Amperometric Biosensors Based on Oxidoreductases Immobilized in Photopolymerized Poly(ethylene glycol) Redox Polymer Hydrogels

3D Printing: An Emerging Technology for Biocatalyst Immobilization

Employment of enzymes as biocatalysts offers immense benefits across diverse sectors in the context of green chemistry, biodegradability, and sustainability. When compared to free enzymes in solution, enzyme immobilization proposes an effective means of improving functional efficiency and operational stability. The advance of printable and functional materials utilized in additive manufacturing, coupled with the capability to produce bespoke geometries, has sparked great interest towards the 3D printing of immobilized enzymes. Printable biocatalysts represent a new generation of enzyme immobilization in a more customizable and adaptable manner, unleashing their potential functionalities for countless applications in industrial biotechnology. This review provides an overview of enzyme immobilization techniques and 3D printing technologies, followed by illustrations of the latest 3D printed enzyme-immobilized industrial and clinical applications. The unique advantages of harnessing 3D printing as an enzyme immobilization technique will be presented, alongside a discussion on its potential limitations. Finally, the future perspectives of integrating 3D printing with enzyme immobilization will be considered, highlighting the endless possibilities that are achievable in both research and industry. This article is protected by copyright. All rights reserved.

Functionalised microscale nanoband edge electrode (MNEE) arrays: the systematic quantitative study of hydrogels grown on nanoelectrode biosensor arrays for enhanced sensing in biological media

Nanoelectrodes and nanoelectrode arrays show enhanced diffusion and greater faradaic current densities and signal-to-noise ratios compared to macro and microelectrodes, which can lead to enhanced sensing and detection. One example is the microsquare nanoband edge electrode (MNEE) array system, readily formed through microfabrication and whose quantitative response has been established electroanalytically. Hydrogels have been shown to have applications in drug delivery, tissue engineering, and anti-biofouling; some also have the ability to be grown electrochemically. Here, we combine these two emerging technologies to demonstrate the principles of a hydrogel-coated nanoelectrode array biosensor that is resistant to biofouling. We first electrochemically grow and analyze hydrogels on MNEE arrays. The structure of these gels is shown by imaging to be electrochemically controllable, reproducible and structurally hierarchical. This structure is determined by the MNEE array diffusion fields, consistent with the established hydrogel formation reaction, and varies in structural scale from nano (early time, near electrode growth) to micro (for isolated elements in the array) to macro (when there is array overlap) with distance from the electrode, forming a hydrogel mesh of increasing density on progression from solution to electrode. There is also increased hydrogel structural density observed at electrode corners, attributable to enhanced diffusion. The resulting hydrogel structure can be formed on (and is firmly anchored to/through) an established clinically relevant biosensing layer without compromising detection. It is also shown to be capable, through proof-of-principle model protein studies using bovine serum albumin (BSA), of preventing protein biofouling whilst enabling smaller molecules such as DNA to pass through the hydrogel matrix and be sensed. Together, this demonstrates a method for developing reproducible, quantitative electrochemical nanoelectrode biosensors able to sense selectively in real-world sample matrices through the tuning of their interfacial properties.

Photolithographically assembled polyelectrolyte complexes as shape-directing templates for thermoreversible gels

Preparation of soft materials with diverse, customized shapes has been a topic of intense research interest. To this end, we have recently demonstrated photolithographic directed assembly as a strategy for customizing polyelectrolyte complex (PEC) shape. This process uses in situ photopolymerization of an anionic monomer in the presence of a cationic polymer, which drives localized PEC formation at the irradiation sites. Here, we show how such photolithographically assembled PECs can serve as structure-directing templates for tailoring the shapes of other soft materials; namely, thermoreversible gels. These templated hydrogels are prepared by adding a thermogelling polymer (agarose) to the anionic monomer/cationic polymer/photoinitiator precursor solutions so that, upon irradiation, custom-shaped PECs form within agarose gel matrices. Once these PECs are formed, the surrounding agarose gels are melted (through heating) and washed away which, upon returning the samples to room temperature, produces interpenetrating PEC/agarose gel networks with photopatterned shapes and dimensions. Dissolution of these sacrificial PEC templates in concentrated NaCl solutions then generates photolithographically templated agarose gels, whose shapes and dimensions match those of their PEC templates. Besides tuning their shapes and sizes, the mechanical properties of these gels can be easily tailored by varying the initial agarose concentrations used. Moreover, this PEC-templated gel synthesis appears to not adversely affect hydrogel cytocompatibility, suggesting its potential suitability for biological and biomedical applications. Though the present study uses only agarose as the model gel system, this PEC-based strategy for customizing gel shape can likely also be applied to other thermoreversible gel networks (e.g., those based on methylcellulose, poloxamers or thermoresponsive chitosan derivatives) and could have many attractive applications, ranging from drug delivery and tissue engineering, to sensing and soft robotics.

