Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Introduction to physical polymer science

Structure–property relationships between material properties and stem cell behaviour are investigated using high-throughput methods. The data identify the optimal substrates within a range of different polymeric surfaces to support the growth and self-renewal of human embryonic stem cells from fully dissociated single cells.

Combinatorial Development of Biomaterials for Clonal Growth of Human Pluripotent Stem Cells

The electrospinning process was used successfully to fabricate nanofibers of poly(ethylene oxide) (PEO) in which multiwalled carbon nanotubes (MWCNT) are embedded. Initial dispersion of MWCNTs in water was achieved using amphiphiles, either as small molecules (sodium dodecyl sulfate, SDS) or as a high molecular weight, highly branched polymer (Gum Arabic). These dispersions provided separation of the MWCNTs and their individual incorporation into the PEO nanofibers by subsequent electrospinning. The focus of this work is on the development of axial orientations in these multicomponent nanofibers. A theoretical model is presented for the behavior of rodlike particles representing CNTs in electrospinning. Initially the rods are randomly oriented, but due to the sinklike flow in a wedge they are gradually oriented mainly along the stream lines, so that straight CNTs are almost oriented upon entering the electrospun jet. The degrees of orientation of polymer, surfactant, and MWCNT were studied using X-ray dif...

Carbon Nanotubes Embedded in Oriented Polymer Nanofibers by Electrospinning

The effect of nanosilica on mechanical, thermal and morphological properties of epoxy coating

Controlling alginate oxidation conditions for making alginate-gelatin hydrogels.

An investigation of dynamic mechanical, thermal, and electrical properties of housing materials for outdoor polymeric insulators

Improvement of ionic conductivity and performance of quasi-solid-state dye sensitized solar cell using PEO/PMMA gel electrolyte

Two series of polyurethane (PU), based on polycaprolactone (PCL) as soft segments with two different molecular weights (2000 and 530 Da), and hexamethylene diisocyanate (HDI) and 1,4-butandiol (BDO) as hard segments were synthesized to fabricate 
curcumin-loaded electrospun nanofibrous PCL-based PU substrate. Chemical structures of the synthesized PUs were characterized by FTIR and NMR spectroscopy techniques. The thermal properties were analyzed by differential scanning calorimetry (DSC) and surface hydrophilicity was studied by static contact angle and bulk hydrophilicity was evaluated by water uptake test. Thereafter, bead-free PU nanofiberous substrate containing curcumin was fabricated by electrospinning and morphology of the mats was observed by scanning electron microscopy (SEM). Mechanical properties of the electrospun mats in comparison with polymeric films were assessed by a universal test machine. The in vitro release of curcumin was studied by UV–Vis spectroscopy. The optical density of the bacterial solutions was used to evaluate the antibacterial activity of the curcumin-loaded nanofibrous mats against Escherichia coli (E-coli ATCC: 25922). The results showed that curcumin-loaded PU synthesized by PCL with molecular weight of 2000 Da displayed better mechanical properties as well as better antibacterial properties in wound dressing application.

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Morteza Ehsani

Papers

The effect of nanosilica on mechanical, thermal and morphological properties of epoxy coating

Controlling alginate oxidation conditions for making alginate-gelatin hydrogels.

An investigation of dynamic mechanical, thermal, and electrical properties of housing materials for outdoor polymeric insulators

Improvement of ionic conductivity and performance of quasi-solid-state dye sensitized solar cell using PEO/PMMA gel electrolyte

Fabrication of curcumin-loaded electrospun nanofiberous polyurethanes with anti-bacterial activity.