Smart crowding in poly-Acrylamide/Poly(N-isopropylacrylamide) composite hydrogels, studied by two-focus fluorescence correlation spectroscopy

The diffusion in complex media is of high interest for a broad range of applications. Fluorescence correlation spectroscopy (FCS) is often used to study diffusion in complex media, such as semi diluted polymer solutions, living cells or complex, heterogeneous hydrogel structures. In these media the refractive index usually differs from that of the immersion medium and is potentially changing across the sample. The two-focus fluorescence correlation spectroscopy (2fFCS) is known to be robust against the refractive index mismatch. In this work the 2fFCS is used to measure the diffusion in such complex media and the results are compared to single-focus FCS measurement results on the same samples to demonstrate the potential impact of the refractive index mismatch on the single-focus FCS measurement results. The diffusion of tagged dextran tracers in water, dilute dextran solutions, acrylamide monomer solutions, poly-acrylamide polymer solutions, and a cross-linked polyacrylamide hydrogel is probed by 2fFCS. In these experiments, both the refractive index and the potential topological constraint and thermodynamic interaction to the probe diffusion is varied, and pairs of samples with same refractive indexes but different compositions are compared. Whereas 2fFCS shows no anomalous diffusion in any of them, single-focus FCS indicates anomalous diffusion. In particular, the values of the stretching exponent of the fluorescence autocorrelation function, which is often interpreted to reflect the extent of anomaly of diffusion, does not vary systematically with the extent of topological or thermodynamic complexity of the different matrixes, but with their refractive index. This shows that apparent anomalous diffusion in FCS is at risk to be the result of refractive index mismatch rather than reflecting truly complex diffusion.Furthermore the 2fFCS has been used in a spatial resolved mode to study the diffusion in core-shell particles and thermo-responsive composite hydrogels.The diffusion of payloads within core–shell carrier particles is of major relevance for drug-delivery applications. We use spatially resolved two-focus fluorescence correlation spectroscopy to quantify the diffusivity of different dextran molecules and colloids within carrier particles composed of a temperature-responsive poly(N-isopropyl-acrylamide) (PNIPAM) shell that surrounds a temperature-insensitive poly(acrylamide) (PAAM) core. The deswelling of the shell that occurs upon heating above the lower critical solution temperature of PNIPAM slightly slows down the diffusion of these tracer oligomers near the core-shell interface. By contrast, the mobility of the tracers inside the core is not affected by deswelling of the shell. This finding assures absence of artifacts such as adsorption of the guests to the amphiphilic shell polymer, supporting the utility of these microgel carriers in encapsulation and controlled release applications.Thermosensitive composite hydrogels that consist of a PAAM hydrogel matrix with embedded micrometre-sized PNIPAM microgel beads are promising models for complex, heterogeneous gels and living cells. The coupling of the microgel beads with the gel matrix and the formation of interpenetrating networks inside the microgels had been investigated by 2fFCS. This technique serves to study the effects of the heterogeneous structure of the composite hydrogels on the diffusive mobility of nanoscopic dextran tracers within the gels. The investigations reveal that the formation of interpenetrating networks inside the embedded microgel beads depends on their cross-link density: whereas interpenetrating networks are formed inside weakly cross-linked beads, they are not formed inside strongly cross-linked beads. If the formation of interpenetrating networks occurs, the temperature-dependent swelling and deswelling of the beads is obstructed. In addition, the mobility of dextran tracers inside the embedded microgel beads is hindered compared to those in free beads and in the surrounding gel matrix. Surprisingly, the surrounding PAAM hydrogel matrix swells inhomogeneously when the embedded PNIPAM beads collapse upon heating. This indicates the formation of pores near the surface of the collapsed beads, offering promising means to tailor composite hydrogels for applications such as membranes with tunable permeability. This experiment also demonstrates the utility of 2fFCS to study spatially resolved diffusion in complex environments, which is of great interest in biomaterials research