Chemically Modified Hydrogel-Filled Nanopores: A Tunable Platform for Single-Molecule Sensing

Label-free, single-molecule sensing is anideal candidate for biomedical applications that rely on the detection of low copy numbers in small volumes and potentially complex biofluids. Among them, solid-state nanopores can be engineered to detect single molecules of charged analytes when they are electrically driven through the nanometer-sized aperture. When successfully applied to nucleic acid sensing, fast transport in the range of 10-100 nucleotides per nanosecond often precludes the use of standard nanopores for the detection of the smallest fragments. Herein, hydrogel-filled nanopores (HFN) are reported that combine quartz nanopipettes with biocompatible chemical poly(vinyl) alcohol hydrogels engineered in-house. Hydrogels were modified physically or chemically to finely tune, in a predictable manner, the transport of specific molecules. Controlling the hydrogel mesh size and chemical composition allowed us to slow DNA transport by 4 orders of magnitude and to detect fragments as small as 100 base pairs (bp) with nanopores larger than 20 nm at an ionic strength comparable to physiological conditions. Considering the emergence of cell-free nucleic acids as blood biomarkers for cancer diagnostics or prenatal testing, the successful sensing and size profiling of DNA fragments ranging from 100 bp to >1 kbp long under physiological conditions demonstrates the potential of HFNs as a new generation of powerful and easily tunable molecular diagnostics tools.

Design of an Os Complex-Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells

Multistep synthesis and electrochemical characterization of an Os complex-modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 M KCl) is demonstrated. The careful selection of bipyridine-based ligands bearing N,N-dimethylamino moieties and an amino-linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6-coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os(3+/2+) redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the FAD-dependent dehydrogenase domain of cellobiose dehydrogenase, low-potential glucose biosensors could be obtained with negligible co-oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase-based biocathode, enzymatic biofuel cells with open-circuit voltages of up to 0.54 V were obtained.

A stimuli-responsive nanoparticulate system using poly(ethylenimine)-graft-polysorbate for controlled protein release

Proteins have emerged as an important class of therapeutic agents due to their high specificity in their physiological actions. Over the years, diverse protein carriers have been developed; however, some concerns, such as the relatively low loading efficiency and release sustainability, have limited the efficiency of protein delivery. This study reports the use of hydrogel nanoparticles based on a novel copolymer, poly(ethylenimine)-graft-polysorbate (PEIP), as effective protein carriers. The copolymer is fabricated by grafting poly(ethylenimine) (PEI) with polysorbate 20 using carbonyldiimidazole chemistry. Its cytotoxicity is much lower than that of unmodified PEI in RGC5 and HEK293 cells. In comparison with nanoparticles formed by unmodified PEI, our nanoparticles are not only more efficient in cellular internalization, as indicated by the 5- to 6-fold reduction in the time they take to cause 90% of cells to exhibit intracellular fluorescence, but also give a protein loading efficiency as high as 70-90%. These, together with the salt-responsiveness of the nanoparticles in protein release and the retention of the activity of the loaded protein, suggest that PEIP and its hydrogel nanoparticles warrant further development as protein carriers for therapeutic applications.

Organometallic polymers for electrode decoration in sensing applications

Macromolecules containing metals combine the processing advantages of polymers with the functionality offered by the metal centers. The developments in the area of electrochemical chemo/biosensors based on organometallic polymers are reviewed.

Synthesis of pH-responsive hydrogel thin films grafted on PCL substrates for protein delivery

A new visible light induced graft polymerization method was utilized to prepare pH-sensitive hydrogel layers covalently attached to polymer substrates for drug delivery.

