Water soluble macromers are modified by addition of free radical polymerizable groups, such as those containing a carbon-carbon double or triple bond, which can be polymerized under mild conditions to encapsulate tissues, cells, or biologically active materials. The polymeric materials are particularly useful as tissue adhesives, coatings for tissue lumens including blood vessels, coatings for cells such as islets of Langerhans, coatings, plugs, supports or substrates for contact with biological materials such as the body, and as drug delivery devices for biologically active molecules.

Gels for encapsulation of biological materials

Hydrogels of polymerized and crosslinked macromers comprising hydrophilic oligomers having biodegradable monomeric or oligomeric extensions, which biodegradable extensions are terminated on free ends with end cap monomers or oligomers capable of polymerization and cross linking are described. The hydrophilic core itself may be degradable, thus combining the core and extension functions. Macromers are polymerized using free radical initiators under the influence of long wavelength ultraviolet light, visible light excitation or thermal energy. Biodegradation occurs at the linkages within the extension oligomers and results in fragments which are non-toxic and easily removed from the body. Preferred applications for the hydrogels include prevention of adhesion formation after surgical procedures, controlled release of drugs and other bioactive species, temporary protection or separation of tissue surfaces, adhering of sealing tissues together, and preventing the attachment of cells to tissue surfaces.

Photopolymerizable biodegradable hydrogels as tissue contacting materials and controlled-release carriers

The biotechnological utilization of cheese whey: A review

Polyhydroxyalkanoates (PHAs) are gaining increasing attention in the biodegradable polymer market due to their promising properties such as high biodegradability in different environments, not just in composting plants, and pro- cessing versatility. Indeed among biopolymers, these biogenic polyesters represent a potential sustainable replacement for fossil fuel-based thermoplastics. Most commercially available PHAs are obtained with pure microbial cultures grown on renewable feedstocks (i.e. glucose) under sterile conditions but recent research studies focus on the use of wastes as growth media. PHA can be extracted from the bacteria cell and then formulated and processed by extrusion for production of rigid and flexible plastic suitable not just for the most assessed medical applications but also considered for applications includ- ing packaging, moulded goods, paper coatings, non-woven fabrics, adhesives, films and performance additives. The present paper reviews the different classes of PHAs, their main properties, processing aspects, commercially available ones, as well as limitations and related improvements being researched, with specific focus on potential applications of PHAs in packag- ing.

http://www.expresspolymlett.com/letolt.php?file=EPL-0005219&mi=cd

Polyhydroxyalkanoate (PHA): Review of synthesis, characteristics, processing and potential applications in packaging

Immobilization technologies and support materials suitable in alcohol beverages production: a review

The immobilization of whole cells: Engineering principles

Biosynthesis and characterization of hydroxybutyrate-hydroxycaproate copolymers.

Pseudomonas putida cells were grown in confined volumes in dual-membrane immobilized cell reactors constructed from microporous polyethylene hollow fibers and silicone rubber tubules as a model system for the study of mass transport in microbial aggregates. Local cell concentrations in the reactors reached 300 g dry mass/L. Pulse-chase radioisotope labeling with (35)SO(4) (2-) was used to estimate the rates of cell mass synthesis and degradation. Sulfur incorporation consistently exceeded sulfur release, implying that the cell mass concentration continually increases. The location and size of the cell growth region was determined using liquid emulsion autoradiography of thin sections prepared from labeled reactors. Cell growth occurs in a region less than 25 mum in depth adjacent to the oxygen supply, and the expansion of the cells caused by cell growth promotes convection of the cell mass into regions of the reactor where starving cells accumulate. The combination of mass-balance and spatial distribution measurements that can be made using radioisotope tracers provides a versatile method for determining metabolic rates and limitations caused by mass transfer in immobilized cell reactors.

Autoradiographic determination of mass‐transfer limitations in immobilized cell reactors

Escherichia coli K-12 cells were grown in a confined volume using microporous hollow fiber membranes. The local cell concentrations in the reactors were above 400 g dry mass/L, in excess of the predicted limit based on the specific volume of free cells determined by tracer exclusion. Cell mass synthesis and degradation rates in these reactors were measured using radioisotope labeling with (35)S. Net accumulation of cell material persisted at these high cell densities. The rates of substrate uptake and cell growth were predicted from the theory of reaction and diffusion assuming that kinetics of cell metabolism are identical for free-living and immobilized cells. This theory was tested by comparison of overall rates and by the size of the region in which cell growth occurred, measured by autoradiography. A yield coefficient of 4 +/- 1 mol sulfur/mol glucose was measured, in agreement with the value determined for free-living cells in similar conditions. Cell growth occurs in a thin layer (10-30 microm), at a rate similar to the growth rate for free cells. Volume expansion by the cells as a consequence of proliferation induces convection of cell mass out of the growth region into a region of the reactor filled with starving cells, which then accumulate in the reactor. The combination of mass-balance and spatial distribution measurements made possible by the use of radioisotope labeling enables a direct test for mass transfer limitations, the determination of the intrinsic cell kinetics, and noninvasive measurements of cell growth in immobilized cell reactors.

Cell mass synthesis and degradation by immobilized Escherichia coli.

An abundance of techniques has been developed for the immobilization of living cells with retention of their enzymatic activities. The well-established mathematical theory of transport and reaction in permeable media should be applicable to the prediction of local and overall reaction rates in all of these systems if we presume that the local values of transport parameters (diffusivities) and reaction rate constants can be obtained or estimated. It is important to recognize that these values will depend not only on the local concentration of cells, but also on the physical and metabolic state of the cells. New techniques must be developed for the characterization of the metabolic state of immobilized cells. Two classes of methods are particularly required: noninvasive methods, which can be used to obtain information during operation of a reactor, and methods that can give information about the spatial variations in cell properties within an immobilized cell layer. As a first step toward a complete description of the metabolism of bacteria immobilized at high cell densities, we have applied a variety of techniques that allow the characterization of several distinct features of cell metabolism. These techniques include radioisotope labeling and autoradiography with ’5S as a tracer for protein synthesis, electron microscopy, and ”P nuclear magnetic resonance (NMR) spectroscopy. These techniques have been applied to the study of a single model system-the anaerobic growth of Escherichia coli contained by microporous hollow fiber membranes.

Steven F. Karel

Papers

The immobilization of whole cells: Engineering principles

Biosynthesis and characterization of hydroxybutyrate-hydroxycaproate copolymers.

Autoradiographic determination of mass‐transfer limitations in immobilized cell reactors

Cell mass synthesis and degradation by immobilized Escherichia coli.

The behavior of immobilized living cells. Characterization using isotopic tracers.