1.1 The Requirements Imposed by Application on Material Structures Intended as Tissue Engineering Scaffolds
The discovery of the multipotent nature of different kinds of stem cells opened new horizons for therapeutics for surgery and for medicine in general. Current treatments for diseased or injured tissues or organs range from transplantation from allogenic or xenogenic donors or reconstruction by transfer of autologous tissues to the use of nonbiological either implanted or extracorporeal devices. None of these strategies is free of inconveniencies (shortage and effectiveness of donors, clinical complications, need of immunosuppressive drugs, tumors formation, etc.). The hope to understand stem cells in their behavior up to the point of directing their differentiation toward desired lineages is at the base of the different new therapeutical strategies encompassed by regenerative medicine.
This process of cell differentiation is triggered and governed by a multiplicity of factors and stimuli, which cells receive from an immediate environment made from other interacting cells and from the extracellular matrix (ECM). In cases of severe loss or degeneration of tissue, the sites of intended regeneration have lost their basic structures, and thus new grafted cells, even if having the right properties in vitro, fail to regenerate functional tissue in vivo. At this point, synthetic tridimensional structures, so-called scaffolds, may be of help, by providing grafted cells with a niche and adequate mechanical and chemical stimuli. As an example, cardiac tissue regeneration in cases of myocardial infarction and subsequent ventricular remodeling has been recently addressed by cell transplantation and cell sheet engineering, with a variety of cell populations and supply methods [1, 2]. Common difficulties found include lack of functional integration and a low survival of the grafted cells. These shortcomings could be overcome by means of biomaterials into which the cells would be encapsulated [3, 4].
Generally speaking, scaffolds must assist the regeneration process, performing as an artificial cellular environment during some stages of the tissue regeneration [5, 6]. Either in vitro or in vivo, they must replace as best as possible some of the functions of the ECM: they must (i) contribute to the structural and mechanical integrity of the diseased tissue, (ii) serve as a means of transport of nutrients and wastes and facilitate vascularization, (iii) act as a spatial guide for cell spreading and organization, and (iv) transduce mechanical or biochemical stimuli, and eventually transport, store, and deliver active molecules that effect the expression of the phenotype. Besides these functions, in defining the requirements on materials intended as scaffolds, two other sets of factors must be taken into account: those deriving from the specificity of the application (in vitro or in vivo, temporal or permanent, etc.) and those related to processability and manufacture (sizes and shapes of the implants, sterilization procedures).
Function, specific application and processability considerations thus define a number of requisite properties of mechanical, physicochemical, biological, and structural nature. From the mechanical side, strength (resistance to failure) and stiffness (characterized by shear, tensile, or compressive moduli) are the most important properties to be addressed. Modulus values as different as those of brain and bone determine a wide interval of magnitude, and mechanotransduction of signals to the cells depends significantly on this property, especially on the surface moduli. The most important physicochemical properties of scaffold materials are their degradable or stable nature, their permeability and diffusivity to fluids and gases, and their hydrophilic or hydrophobic nature. Material surfaces possess also specific biologically relevant properties: their chemical functionalities may be directly involved in the surface nucleation of compounds such as bone hydroxyapatite, or they may adsorb ECM proteins in different conformations, thus affecting cell adhesion, spreading, and proliferation. Lastly, microstructural properties of the materials, such as their pore volume fraction, pore connectivity and geometry (shape, dimensions of the pores, regularity) are critical for the scaffoldâs final performance. The scaffoldâs ability to host cells in required numbers, to allow vascularization throughout it, or to guide and organize spatially cell growth in specific ways, depends crucially on these properties.
Our ways to meet these mechanical, physicochemical, biological, and structural requirements is through bulk and surface chemistry for the first three, and through different porogenic techniques for the fourth. A material with a given overall chemical composition may be, furthermore, in very different physical states: it may be a random or a block copolymer, it may be an interpenetrated network, it may be semicrystalline or amorphous, vitreous or rubbery under physiological conditions. These possibilities are afforded by polymerization chemistry and/or subsequent processing or treatment, and make polymers such unique materials for tissue engineering applications.
The intended end uses of the scaffolds are widely different. Scaffolds may be implanted empty (acellular) when they are expected to be invaded and colonized by the cells in a short period of time; otherwise, it is necessary to seed the scaffolds with the appropriate cells before implantation or even to preculture them in vitro within the scaffold [7]. A bioreactor can be helpful in this stage to recreate in vitro some dynamic conditions and the mechanical and/or chemical stimuli that the cells would receive in vivo. Scaffolds may be chemically modified in order to direct cell anchorage or differentiation, through addition of proteins, peptides, growth factors (GFs), hormones, enzymes, or other regulators of the cell behavior [8â11]. Several methods for the controlled release of factors from scaffolds have been developed [12â14].
Polymer materials are especially suited to interface with cells. Being formed by long chain molecules, they share some basic properties with biological macromolecules. At the most fundamental level, both kinds of molecules deform with the inertial mechanism of conformational change, which gives rise to molecular dynamics with characteristic relaxation phenomena at long time scales. Moreover, both biological and synthetic macromolecules are able to exhibit structure at a subnanometer, molecular level (the local arrangement of different chemical monomers) and at a supramolecular, nano- to micrometer level: phase-separated domains, crystalline domains. And the more complex multimolecular associations leading to the macroscopic network structure of the ECM can be mimicked to some extent by the porous architecture of the polymer scaffolds. This represents a third level of structure, with typical dimensions ranging from tens to hundreds of microns.
