Author, editor; Tilda Barliya PhD
Tissue Engineering is an emerging multidisciplinary field involving biology, medicine, and engineering that is likely to revolutionize the ways we improve the health and quality of life for millions of people by restoring, maintaining, or enhancing tissue and organ function. Tissue engineering emerged as organ transplantation is limited by the number of available donors and high cost process, leaving thousands of people each year on the transplant waiting lists in the United States alone. Many die before an organ donor becomes available. Dr. Tal Dvir from the Langer’s lab at MIT have summarized this topic in his review (2. http://nextbigfuture.com/2011/01/nanotechnology-strategies-for-tissue.html)
Tissue engineering aims at developing functional substitutes for damaged tissues and organs, a process that involves the use of a combination of cells, engineering and material methods, including suitable biochemical and chemical factors to improve or replace biological functions. Rather than simply introducing cells into a diseased area to repopulate a defect and/or restore function, in tissue engineering the cells are often seeded in or onto biomaterials (scaffolds) before transplantation.
These biomaterial scaffold allows cells to attach and reorganize to form functional tissue by proliferating, synthesizing extracellular matrix, and migrating along the implant path (1,2,3) Figure 1.
Until recently, it was believed that the macroporous features of scaffolds used in tissue engineering mimicked the dimension scale of the extracellular matrix (ECM), and that the matrix itself (natural or artificial) only served as a support for the cells; morphogenesis was controlled passively by defining tissue boundaries. Emphasis was placed on critical engineering and material issues, such as improving mass transfer into the core of the cell constructs and designing biocompatible and biodegradable scaffolds with mechanical properties suitable for engineering various tissues. As the field evolved, attention focused on the biology of the scaffolds and how they affect various cell types.
Tissue engineers had recognized that some of the widely used scaffolds do not fairly recapitulate the cell microenvironment and that the ECM is a dynamic and hierarchically organized nanocomposite that regulates essential cellular functions such as:
Nanotechnological tools for tissue engineering may help design advanced nanocomposite scaffolds that can better mimic the ECM and eventually assemble more complex and larger functional tissues. In order to generate a functional tissue, effective organization of cells in the tissue is required with similar morphology and physiology of the parental tissue.
Morphogenesis in the three-dimensional (3D) scaffold should occur in a similar way to natural organ development. The cells reorganize owing to interaction with the ECM on the basis of:
- mechanical properties (such as matrix stiffness, elasticity and viscosity)
- concentration gradients of immobilized growth factors
- ECM molecules.
Recently, Ott and co-workers (4) reported a study emphasizing the importance of the ECM structure in guiding the seeded cells and promoting morphogenesis. Rat hearts were decellularized by perfusion of detergents to preserve the underlying ECM and then reseeded with cardiac and endothelial cells (4). The cells migrated and self-organized in their natural location in the matrix and by day 8, under physiological load and electrical stimulation, the constructs were able to generate pump function (4). The importance of the ECM was shown for:
So why is the Extracellular Martix (ECM) so important?
The ECM is composed of an intricate interweaving of protein fibres such as fibrillar collagens and elastins, ranging from 10 to several hundreds of nanometres. The mesh is covered with nanoscale adhesive proteins such as laminin and fibronectin that provide specific binding sites for cell adhesion (interacting with integrins, cadherins and so forth) and have been shown to regulate important cell behaviours such as growth, shape, migration and differentiation. Polysaccharides such as hyaluronic acid and heparan sulphate fill the interstitial space between the fibres and act as a compression buffer against the stress placed on the ECM or serve as a growth factor depot (Figure 2).
Scaffold design considerations
The ECMs of various tissues in the body differ in the composition and spatial organization of the collagens, elastins, proteoglycans and adhesion molecules, to maintain specific tissue morphologies and organ specific shape and function, and to supply specific instructive cues. Therefore, the design considerations for scaffolds should vary according to the desired engineered tissue. For example, the biochemical, electrical and mechanical functions of the heart are uniquely dependent on their biological nanostructures. The heart’s 3D ECM network is composed of an intricate, micro- and nanoscale interweaving pattern of fibrillar collagen and elastin bundles that form a dense, elastic network with proteoglycans and with adhesive and non-adhesive molecules. In this defined mesh, the cardiomyocytes are forced to couple mechanically to each other, to form elongated and aligned cell bundles that interact with each other or with neighbouring capillaries and nerves.
Post-isolation cells lose their ultrastructural elongated morphology and their interaction with their surroundings, and they adopt a random distribution on the flat surface of the scaffold, which compromises many of their physiological properties. Therefore, the structure and support of the ECM is crucial. See Figure 2.
Limitations of the ECM:
- Weak mechanical properties
- Lack of electrical conductivity
- Absence of adhesive and micoenvironment- defining moieties
- Inability of cells to self-assemble to 3D tissue structure.
The rational behind incorporating nanostructures is to compensate for other scaffold limitations (Table 2) Ref.2
- cell responses to micro- and nanopatterned topographical cues
- cell responses to patterned biochemical cues
- controlled 3D scaffolds
- patterned tissue vascularization
- electromechanical regulation (conductivity). of tissue assembly and function
Nanostructures can be used to record the electronic signals that are transmitted through cells such as neurons and cardiomyocytes. One way to record these signals is by lithographically defining nanostructures as field-effect transistors, which are sensitive to local electric field changes. In particular, silicon nanowire transistors are useful for measuring extracellular signals because they exhibit particularly exquisite field-effect sensitivity compared with conventional, planar devices; they are just tens of nanometres in diameter and can therefore interface with cells and tissue at a subcellular level; and they show nanotopographic features that encourage tight interfaces with biological systems.
This introduction reviewed some of the aspects required for tissue engineering with the affiliation to nanotechnology. In the next post, we will dive deeper into a specific tissue organ, the bioengineering aspect and how nanotechnology strategies may improve the design and outcome.
2. Dvir T., Timko BR., Kohane DS., and Langer R. Nanotechnological strategies for engineering complex tissues. Nature Nanotechnology 2010; 12():. http://nextbigfuture.com/2011/01/nanotechnology-strategies-for-tissue.html
4. Ott, H. C. et al. Perfusion-decellularized matrix: Using nature’s platform to engineer a bioartificial heart. Nature Med. 14, 213–221 (2008).
5. Boyang Zhang, Yun Xiao, Anne Hsieh, Nimalan Thavandiran and Milica Radisic. Micro- and nanotechnology in cardiovascular tissue engineering. Nanotechnology 2011; 22(49): 494003