Polyelectrolyte multilayers prepared on hydrogel surfaces

Stable and selective permeable hydrogel microcapsules for high-throughput cell cultivation and enzymatic analysis

BACKGROUND

Miniaturization of biochemical reaction volumes within artificial microcompartments has been the key driver for directed evolution of several catalysts in the past two decades. Typically, single cells are co-compartmentalized within water-in-oil emulsion droplets with a fluorogenic substrate whose conversion allows identification of catalysts with improved performance. However, emulsion droplet-based technologies prevent cell proliferation to high density and preclude the feasibility of biochemical reactions that require the exchange of small molecule substrates. Here, we report on the development of a high-throughput screening method that addresses these shortcomings and that relies on a novel selective permeable polymer hydrogel microcapsule.

RESULTS

Hollow-core polyelectrolyte-coated chitosan alginate microcapsules (HC-PCAMs) with selective permeability were successfully constructed by jet break-up and layer-by-layer (LBL) technology. We showed that HC-PCAMs serve as miniaturized vessels for single cell encapsulation, enabling cell growth to high density and cell lysis to generate monoclonal cell lysate compartments suitable for high-throughput analysis using a large particle sorter (COPAS). The feasibility of using HC-PCAMs as reaction compartments which exchange small molecule substrates was demonstrated using the transpeptidation reaction catalyzed by the bond-forming enzyme sortase F from P. acnes. The polyelectrolyte shell surrounding microcapsules allowed a fluorescently labelled peptide substrate to enter the microcapsule and take part in the transpeptidation reaction catalyzed by the intracellularly expressed sortase enzyme retained within the capsule upon cell lysis. The specific retention of fluorescent transpeptidation products inside microcapsules enabled the sortase activity to be linked with a fluorescent readout and allowed clear separation of microcapsules expressing the wild type SrtF from those expressing the inactive variant.

CONCLUSION

A novel polymer hydrogel microcapsule-based method, which allows for high-throughput analysis based on encapsulation of single cells has been developed. The method has been validated for the transpeptidation activity of sortase enzymes and represents a powerful tool for screening of libraries of sortases, other bond-forming enzymes, as well as of binding affinities in directed evolution experiments. Moreover, selective permeable microcapsules encapsulating microcolonies provide a new and efficient means for preparing novel caged biocatalyst and biosensor agents.

Controlled release of vascular endothelial growth factor from alginate hydrogels nano-coated with polyelectrolyte multilayer films

Alginate (ALG) hydrogels incorporating vascular endothelial growth factor (VEGF) were nano-coated with polyelectrolyte multilayer (PEM) films composed of chitosan (CT) and dextran sulfate (DEX) in order to control the VEGF release from the hydrogels. When non- and nano-coated ALG hydrogels containing VEGF were incubated in phosphate-buffered saline (PBS) at 37 degrees C for the prescribed times, the nano-coated hydrogels were stable, even after incubation for a week, whereas the non-coated hydrogels collapsed after 6 h. The release profile of VEGF from the non- and nano-coated ALG hydrogels was evaluated by an enzyme-linked immunosorbent assay (ELISA). Although all of the VEGF incorporated into the non-coated ALG hydrogels was released within 6 h by the collapse, only a few percent of the VEGF incorporated in the nano-coated hydrogels was released. Furthermore, the incorporated VEGF in the nano-coated hydrogels was released continuously, even after a month, without any initial burst release. The release percentage of VEGF was easily controlled by the PEM film thickness on the surface of the ALG hydrogels. The VEGF released from the nano-coated hydrogels retained its activity without denaturation. Consequently, the nano-coating of hydrogel surfaces with PEM films may be useful for controlled and sustained drug-delivery systems.

Free-standing membranes via covalent cross-linking of polyelectrolyte multilayers with complementary reactivity

Polyelectrolyte multilayers were prepared by the layer-by-layer (LbL) technique from polyanions bearing aldehyde and polycations with 4-methylpyridinium moieties. The aldol reaction of these complementary reactive groups can be followed by the formation of fluorescent merocyanine dyes, resulting in cross-linked, ultrathin polymer films. The efficient stabilization of the polymer films allows for their intact removal from high surface energy supports, such as glass or surface oxidized silicon wafers, by simple treatment with salt solutions, yielding free-standing membranes. Increasing separation of the reactive polycation and polyanion layers with layers of inert polycation and polyanion analogues only gradually prevents the coupling reaction. From this dependence, polyions assembled in consecutive adsorption layers seem to be able to penetrate into as far as three neighboring layers.

Adsorption of protein-coated lipid droplets to mixed biopolymer hydrogel surfaces: role of biopolymer diffusion

The adsorption of charged particles to hydrogel surfaces is important in a number of natural and industrial processes. In this study, the adsorption of cationic lipid droplets to the surfaces of anionic hydrogels was examined. An oil-in-water emulsion containing cationic beta-lactoglobulin-coated lipid droplets was prepared (d32=0.24 microm, zeta=+74 mV, pH 3.0). An anionic hydrogel containing 0.1 wt % beet pectin and 1.5 wt % agar (pH 3.0) was prepared. Emulsions containing different lipid droplet concentrations (0.3-5 wt %) were brought into contact with the hydrogel surfaces for different times (0-24 h). The adsorption of lipid droplets to the hydrogel surfaces could not be explained by a typical adsorption isotherm. We found that the electrical charge on the nonadsorbed lipid droplets became less positive or even became negative in the presence of the hydrogel and that extensive droplet aggregation occurred, which was attributed to the ability of pectin molecules to diffuse through the hydrogels and interact with the lipid droplets. These results may have important consequences for understanding certain industrial and biological processes, as well as for the design of controlled or triggered release systems.

Layer-by-layer coating of degradable microgels for pulsed drug delivery

Recently, we reported on "self-rupturing" microcapsules which consist of a biodegradable dextran-based microgel surrounded by a polyelectrolyte membrane. Degradation of the microgel increases the swelling pressure in the microcapsules which, when sufficiently high, ruptures the surrounding polyelectrolyte membrane. The membrane surrounding the microgels is deposited using the layer-by-layer (LbL) technique, which is based on the alternate adsorption of oppositely charged polyelectrolytes onto a charged substrate. In this paper, we characterize with confocal microscopy, electrophoretic mobility, scanning electron microscopy and atomic force microscopy in detail the deposition and the properties of the LbL coatings on the dextran microgels. We show that by fine-tuning the properties of both the microgel core and the LbL membrane the swelling pressure which is evoked by the degradation of the microgel is indeed able to rupture the surrounding LbL membrane. Further, we show that the application of an LbL coating on the surface of the microgels dramatically lowers the burst release from the microcapsules and results in massive release at the time the microcapsules rupture.