Ferrocene-functionalized polydopamine as a novel redox matrix for H2O2 oxidation

Simple synthesis of a ferrocene-functionalized polydopamine [poly(DA-Fc)] is described. This redox film displays excellent electrocatalytic oxidation of H₂O₂ in neutral buffer solutions in a Fenton-type reaction, and is found to be selective to H₂O₂ with negligible interference from other small molecules.

Microfabricated Cell-based Biosensor Arrays

Here, we described the fabrication using photolithography of poly(ethylene glycol) (PEG)-based hydrogel microstructures encapsulating viable mammalian cells on glass and silicon substrates. Substrates were treated with 3-(trichlorosilyl) propyl methacrylate to form pendant acrylate group to covalent link the hydrogel microstructure. Cells were encapsulated in arrays of cylindrical hydrogel microstructures 600 and 50 μm in diameter and viability assays demonstrated that encapsulated cells remained viable after photoencapsulation. These microstructures had clearly defined, three-dimensional structure without any residual cells remaining surface and no delamination of hydrogel elements from functionalized substrate occurred in aqueous environment for over a week. By changing spin-coating rates and feature sizes of photomasks, we could create cell-containing microstructures with aspect ratios ranging from 0.12 to 1.4. In case of 50 μm hydrogel microstructure, number of cells could be limited to 1 or 2 cells per element and array consisting of 400 elements could be fabricated in a square of 2 mm². These cell-containinghydrogel microstructures were also successfully fabricated in poly(dimethylsiloxane) microchannels to create optical biosensor arrays of individually addressable single or multiple cell- containing hydrogel microstructures with potential applications in drug screening or pathogen detection.

Three-dimensional hydrogel structures as optical sensor arrays, for the detection of specific DNA sequences

The fabrication and characterization of surface-attached PEG-diacrylate hydrogel structures and their application as sensing platforms for the detection of specific target sequences are reported. Hydrogel structures were formed by a photopolymerization process, using substrate-bound Eosin Y molecules for the production of free radicals. We have demonstrated that this fabrication process allows for control over hydrogel growth down to the micrometer scale. Confocal imaging revealed relatively large pore structures for 25% (v/v) PEG-diacrylate hydrogels, which appear to lie in tightly packed layers. Our data suggest that these pore structures decrease in size for hydrogels with increasing levels of PEG-diacrylate. Surface coverage values calculated for hydrogels immobilized with 21-mer DNA probe sequences were significantly higher compared to those previously reported for 2- and 3-dimensional sensing platforms, on the order of 10(16)molecules cm(-2). Used as sensing platforms in DNA hybridization assays, a detection limit of 3.9 nM was achieved for hybridization reactions between 21-mer probe and target sequences. The ability of these hydrogel sensing platforms to discriminate between wild-type and mutant allele sequences was also demonstrated, down to target concentrations of 1-2 nM. A reduction in the hybridization time down to a period of 15 min was also achieved, while still maintaining confident results, demonstrating the potential for future integration of these sensing platforms within Lab-on-Chip or diagnostic devices.

Cell microarrays based on hydrogel microstructures for the application to cell-based biosensor

Cell-based biosensors constitute a promising field that has numerous applications ranging from pharmaceutical screening to detection of pathogen and toxicant. The trends toward miniaturization of cell-based biosensor continue to spur development of cell microarray integrated into microfluidic devices. For cell-based biosensors to be useful for larger applications, several technical goals must be realized. First, the cell-patterning method used to generate multi-phenotypic array can accommodate multiple cell lines without major losses of cell viability, maintain total isolation of each cell phenotype, provide for the adequate mass transfer of dissolved gases and nutrients, and easy enough to allow for mass production. Second, cells on microarray must be cultured in three-dimensional environment as they do in real tissue to obtain accurate response of cells against target analyte. Third, physiological status of micropatterned cells must be monitored non-invasively. As one solution to satisfy these requirements, we prepare cell microarrays using microfabricated poly(ethylene glycol)(PEG) hydrogel. Arrays of hydrogel microstructures encapsulating one or more different cell phenotypes can be fabricated using photolithography or photoreaction injection molding, and can be incorporated within microfluidic network. Finally, we demonstrate the potential application of cell-containing hydrogel microarrays for toxin detection by monitoring toxin-induced change of cell viability and intercellular enzymatic reaction.

