Tissue engineering, that means the generation of artificial three-dimensional (3D) tissues. Its goal is the development of cell-based substitutes for restoring, maintaining, or improving tissue function. These substitutes should have organ-specific properties w.r.t biochemical activity, microstructure, mechanical integrity and bio-stability. Cell based therapy concept include, the direct transplantation of isolated cells; the implantation of bio-active scaffold for the stimulation of cell growth within the original tissue; and the implantation of a 3D bio-hybrid structure of a scaffold and cultured cells or tissue.
Important aspects for bioreactor design and culture system
Bioreactors are in use for cell proliferation on small and large scale. Isolated and proliferated cells in-vitro generate 3D tissue constructs .
- These bioreactors should enable the control of environmental conditions such as oxygen, tension, pH, temp, shear stress, feeding and sampling.
- It should have automated processing steps
- Should allow proliferation of cells, the seeding of cells onto macro-porous scaffolds, nutrient supply (particularly oxygen) to the new tissue, and mechanical stimulation of developing tissues
- Fixed-bed and fluidized-bed bioreactors is where 3D cultures are performed in. The cells immobilize in macro-porous carriers or in networks of fibers arranged in a column so that they are either packed (fixed-bed) or floating (fluidized-bed)
These types of reactor are very efficient for the long term cultivation of mammalian cells to produce bio-pharmaceuticals such as monoclonal antibodies.
The field relies extensively on the use of porous 3D scaffolds to provide the appropriate environment for the regeneration of tissues and organs. Cells are often implanted or ‘seeded’ into an artificial structure and (occasionally growth factors) capable of supporting three-dimensional tissue formation. These structures, typically called scaffolds.
The term scaffold refers to the 3D biomaterial before cells have been added (in vitro or in vivo).
Materials used as scaffold
- Many different materials (natural and synthetic, biodegradable and permanent) are investigated. Like collagen and some polyesters.
- A commonly used synthetic material is PLA – polylactic acid. This is a polyester which degrades within the human body ing form lactic acid, a naturally occurring chemical which is easily removed from the body.
Scaffold: in vivo or in vitro
To synthesize tissues these cell-seeded scaffolds are cultured in vitro. Which is implanted into an injured site, or are implanted directly into the injured site, using the body’s own systems, inducing regeneration of tissues and cells. This tissue engineering triad is referred to combination of cells, signals and scaffolds.
Tissue engineering triad of cells, signals (provided chemically by growth factors or physically by a bioreactor), and the scaffold which acts as a template for tissue formation by allowing cells to migrate, adhere, and produce tissue.
Regardless of the tissue type, a number of key considerations are important when designing or determining the suitability of a scaffold for use in tissue engineering:
The very first criterion of any scaffold is that it must be biocompatible;
- cells must adhere
- function normally
- and migrate onto the surface and eventually through the scaffold and begin to proliferate before laying down new matrix
- no immune reaction
The objective of tissue engineering is to allow the body’s own cells, over time, to eventually replace the implanted scaffold or tissue engineered construct. Scaffolds and constructs, are not permanent implants. The scaffold must therefore be biodegradable so as to allow cells to produce their own extracellular matrix. The by-products of this degradation should also be non-toxic and able to exit the body without interference with other organs.
It must be strong enough to allow surgical handling during implantation. Producing scaffolds with adequate mechanical properties is one of the great challenges in attempting to engineer bone or cartilage. For these tissues, the implanted scaffold must have sufficient mechanical integrity to function from the time of implantation to the completion of the remodeling process. A further challenge is that healing rates vary with age.
Should have an interconnected pore structure and high porosity to ensure cellular penetration and adequate diffusion of nutrients to cells within the construct and to the extra-cellular matrix formed by these cells. Furthermore, a porous interconnected structure is required to allow diffusion of waste products out of the scaffold.
Another key component is the mean pore size of the scaffold. Cells primarily interact with scaffolds via chemical groups (ligands) on the material surface. Scaffolds synthesized from natural extracellular materials (e.g. collagen) naturally possess these ligands in the form of Arg-Gly-Asp (RGD) binding sequences, whereas scaffolds made from synthetic materials may require deliberate incorporation of these ligands through, for example, protein adsorption.
Bone tissue engineering using 3D
Osseous tissue, known as bone, is made of two different structures; cancellous and cortical bone
- Cancellous, or the inner part of bone, is spongy in nature having 50–90 vol% porosity
- Cortical bone is the dense outer layer of bone with less than 10 vol% porosity
Both types of bone undergo dynamic remodeling, maturation, differentiation, and resorption that are controlled via interactions among osteocyte, osteoblast, and osteo-clast cells
Scaffolds are an integral part of bone tissue engineering
- Scaffolds are (3D) biocompatible structures which can mimic the ECM properties (such as mechanical support, cellular activity and protein production through biochemical and mechanical interactions), and provide a template for cell attachment and stimulate bone tissue formation in vivo.
- At an early stage, bone ingrowth happens at the periphery of scaffolds.
- Porosity: of bone tissue, interconnected porosity is important.
- Open and interconnected pores allow nutrients and molecules to transport to inner parts of a scaffold to facilitate cell in growth, vascularization, as well as waste material removal.
- A minimum pore size between 100 and 150 μm is needed for bone formation . However, enhanced bone formation and vascularization are reported for scaffolds with pore size larger than 300 μm.
- Pore size also plays an important role in ECM production and organization.
- g, Poly (d,l-lactic acid) (PDLLA) scaffolds with pore size 325 and 420 μm led to well-organized collagen I network; whereas, smaller pore size of 275 μm prevented the human osteosarcoma-derived osteoblasts to proliferate, differentiate and produce functional ECM.
Growth factor and drug delivery using 3D printed scaffolds:
There are many growth factors such as vascular endothelial growth factor (VEGF), fibro-blast growth factors (FGFs) and bone morphogenic proteins (BMPs) .That are important in bone tissue engineering. For scaffolds, pore size, connectivity and geometry are effective parameters. Which control drug loading as well as release rates in vivo.
Every day to replace or repair tissue ,thousands of surgical procedures are performed . Through disease or trauma they are damaged . The developing field of tissue engineering (TE) aims to regenerate damaged tissues. By combining cells from the body with highly porous scaffold bio-materials. Which act as templates for tissue regeneration, to guide the growth of new tissue.