Kamila Gawel

Responsive Hydrogels for Label-Free Signal Transduction within Biosensors

Hydrogels have found wide application in biosensors due to their versatile nature. This family of materials is applied in biosensing either to increase the loading capacity compared to two-dimensional surfaces, or to support biospecific hydrogel swelling occurring subsequent to specific recognition of an analyte. This review focuses on various principles underpinning the design of biospecific hydrogels acting through various molecular mechanisms in transducing the recognition event of label-free analytes. Towards this end, we describe several promising hydrogel systems that when combined with the appropriate readout platform and quantitative approach could lead to future real-life applications.

Sol-Gel Synthesis of Non-Silica Monolithic Materials

Monolithic materials have become very popular because of various applications, especially within chromatography and catalysis. Large surface areas and multimodal porosities are great advantages for these applications. New sol-gel preparation methods utilizing phase separation or nanocasting have opened the possibility for preparing materials of other oxides than silica. In this review, we present different synthesis methods for inorganic, non-silica monolithic materials. Some examples of application of the materials are also included.

Toehold of dsDNA exchange affects the hydrogel swelling kinetics of a polymer–dsDNA hybrid hydrogel

Hydrogel-based label-free nucleic acids sensor materials using high resolution interferometric readout of hydrogel swelling changes was prepared and characterized with respect to molecular design parameters. The DNA-sensitive hydrogel comprised sensing (S) and blocking (B) oligonucleotide pairs copolymerized with the network and formed reversible crosslinks in addition to stable covalent ones. Oligonucleotide probes (P) complementary to S with longer complementary regions comparing to B results in competitive replacement of S–B strands. The associated destabilization of DNA crosslinks results in changes of the hydrogel swelling. S–B dioligonucleotidic crosslinkers were designed with 8, 12 and 16 basepairs in complementary regions and were destabilized by probes with matching sequences with 2, 6 and 10 excess basepairs in the so-called “toe-hold” region. The kinetics of hydrogel swelling was tested at room and body temperature for hydrogels with different covalent crosslink densities. In general, the swelling kinetics was faster for longer “toe-holds” and slower for the longer and more stable blocking sequence. An increase of the covalent crosslink density resulted in a decrease of the swelling rate. The swelling of the hydrogel was faster the higher the temperature and closer to the S–B melting point. The following work shows direct correlation between the kinetics of strand displacement reaction and hydrogel swelling rates and provides a direction for further application of DNA-sensitive hydrogels.

Logic swelling response of DNA–polymer hybrid hydrogel

An oligonucleotide–polymer hybrid hydrogel displaying swelling response in a logical AND and OR fashion depending on specific stimuli by probe oligonucleotides was prepared. The hydrogel acts as a transducer of the specific recognition of DNA sequences into micro- and macroscale mechanics. Input oligonucleotide probes induce hydrogel swelling ratio at two signal levels.

High resolution interferometry as a tool for characterization of swelling of weakly charged hydrogels subjected to amphiphile and cyclodextrin exposure.

A high resolution interferometric technique was used to determine swelling behavior of weakly charged polyacrylamide hydrogels in the presence of oppositely charged surfactants and subsequent exposure to cyclodextrins. Hydrogels of copolymerized acrylamide and 2-acrylamido-2-methyl-1-propanesulfonic acid (0.22, 0.44, 0.88 mol%) crosslinked with bisacrylamide (3, 6, 12 mol%) were employed. The equilibrium swelling and swelling kinetics of the hydrogels were determined with 2nm resolution of the optical length and sampled at approximately 1 Hz. These properties were determined for the hydrogels exposed to cationic surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) at concentrations from 10(-7) up to 2×10(-3)M. The distribution of surfactant within one AAM-co-AMPSA hydrogel equilibrated in CTAB/perylene solution was investigated by confocal laser scanning microscopy. Hydrogels equilibrated at selected surfactant concentrations were subsequently exposed to cyclodextrins (α-CD, β-CD, methyl-β-CD and γ-CD) forming inclusion complexes with the surfactants. The results show different types of behavior for the two surfactants used, arising from the difference in the length of surfactant hydrophobic tail. The changes in the surfactant induced swelling of the hydrogels are suggested to arise from the net effect of electrostatic screening of sulfonic acid-amide group interactions and surfactant micellization. Hydrogels with the largest charge density and the lowest crosslink density yielded the most pronounced changes in swelling properties on exposure to DTAB or CTAB. The hydrogels displayed swelling kinetics on stepwise changes in surfactant concentrations that depended on the surfactant concentration range. The high resolution monitoring of hydrogel swelling associated with supramolecular complex formation in three-component systems hydrogel-amphiphilic molecule-cyclodextrin provides more details on the swelling behavior than previously disclosed.

Swelling of a hemi-ellipsoidal ionic hydrogel for determination of material properties of deposited thin polymer films: an inverse finite element approach

Selective deposition of polymers at the surface of an ionic hydrogel is conventionally used to tailor properties of the composite material for application in for instance drug release and cell encapsulation. Here we describe a method for determination of the mechanical properties of a thin polymer film deposited on an ionic hydrogel core. The ionic strength-dependent hydrogel swelling is affected by the cross-link density and thickness of the deposited polymer layer. A hemi-ellipsoidal geometry of the hydrogel, corresponding to that employed in proof-of-concept experiments, is used to enforce biaxial deformation of the deposited layer when the ionic hydrogel core is equilibrated at various ionic strengths. The ionic strength dependent equilibrium swelling ratio of the hydrogel with the deposited polymer film is modeled using a finite element approach. The free energy of the hydrogel core includes contributions accounting for polymer mixing, elastic deformation of the network and the Donnan equilibrium. The latter type of contribution is not included in the neutral thin layer in the present study. Adding the polymer multilayer/shell at the surface reveals that the ionic strength-dependent swelling constraint is more pronounced the thicker and stiffer the film is. Combining thickness measurements of the polymer film with high resolution interferometric determination of reduction in swelling capacity of ionic hydrogels, an equivalent elastic property of the polymer layer is obtained using inverse finite element analysis. In the proof-of-concept experiments, analysis of data obtained for chitosan–alginate multilayers composed of four and eight polymer bilayers deposited on an anionic acrylamide-based hydrogel core suggests that these bilayers show an elastic stiffness one order of magnitude larger than that of the hydrogel core.

Swelling Dynamics of a DNA-Polymer Hybrid Hydrogel Prepared Using Polyethylene Glycol as a Porogen

DNA-polyacrylamide hybrid hydrogels designed with covalent and double-stranded (dsDNA) crosslinks respond to specific single-stranded DNA (ssDNA) probes by adapting new equilibrium swelling volume. The ssDNA probes need to be designed with a base pair sequence that is complementary to one of the strands in a dsDNA supported network junction. This work focuses on tuning the hydrogel swelling kinetics by introducing polyethylene glycol (PEG) as a pore-forming agent. Adding PEG during the preparation of hydrogels, followed by removal after polymerization, has been shown to improve the swelling dynamics of DNA hybrid hydrogels upon specific ssDNA probe recognition. The presence of porogen did not influence the kinetics of osmotic pressure-driven (2-acrylamido-2-methylpropane sulfonic acid)-co-acrylamide (AMPSA-co-AAm) hydrogels’ swelling, which is in contrast to the DNA-sensitive hydrogels. The difference in the effect of using PEG as a porogen in these two cases is discussed in view of processes leading to the swelling of the gels.