Layer-by-layer coated digitally encoded microcarriers for quantification of proteins in serum and plasma.

The "layer-by-layer" (LbL) technology has been widely investigated for the coating of flat substrates and capsules with polyelectrolytes. In this study, LbL polyelectrolyte coatings applied at the surface of digitally encoded microcarriers were evaluated for the quantitative, sensitive, and simultaneous detection of proteins in complex biological samples like serum, plasma, and blood. LbL coated microcarriers were therefore coupled to capture antibodies, which were used as capture agents for the detection of tumor necrosis factor (TNF-alpha), P24, and follicle stimulating hormone (FSH). It was found that the LbL coatings did not disassemble upon incubating the microcarriers in serum and plasma. Also, nonspecific binding of target analytes to the LbL coating was not observed. We showed that the LbL coated microcarriers can reproducibly detect TNF-alpha, P24, and FSH down to the picogram per milliliter level, not only in buffer but also in serum and plasma samples. Microcarrier-to-microcarrier intratube variations were less then 30%, and interassay variations less than 8% were observed. This paper also shows evidence that the LbL coated digitally encoded microcarriers are ideally suited for assaying proteins in "whole" blood in microfluidic chips, which are of high interest for "point-of-care" diagnostics.

Anomalous photobleaching in fluorescence recovery after photobleaching measurements due to excitation saturation--a case study for fluorescein.

In this study we examine the implications of excitation saturation on fluorescence recovery after photobleaching (FRAP) experiments. In particular we present both experimental and theoretical evidence that fluorescein, one of the most frequently used fluorophores in FRAP, does not always comply with the basic assumptions that are made in many FRAP models: an invariant bleaching illumination intensity distribution (BID) in combination with first-order photobleaching kinetics. High light intensity levels, which are typical for the photobleaching phase of FRAP experiments, can cause excitation saturation of fluorescein in the excited triplet state. We show by experiments and computer simulations that under such saturating conditions the higher-order diffraction maxima of the BID substantially contribute to the photobleaching process and can no longer be neglected. As a result, the bleached regions are larger than expected theoretically from the FRAP models. Although this effect is not always directly evident from the FRAP experiments, neglecting it may shift the calculated diffusion coefficient by as much as over one order of magnitude. We present a discussion on the implications of this saturation effect on various types of FRAP models.

Self-exploding lipid-coated microgels.

Self-exploding microparticles show potential for advanced delivery of certain therapeutics. This study evaluates (1) whether degrading biodegradable dextran hydroxyethyl methacrylate (dex-HEMA) microgels can be coated by a lipid membrane and (2) whether the surrounding membrane can be ruptured by the increasing swelling pressure of the degrading microgel. We found that adsorption of charged liposomes to oppositely charged dex-HEMA microgels provides efficient coating of the microgels, whereby microparticles with a "core-shell" structure were clearly obtained. Especially, we could confirm experimentally that the swelling pressure increase of degrading dex-HEMA microgels can destroy the lipid membrane surrounding the microgels.

"Programmed polymeric devices" for pulsed drug delivery.

Pharmaceutical research strives to design drug delivery systems that respond to therapeutic needs. Considering the facts that physiologic parameters (e.g., heart rate, blood pressure, and plasma concentration of hormones, plasma proteins, and enzymes) display constancy over time, drug delivery systems with a constant release profile have been designed. However, because of circadian rhythms in physiologic parameters and pathologic conditions (e.g., asthma, angina pectoris), the conventional paradigm concerning drug concentrations "the flatter the better" may not be what the organism may need. Instead, to correlate with our biological needs, "precisely timed drug delivery," which could be accomplished with "programmable dosage forms," is required. Precisely timed drug delivery may maximize therapeutic efficacy, may minimize dose frequency, and may reduce toxicity by avoiding side effects and drug tolerance. This paper outlines the concepts that have been proposed to release drugs in a pulsed manner from pharmaceutical devices.

Tailoring the swelling pressure of degrading dextran hydroxyethyl methacrylate hydrogels.

Swelling pressure measurements were performed on degrading dextran hydroxyethyl methacrylate (dex-HEMA) hydrogels. In these networks, the cross-links are hydrolyzable carbonate ester bonds formed between methacrylate groups and dextran molecules. It is demonstrated that dex-HEMA gels made in the presence of a known amount of free dextran chains exhibit osmotic properties similar to those of partially degraded dex-HEMA gels. The swelling pressure, Pi(sw), of degrading dex-HEMA gels is controlled primarily by the cross-linked dex-HEMA polymer and the free dextran molecules, while the contribution of short poly-HEMA fragments (produced in the degradation process) is negligible. It is found that Pi(sw) only slightly changes during the first 15 days of degradation. Close to the end of the degradation process, however, a much faster increase in Pi(sw) is observed. The swelling pressure profile of these gels strongly depends on the concentration of the cross-linked dex-HEMA and its chemical composition (amount of HEMA groups per 100 glucose units).

A comparison between the use of dynamic mechanical analysis and oscillatory shear rheometry for the characterisation of hydrogels.

In this study the use of dynamic mechanical analysis (DMA) for the mechanical characterisation of pharmaceutical hydrogels was evaluated. DMA was used in two different modes, the "controlled force" and the "multi-strain" (MS). The results obtained on dextran methacrylate hydrogels of various compositions were compared to those obtained using an oscillatory shear rheometer. The best agreement was found between the MS-DMA and the rheometer results. The moduli measured in MS-DMA were extrapolated towards zero compression to obtain the modulus of the hydrogels. This procedure resulted in good agreement with the data obtained with the rheometer. Hydrogels were analysed after swelling to equilibrium with both methods, DMA and rheology. A scaling between the elastic modulus (G') and the equilibrium swollen polymer volume fraction (v(2,s)) could be found, although the best correlation between G' and v(2,s) was obtained with the rheometer.