacement heart valves can be made from the residual biological scaffold left after cellular material has been removed, and can be freeze-dried and stored for later use.

Researchers at Leibniz Universität Hannover (Germany) and Hannover Medical School dissected porcine hearts to investigate the structure, biomechanical properties, and elasticity of freeze-dried decellularized porcine pulmonary heart valves. To do so, the tissues were first decellularized and then separated in three groups; without lyoprotectant; with 5% sucrose; and with a mixture of 2.5% sucrose and 2.5% hydroxyl ethylene starch (HES). They then underwent freeze-drying.

The results showed that freeze-drying in the absence of lyoprotectants resulted in the largest pore sizes, but caused an overall more disintegrated appearance of the histological architecture of the porcine valves, especially between the fibrosa and the ventricularis layers. The elastic modulus of freeze-dried tissue without protectants was reduced, and resembled that of decellularized tissue. Freeze-dried tissue with lyoprotectants had a looser network of collagen and elastic fibers with bigger pore sizes, and showed pores of intermediate sizes between the decellularized tissue and the unprotected freeze-dried samples.

In all, tissue freeze-dried with sucrose looked more intact and displayed less porosity than tissue freeze-dried using a sucrose/HES mixture, whereas no significant differences in biomechanical properties were observed. The researchers concluded that the increased pore size of freeze-dried tissue could be advantageous for the adherence of circulating stem cells. The biomechanical tests show that decellularization significantly affected the biomechanical properties and elastic modulus of the tissues, but that the biomechanical properties of the valves freeze-dried with sucrose more closely resembled those of native tissue. The study was published online ahead of print on January 7, 2012, in Tissue Engineering, Part C: Methods.

“Mechanical valve prostheses bear a high risk of thromboembolic events and bleeding after implantation,” wrote lead author PhD candidate Shangping Wang, MSc, of the Leibniz Institute of Multiphase Processes. “Tissue engineering, the regeneration of tissue with the aid of support structures, may provide alternative solutions for mechanical valves.”

Freeze-drying is a technique where water is removed from frozen material under reduced pressure, first by sublimation of the freezable water fraction (primary drying), followed by desorption of the unfrozen water (secondary drying).Freeze-drying, however, is damaging to biomolecules and can cause a collapse of the extracellular matrix (ECM) structure and denaturation of sensitive proteins, and thus it requires protective additives. The rationale for using a mixture of sucrose and HES is that sucrose forms hydrogen bonds with tissue proteins in the dried state, whereas HES is used to elevate the glass transition temperature during the freeze-drying procedure.

Related Links:
Leibniz Universität Hannover
Hannover Medical School