Stimuli-responsive hydrogels are materials with great potential for development of active functionalities in fluidics and micro-fluidics. Based on the current state of research on pH sensors, hydrogel sensors are described qualitatively and quantitatively for the first time. The review introduces the physical background of the special properties of stimuli-responsive hydrogels. Following, transducers are described which are able to convert the non-electrical changes of the physical properties of stimuli-responsive hydrogels into an electrical signal. Finally, the specific sensor properties, design rules and general conditions for sensor applications are discussed.

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Review on Hydrogel-based pH Sensors and Microsensors.

Intrinsic viscosities and unperturbed dimensions of long chain molecules

The formation of densely crosslinked networks by chain crosslinking photopolymerization is discussed in relation to selected applications in the electronics industry. All of these applications make use of the high speed and of the latitude to meet other requirements by variation of the chemical structure of the monomers. The selection comprises:

1.

The coating of optical fibers;




2.

The replication of optical discs;




3.

The replication of aspherical lenses, used for laser read-out of these discs.

Network formation by chain crosslinking photopolymerization and some applications in electronics

Hydrogels were the first biomaterials rationally designed for human use. Beginning with the pioneering work of Wichterle and Lim on three-dimensional polymers that swell in water, we review the design, synthesis, properties, and applications of hydrogels. The field of hydrogels has moved forward at a dramatic pace. The development of suitable synthetic methods encompassing traditional chemistry to molecular biology has been used in the design of hydrogels mimicking basic processes of living systems. Stimuli-sensitive hydrogels, hydrogels with controlled degradability, genetically engineered poly(amino acid) polymers reversibly self-assembling in precisely defined three-dimensional structures, and hybrid polymers composed of two distinct classes of molecules are just some examples of these exciting novel biomaterials. The biocompatibility of hydrogels and their applications from implants to nanomaterials are also reviewed. Copyright © 2007 Society of Chemical Industry

Hydrogels as smart biomaterials

Physicochemical characterization of chitosan nanoparticles: electrokinetic and stability behavior

HIGH-MOLECULAR polypeptides containing only one kind of amino-acid are prepared by polymerization of N-carboxylic-α-amino-acid anhydrides1 (α-amino-acid-NCA). The determination of the heat of combustion of these compounds and of the corresponding amino-acids offers for the first time the possibility of estimating quantitatively the energetics of formation of high-molecular peptide.

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Energetics of peptide formation.

Die stabilisierende Wirkung des Hydrochinons auf die Wärmepolymerisation des Styrols

Spectroscopic measurements at 335 nm on polymerizing styrene have been carried out with spontaneously polymerizing as well as dibenzoyl peroxide (BPO)-initiated bulk systems. The evidence for the formation of two Diels-Alder-intermediates, 1a and 1b, out of which only 1a seems to be capable to produce initiating radicals by reaction with the monomer, can also be derived from the development of the absorption, characteristic for the intermediates, during the BPO-initiated polymerization. From the dependence of the overall rate constants of consumption of 1a, k2,a[M], and 1b, k2,b[M], on the rate of polymerization the reactivity of 1a and 1b towards polymer radicals can be estimated. Whereas 1b, the less reactive intermediate, seems to be mainly consumed by polymer radicals through copolymerization, chain transfer being unimportant, (the rate constant of consumption of 1b by polymer radicals, k3,b, relative to the constant of chain propagation kp ranges from 2,1 at 60°C to 3,2 at 80°C) the corresponding ratio for 1a, k3,a/kp, which goes from 113 at 80°C to 143 at 60°C, is supposed to represent primarily the constant of chain transfer of 1a in styrene polymerization. Under the condition of spontaneous polymerization the reaction with polymer radicals contributes 16% (for 1a) and 5–8,5%, depending on temperature (for 1b), to the overall rate of consumption of the intermediates. The stationary concentration of 1a, the isomer which accordingly is solely responsible for initiation as well as chain transfer in spontaneous styrene polymerization, is calculated to range from 5,2·10−6 mol l−1 at 60°C to 8,5·10−6 mol l−1 at 80°C.

Spectroscopic measurements on spontaneously polymerizing styrene, 2. The estimation of the reactivity of the two Diels‐Alder‐isomers towards polymer radicals

Polymerisationsanregung durch Elektrolyse

The mechanism of the bimolecular reaction between the model radicals for polystyryl: 1-phenylethyl, 1-phenylpropyl, and 1,3-diphenylpropyl radical is investigated by means of gas chromatography. The radicals are generated either at 80°C by thermal or at 20°C by photochemical decomposition of the corresponding azoalkanes in benzene solution. At 80°C 7,5% of the 1-phenylethyl radicals, 12,9% of the 1-phenylpropyl radicals, and 13,5% of the 1,3-diphenylpropyl radicals undergo disproportionation. Since the structure of these model radicals approaches that of the growing polystyrene chain in this order one may expect polystyryl radicals to disproportionate to about the same extent as 1,3-diphenylpropyl radicals. In photolysis experiments evidence for ortho coupling of 1-phenylalkyl radicals is found. Photoisomerisation of this ortho coupling product leads to the formation of a small amount of a thermally stable unsymmetrical dimer.

Johann Wolfgang Breitenbach

Papers

Energetics of peptide formation.

Die stabilisierende Wirkung des Hydrochinons auf die Wärmepolymerisation des Styrols

Spectroscopic measurements on spontaneously polymerizing styrene, 2. The estimation of the reactivity of the two Diels‐Alder‐isomers towards polymer radicals

Polymerisationsanregung durch Elektrolyse

Kombination und disproportionierung von modellradikalen für die wachsende polystyrolkette