Electrochemical glucose sensors--developments using electrostatic assembly and carbon nanotubes for biosensor construction

In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of an electrochemical sensor for glucose in blood plasma. In their application, Clark and Lyons describe an electrode in which a membrane permeable to glucose traps a small volume of solution containing the enzyme adjacent to a pH electrode, and the presence of glucose is detected by the change in the electrode potential that occurs when glucose reacts with the enzyme in this volume of solution. Although described nearly 50 years ago, this seminal development provides the general structure for constructing electrochemical glucose sensors that is still used today. Despite the maturity of the field, new developments that explore solutions to the fundamental limitations of electrochemical glucose sensors continue to emerge. Here we discuss two developments of the last 15 years; confining the enzyme and a redox mediator to a very thin molecular films at electrode surfaces by electrostatic assembly, and the use of electrodes modified by carbon nanotubes (CNTs) to leverage the electrocatalytic effect of the CNTs to reduce the oxidation overpotential of the electrode reaction or for the direct electron transport to the enzyme.

Enzyme containing redox polymer networks for biosensors or biofuel cells: a photochemical approach

A photochemical approach to the generation of (microstructured) redox hydrogels with incorporated enzymes is presented and evaluated with respect to its potential in biosensor and biofuel cell applications. For this, poly(dimethylacrylamide) polymers containing both electroactive ferrocene moieties and photoreactive benzophenone groups are synthesized and deposited as thin films on electrode surfaces. Upon short irradiation with UV light, the polymer layer cross links and becomes firmly adhered to the glassy carbon electrodes. If glucose oxidase is mixed into the polymer solution prior to coating, then glucose-oxidizing electrodes with very high catalytic current responses are obtained. The influence of multivalent ions and proteins on the performance of the electrocatalytic films is studied. It is found that the interaction between bivalent HPO(4)(2-) and the oxidized redox moieties can shorten the lifetime of the redox electrodes significantly whereas the same electrodes are quite stable in the presence of monovalent ions and the reduced form of the mediator. Coating a thin, covalently attached poly(dimethylacrylamide) protective layer onto the redox polymer networks can greatly reduce the adsorption of proteins onto the surfaces and improve the long-term stability of the electrodes in physiological environments. Because the adsorption of proteins onto unprotected surfaces is one of the major causes of bioelectrode failure, this aspect is expected to contribute to the design of more biostable sensors and fuel cells.

Enzyme-containing hydrogel micropatterns serving a dual purpose of cell sequestration and metabolite detection

The integration of sensing elements with small groups of cells is a critical step towards miniaturization of cell cultivation and analysis. This paper describes the development of an optical, enzyme-based biosensor for local detection of hydrogen peroxide (H(2)O(2)) secreted by stimulated macrophages. Photolithographic patterning of horseradish peroxidase (HRP)-containing poly (ethylene glycol) (PEG) hydrogel microstructures was used to create sensing structures on the glass surface. Importantly, enzyme-entrapping hydrogel micropatterns did not support protein or cell deposition and allowed to guide attachment of macrophages next to the sensing elements. Amplex Red, an organic molecule that becomes fluorescent in the presence of H(2)O(2) and HRP, was either immobilized inside hydrogel elements alongside enzyme molecules or added into the cell culture media during cell activation. The production of H(2)O(2) after mitogenic stimulation of macrophages resulted in appearance of fluorescence in the HRP-containing hydrogel microstructures, with fluorescence intensity being a strong function of analyte concentration. The novel cell culture system with integrated sensing elements described here may be enhanced in the future by incorporating additional biorecognition elements to enable multi-metabolite detection at the site of a cell.

Effects of electrolyte and pH on the behavior of cross-linked films of ferrocene-modified poly(ethylenimine)

