Functional polymeric foams

Open-pored polymeric foams are interesting materials for a variety of applications, especially as adsorber materials for substance separation or substance accumulation, as carrier material for chemo- or biocatalysis and as three-dimensional carriers for mammalian cells or microorganisms. Foams made conductive by suitable additives are even suitable as electrode material for biofuel cells. For the above-mentioned applications, the inner surface of the pores must usually be equipped with functional groups or molecules. Depending on the pore structure and the type of polymer used for the foam, this has not been possible to date, or only at great expense.

We have developed a one-step synthesis strategy for the simple production of macroporous polymer foams with functional groups that can be easily implemented via so-called click reactions.

For this purpose, a cross-linked polymer foam containing azide functions was produced using an emulsion polymerization process (high internal phase emulsion). The organic phase contains the monomer, the crosslinker and a surfactant. The aqueous phase, which is placed in a reaction vessel together with the organic phase, contains the initiator for the radical polymerization (Fig. 2).

The manufactured material shown as an example in Figure 2 has a porosity of approx. 90 percent. The structure with open macropores in the range of approx. 5-10 µm and micropores below 1 µm is shown in the scanning electron microscope image in Figure 3.

Azide functions can react with alkyne functions under very mild conditions, such as those required by biomolecules, and without side reactions (click reaction). We were able to show the binding of low-molecular compounds and also polymers to the inner surface of the synthesized foam via an azide-alkine cycloaddition (Figure 4) using the examples of propargyl alcohol (Figure 4A) and propargyl poly(methyl methacrylate) (Figure 4B).

For applications of the material as a carrier for cells, its biocompatibility is a necessary prerequisite. First tests with human skin cells on the PMMA-modified foams showed improved cell adhesion after an incubation period of 24 hours compared to the unmodified foam. Live-dead staining of human cells cultivated on the surface for 24 hours showed a high live cell count.

In contrast to conventional polymer foams, the finished polymer is not foamed here. The material therefore has all the advantages of a synthetically producible polymer. Due to its cross-linked structure, the material is stable against organic solvents. In addition, the freedom in the choice of chemical building blocks allows for the targeted adjustment of desired properties such as hydrophilicity or elasticity.

Once the feasibility of producing easily modifiable polymer foams with suitable porosity has been demonstrated in a one-step synthesis, the macroporous material will be further developed so that it can be used for colonization with microbial cells for substance enrichment.

We would like to thank the Fraunhofer-Gesellschaft for funding the project "Bacteria behind bars to recover phosphorus" as part of the Fraunhofer Symposium Netzwert 2011.