College 1 – “An introduction to Tissue Engineering” – 22nd of November 2012

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 Role of the scaffold will become less important, sine cells makes their own matrix.
 “Cells like being loaded”
 Any device that provides the transport system for nutrients to cultured cells and allows the efficient withdrawal of toxic wastes and inhibitory metabolic by-products.

  • Applications
    Developed for a range of biotechnology applications. For example, are routinely used for : microbial cell production of biologicals (e.g. penicillin), fermentation processes and waste management where there is close monitoring of culture conditions.

  • Features
    Cell culture vessel (Rolling bottles, Rotating Wall Vessel, Airlift, Hollow fibre perfusion)
    External Loop components (Peristaltic Pump, Manifold, Oxygenator, Medium and waste reservoir)
     Oxygenation can be done by different methods:
    1. Conventional methods of oxygenation disturb the medium and damage the cells.
    2. Membrane oxygenation is adequate. Gas diffusion through a silicone membrane could be improved by decreasing the membrane thickness or more porous. (An example of this technology is the extra-corporeal oxygenators used in heart-lung machine)
    Design Features (High productivity at reduced cost, High and consistent product quality, Simple validation ,Monitoring and control of pH, pO2 and pCO2, cells, infection and products Batch, fed batch and continuous e.g. chemostat, perfusion)

  • Considerations
    Medium flow via diffusion, osmosis and perfusion  produce shear stresses
    Shear stress and turbulence caused by friction between fluid particles, due to fluid viscosity Mass transfer is increased by agitating, (if this agitation is too much or if the cells are particularly shear sensitive, a reduction in net growth is observed, due to damage from fluid-mechanical forces)
    Hydrodynamic Forces - moderate forces are required to maintain cells in suspension and provide adequate mass transport for nutrients and waste products. Techniques has been developed to shield cells.

  • Culture systems:

      • Static cell cultures e.g. petri dishes, flat culture flasks

      • Static matrix cultures e.g. 3D constructs

      • Roller bottles  on a plate the roller bottle will go slower.

      • Airlift Bioreactors e.g. microcarriers beads on air bubbles

      • Stirred suspension carriers e.g. magnetic stirrers

      • Hollowed perfused fibre systems e.g. Vortex

      • Microgravity based bioreactors e.g. HARV, RWV

      • Others e.g. Spinner flasks yield turbulent mixing

  • Microgravity:
    If cells remain buoyant in medium, these forces could be greatly reduced. This can be achieved by culturing cells in microgravity. To simulate microgravity, a cylindrical cultivation chamber completely filled with medium rotates horizontally, so that the net sum of all gravitational forces are zero - a state of continuous “free-fall”. Medium and chamber wall rotate as a solid body at the same angular velocity, eliminating shear forces at the interface.
     Cells grow suspended and evenly dispersed in the chamber.
     Better to get access to the nutrients and to get rid of the waste.
     Aspect ratio is important design feature.

  • When something went wrong during the experiments in the past they remove the device and then examined the sample. Nowadays this can be done during the experiments with optical devices.

  • Cell-polymer-bioreactor system:
    The results for freshly harvested bovine chondrocytes, put in a bioreactor cultivation, implanted in a knee joint, followed for 40 days:

    “Nearest data that replicate the graph of college 1 (29 November)”. The turnover of collagen takes more time. May be shear force needed for collagen?
     The right mix of growth factors will reduce the doubling time, so breading cells will take less time.

    Patients will have to be screened since the designed tissues will not be universally applicable. It will not be effective for all genotypes.
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