Ferrocene redox polymers based on the coupling of ferrocenecarboxaldehyde to both linear and branched poly(ethylenimine) (PEI) have been prepared to investigate the effects of pH, electrolyte, and cross-linking on electron charge transport and film swelling. The redox behavior of both ferrocene-modified linear PEI and ferrocene-modified branched PEI was investigated by cyclic voltammetry, while electron diffusion coefficients reported for PEI-based redox polymers were determined by electrochemical impedance spectroscopy. In phosphate solutions at pH>7, cross-linked films of both redox polymers exhibited multiple redox wave behavior and were unstable. In contrast, in non-phosphate solutions, cross-linked films exhibited stable electrochemical behavior and fast electron transfer in solutions with pH<11. Gel swelling experiments suggested that the multiple wave behavior and instability exhibited in either phosphate solutions or at high pH in non-phosphate solutions were related to a combination of film collapse and electrolyte binding within the hydrogel. The electron diffusion coefficients for these polymers are on the order of 10-8 (mol cm(-2) s(-1/2)), which are approximately 40 times greater than other ferrocene-modified polymers. Incorporation of the enzyme, glucose oxidase, into these films demonstrated that these redox polymers were able to electrically communicate with the enzyme's flavin adenine dinucleotide (FAD) redox centers. Glucose sensors based on these films exhibited enzyme saturation current densities that ranged from 240 to 480 microA/cm2 in response to glucose, which were dependent upon the supporting electrolyte and pH. The sensitivity of these sensors at 5 mM glucose ranged from 10 to 48 microA.cm(-2).mM(-1).

Directed growth of poly(isobenzofuran) films by chemical vapor deposition on patterned self-assembled monolayers as templates

This paper describes a method to direct the formation of microstructures of poly(isobenzofuran) (PIBF) by chemical vapor deposition (CVD) on chemically patterned, reactive, self-assembled monolayers (SAMs) prepared on gold substrates. We examined the growth dependence of PIBF by deposition onto several different SAMs each presenting different surface functional groups, including a carboxylic acid, a phenol, an alcohol, an amine, and a methyl group. Interferometry, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and optical microscopy were used to characterize the PIBF films grown on the various SAMs. Based on the kinetic and the spectroscopic analyses, we suggest that the growth of PIBF is surface-dependent and may follow a cationic polymerization mechanism. Using the cationic polymerization mechanism of PIBF growth, we also prepared patterned SAMs of 11-mercapto-1-undecanol (MUO) or 11-mercaptoundecanoic acid (MUA) by microcontact printing (microCP) on gold substrates as templates, to direct the growth of the PIBF. The directed growth and the formation of microstructures of PIBF with lateral dimensions of 6 microm were investigated using atomic force microscopy (AFM). The average thickness of the microstructures of PIBF films grown on the MUO and the MUA patterns were 400 +/- 40 nm and 490 +/- 40 nm, respectively. SAMs patterned with carboxylic acid salts (Cu2+, Fe2+, or Ag+) derived from MUA led to increases in the average thickness of the microstructures of PIBF by 10%, 12%, or 27%, respectively, relative to that of control templates. The growth dependence of PIBF on the various carboxylic acid salts was also investigated using experimental observations of the growth kinetics and XPS analyses of the relative amount of metal ions present on the template surfaces.

Fabrication of cell-containing hydrogel microstructures inside microfluidic devices that can be used as cell-based biosensors

This paper describes microfluidic systems containing immobilized hydrogel-encapsulated mammalian cells that can be used as cell-based biosensors. Mammalian cells were encapsulated in three-dimensional poly(ethylene glycol)(PEG) hydrogel microstructures which were photolithographically polymerized in microfluidic devices and grown under static culture conditions. The encapsulated cells remained viable for a week and were able to carry out enzymatic reactions inside the microfluidic devices. Cytotoxicity assays proved that small molecular weight toxins such as sodium azide could easily diffuse into the hydrogel microstructures and kill the encapsulated cells, which resulted in decreased viability. Furthermore, heterogeneous hydrogel microstructures encapsulating two different phenotypes in discrete spatial locations were also successfully fabricated inside microchannels.

A Laccase-Wiring Redox Hydrogel for Efficient Catalysis of O2 Electroreduction

Laccase was earlier wired to yield an O2 electroreduction catalyst greatly outperforming platinum and its alloys. Here we describe the design, synthesis optimization of the composition, and characterization of the +0.55 V (AgAgCl) laccase-wiring redox hydrogel, with an apparent electron diffusion coefficient (D(app)) of 7.6 x 10(-7) cm2 s(-1). The high D(app) results in the tethering of redox centers to the polymer backbone through eight-atom-long spacer arms, which facilitate collisional electron transfer between proximal redox centers. The O2 flux-limited, true-area-based current density was increased from the earlier reported 560 to 860 microA cm(-2). When the O2 diffusion to the 7-microm-diameter carbon fiber cathode was cylindrical, half of the O2 flux-limited current was reached already at 0.62 V and 90% at 0.56 V vs Ag/AgCl, merely -0.08 and -0.14 V versus the 0.7 V (Ag/AgCl) reversible O2/H2O half-cell potential at pH 5.