Diffusion of guest molecules within sensitive core–shell microgel carriers

The diffusion of payloads within core shell carrier particles is of major relevance for drug delivery applications. We use spatially resolved two focus fluorescence correlation spectroscopy to quantify the diffusivity of different dextran molecules and colloids within carrier particles composed of a temperature responsive poly N isopropylacrylamide PNIPAM shell that surrounds a temperature insensitive polyacrylamide core. The deswelling of the shell that occurs upon heating above the lower critical solution temperature of PNIPAM slightly slows down the diffusion of these tracer oligomers near the core shell interface. By contrast, the mobility of the tracers inside the core is not affected by deswelling of the shell. This finding assures absence of artifacts such as adsorption of the guests to the amphiphilic shell polymer, supporting the utility of these microgel carriers in encapsulation and controlled release application

Spatially Resolved Tracer Diffusion in Complex Responsive Hydrogels

Thermosensitive composite hydrogels that consist of a poly­(acrylamide) hydrogel matrix with embedded micrometer-sized poly­(<i>N</i>-isopropylacrylamide) microgel beads are promising models for complex, heterogeneous gels. We investigate the coupling of the microgel beads with the gel matrix and the formation of interpenetrating networks inside the microgels by confocal two-focus fluorescence correlation spectroscopy (2fFCS). This technique serves to study the effects of the heterogeneous structure of the composite hydrogels on the diffusive mobility of nanoscopic dextran tracers within the gels. Our investigations reveal that the formation of interpenetrating networks inside the embedded microgel beads depends on their cross-link density: whereas interpenetrating networks are formed inside weakly cross-linked beads, they are not formed inside strongly cross-linked beads. If the formation of interpenetrating networks occurs, the temperature-dependent swelling and deswelling of the beads is obstructed. In addition, the mobility of dextran tracers inside the embedded microgel beads is hindered compared to those in free beads and in the surrounding gel matrix. Surprisingly, the surrounding poly­(acrylamide) hydrogel matrix swells inhomogeneously when the embedded poly­(<i>N</i>-isopropylacrylamide) beads collapse upon heating. This indicates the formation of pores near the surface of the collapsed beads, offering promising means to tailor composite hydrogels for applications as membranes with tunable permeability. Our experiments also demonstrate the utility of 2fFCS to study spatially resolved diffusion in complex environments, which is of great interest in biomaterials research

Economic evaluations of pharmacogenetic and genomic screening programs: update of the literature

Pharmacogenetics and pharmacogenomics show great potential for developing individual treatment modalities to achieve optimal therapy effectiveness. Economic analyses are performed to determine whether pharmacogenetic screening strategies provide good value for money. The current review provides an update of published economic studies. Economic analyses of pharmacogenetic screening programs published between 2000 and July 2010 were included in the review. Information was extracted on research area, genetic information, type of economic analysis, key aspects of adherence to economic guidelines, costs and commercial availability of genetic tests, and the role of the funding party. A total of 42 economic studies on pharmacogenetic screening strategies were included. Over time, more cost-utility analyses were performed, longer time windows were employed, and more extensive sensitivity analyses were conducted. Considerable differences in costs of screening tests for the same polymorphism were found, which often, but not always, had a large influence on the costs of screening strategies. Most studies were conducted from an academic or hospital perspective without direct links to pharmaceutical or diagnostic manufacturers. The quality of economic analyses of pharmacogenetic screening programs has improved over time. However, input variables are not always clearly described. In particular, substantial variation exists in the reported costs of the pharmacogenetic tests. Often these test costs are considered a major cost driver and could therefore be of particular importance for the interpretation of cost-effectiveness results. Furthermore, the economic studies seem to be conducted to increase awareness of possibilities and perspectives of genetic testing rather than to influence policy decisions on reimbursement. Drug Dev Res 71: 492-501, 2010. (C) 2010 Wiley-Liss, Inc