Glucose measurements with sensors and ultrasound

Accurate monitoring of the blood glucose level in diabetics is essential in preventing complications. Generally, conventional over-the-counter glucose meters require frequent painful finger punctures to obtain samples, which makes a noninvasive method preferable. The purpose of this study was to demonstrate that glucose levels can be measured transdermally with the combination of the low-profile cymbal array and an electrochemical glucose sensor consisting of amperometric electrodes and a novel glucose oxidase hydrogel. Interstitial fluid glucose concentrations can be determined with the electrochemical glucose sensor after the skin is made permeable to glucose by ultrasound (US) (20 kHz) with the thin (< 7 mm) and light (< 22 g) cymbal array. Using this array to deliver insulin into hyperglycemic rats, our previous experiments demonstrated that blood glucose levels would decrease 233.3 mg/dl with 5 min of US exposure. With the sensor and array, our goal was to determine the glucose levels of hyperglycemic rats noninvasively and evaluate the possible bioeffects. A total of 12 anesthetized rats were placed into two groups (US exposure group and control group) and the array (I(SPTP) = 100 mW/cm(2)) with a saline reservoir operating for 20 min was affixed to the abdomen. The array was removed and an electrochemical glucose sensor was placed on the exposed area to determine the glucose concentrations through the skin. Comparison was made using a commercial glucose meter with the blood collected from a jugular vein. The average blood glucose level determined by the sensor was 356.0 +/- 116.6 mg/dl, and the glucose level measured by the commercial glucose meter was 424.8 +/- 59.1 mg/dl. These results supported the use of this novel system consisting of the electrochemical glucose sensor and the cymbal array for glucose monitoring.

Direct Fabrication and Harvesting of Monodisperse, Shape-Specific Nanobiomaterials

A versatile "top-down" method for the fabrication of particles, Particle Replication In Nonwetting Templates (PRINT), is described which affords absolute control over particle size, shape, and composition. This technique is versatile and general enough to fabricate particles with a variety of chemical structures, yet delicate enough to be compatible with sophisticated biological agents. Using PRINT, we have fabricated monodisperse particles of poly(ethylene glycol diacrylate), triacrylate resin, poly(lactic acid), and poly(pyrrole). Monodisperse particle populations, ranging from sub-200 nm nanoparticles to complex micron-scale objects, have been fabricated and harvested. PRINT uses low-surface energy, chemically resistant fluoropolymers as molding materials, which eliminates the formation of a residual interconnecting film between molded objects. Until now, the presence of this film has largely prevented particle fabrication using soft lithography. Importantly, we have demonstrated that PRINT affords the simple, straightforward encapsulation of a variety of important bioactive agents, including proteins, DNA, and small-molecule therapeutics, which indicates that PRINT can be used to fabricate next-generation particulate drug-delivery agents.

In situ study of the thermoresponsive behavior of micropatterned hydrogel films by imaging ellipsometry

A patterned hydrogel was immobilized on a polymer substrate by low-pressure argon plasma treatment using a masking technique. The polymer sample showed a thermoresponsive aggregation behavior in the region of 35-37 degrees C. The micropatterned, thermoresponsive hydrogel film has been characterized with imaging ellipsometry. The characterization was carried out on the dry film as well as on a swollen sample in water. The thermoresponsive behavior was studied in deionized water by temperature-dependent measurements in a solid-liquid cell. Through imaging ellipsometry, it was possible to distinguish the different regions of interest on a micrometer scale and to follow the swelling of the hydrogel part as a function of the temperature. It was possible to visualize the swelling as 3D profiles of Delta at various temperatures. Long-term changes of the sample could also be detected, which cannot be picked up by conventional ellipsometry.

Photopolymerization of poly(ethylene glycol) diacrylate on eosin-functionalized surfaces

We describe a new method that allows photopolymerization of hydrogels to occur on surfaces functionalized with eosin. In this work, glass and silicon surfaces were derivatized with eosin and photopolymerization was carried out using visible light (514 nm). This mild condition may have advantages over methods that use ultraviolet (UV) light (e.g., for encapsulation of cells and proteins, in drug screening, or in biosensing applications). The hydrogel formed on the modified surface is remarkably stable for an extended period of time. The resultant hydrogel was hydrated for more than 18 months without suffering delamination from the substrate surface. This strongly suggests covalent attachment of the hydrogel to the surface. Contact angle titration measurements and X-ray photoelectron spectroscopy analysis of eosin surfaces before and after irradiation in the presence of triethanolamine suggest that the eosin radical is responsible for the covalent attachment of the gel onto the substrate surface. This method allows for 2-D patterning of hydrogels, which is demonstrated here using the microcontact printing technique. However, noncontact photolithography could be used to form similar patterns by directing light through a mask. This method can be easily implemented to form arrays of fluorophores and proteins in situ.

Rational Protein Modification Leading to Resistance of Enzymes to TiO2−UV Irradiation-Induced Inactivation

Photoexcited TiO2 degrades biomolecules such as nucleic acids, cell membrane proteins, and enzymes. Stabilization of enzyme activity against the deactivation caused by the combination of TiO2-UV is essential if we are to develop novel hybrid materials exhibiting photocatalytic and biocatalytic activities useful for decontamination applications. In this paper we describe the stabilization of a model enzyme, chymotrypsin, against TiO2-UV-induced deactivation by conjugating the enzyme with UV-absorbing, carboxyl-terminated oligo[2-[3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl]ethyl methacrylate] [oligo(HBMA)-COOH]. Chymotrypsin was completely deactivated within 3 h, whereas the chymotrypsin-oligo(HBMA) conjugate retained > 50% activity even after 5 h of exposure to TiO2-UV (lambdamax 365 nm). The degree of enzyme stabilization induced by the conjugated UV absorber was 2-fold higher than that from the equivalent number of conjugated PEG chains. Spectroscopic characterizations revealed that chymotrypsin-oligo(HBMA) absorbs UV light and initially resists photoexcitation of TiO2. Modified chymotrypsin also exhibited resistance to changes in the secondary structure during the deactivation. This method of stabilizing enzymes against photodegradation could be also useful in photolithographic enzyme immobilizations for sensors and arrays or for stabilization of any UV-sensitive protein.

Long tethers binding redox centers to polymer backbones enhance electron transport in enzyme "Wiring" hydrogels

A redox hydrogel with an apparent electron diffusion coefficient (D(app)) of (5.8 +/- 0.5) x 10(-)(6) cm(2) s(-)(1) is described. The order of magnitude increase in D(app) relative to previously studied redox hydrogels results from the tethering of redox centers to the backbone of the cross-linked redox polymer backbone through 13 atom spacer arms. The long and flexible tethers allow the redox centers to sweep electrons from large-volume elements and to collect electrons of glucose oxidase efficiently. The spacer arms make the collection of electrons from glucose oxidase so efficient that glucose is electrooxidized already at -0.36 V versus Ag/AgCl, the reversible potential of the redox potential of the FAD/FADH(2) centers of the enzyme at pH 7.2. The limiting current density of 1.15 mA cm(-)(2) is reached at a potential as low as -0.1 V versus Ag/AgCl. The novel redox center of the polymer is a tris-dialkylated N,N'-biimidazole Os(2+/3+) complex. Its redox potential, -0.195 V versus Ag/AgCl, is 0.8 V reducing relative to that of Os(bpy)(2+/3+), its 2,2'-bipyridine analogue.

Poly(ethylene glycol) hydrogel microstructures encapsulating living cells

We present an easy and effective method for the encapsulation of cells inside PEG-based hydrogel microstructures fabricated using photolithography. High-density arrays of three-dimensional microstructures were created on substrates using this method. Mammalian cells were encapsulated in cylindrical hydrogel microstructures of 600 and 50 micrometers in diameter or in cubic hydrogel structures in microfluidic channels. Reducing lateral dimension of the individual hydrogel microstructure to 50 micrometers allowed us to isolate 1-3 cells per microstructure. Viability assays demonstrated that cells remained viable inside these hydrogels after encapsulation for up to 7 days.