Hybrid Sol-gel Glass Research Articles

Sustained Calcium(II)-Release to Impart Bioactivity in Hybrid Glass Scaffolds for Bone Tissue Engineering.

In this study, we integrated different calcium sources into sol-gel hybrid glass scaffolds with the aim of producing implants with long-lasting calcium release while maintaining mechanical strength of the implant. Calcium(II)-release was used to introduce bioactivity to the material and eventually support implant integration into a bone tissue defect. Tetraethyl orthosilicate (TEOS) derived silica sols were cross-linked with an ethoxysilylated 4-armed macromer, pentaerythritol ethoxylate and processed into macroporous scaffolds with defined pore structure by indirect rapid prototyping. Triethyl phosphate (TEP) was shown to function as silica sol solvent. In a first approach, we investigated the integration of 1 to 10% CaCl2 in order to test the hypothesis that small CaCl2 amounts can be physically entrapped and slowly released from hybrid glass scaffolds. With 5 and 10% CaCl2 we observed an extensive burst release, whereas slightly improved release profiles were found for lower Calcium(II) contents. In contrast, introduction of melt-derived bioactive 45S5 glass microparticles (BG-MP) into the hybrid glass scaffolds as another Calcium(II) source led to an approximately linear release of Calcium(II) in Tris(hydroxymethyl)aminomethane (TRIS) buffer over 12 weeks. pH increase caused by BG-MP could be controlled by their amount integrated into the scaffolds. Compression strength remained unchanged compared to scaffolds without BG-MP. In cell culture medium as well as in simulated body fluid, we observed a rapid formation of a carbonated hydroxyapatite layer on BG-MP containing scaffolds. However, this mineral layer consumed the released Calcium(II) ions and prevented an additional increase in Calcium(II) concentration in the cell culture medium. Cell culture studies on the different scaffolds with osteoblast-like SaOS-2 cells as well as bone marrow derived mesenchymal stem cells (hMSC) did not show any advantages concerning osteogenic differentiation due to the integration of BG-MP into the scaffolds. Nonetheless, via the formation of a hydroxyapatite layer and the ability to control the pH increase, we speculate that implant integration in vivo and bone regeneration may benefit from this concept.

Effects of curing and organic content on bioactivity and mechanical properties of hybrid sol–gel glass scaffolds made by indirect rapid prototyping

We employed indirect rapid prototyping templating to fabricate bioactive and macroporous scaffolds for bone regeneration. This templating technique utilizes lost molds made of polycaprolactone by fused deposition modeling, in which the organic/ inorganic hybrid silica sol was filled and cured. Finally, the molds were dissolved and extracted, and the remaining macroporous hybrid glass constructs were recovered. The hybrid glass scaffolds offered a fully interconnected pore structure with 63–72% porosity measured by N2-pycnometry and Hg-intrusion. In bioactive sol–gel glasses one issue is the insufficient and inhomogeneous incorporation of calcium (II) ions. To address this problem we varied the curing conditions and tested the effect of the organic crosslinker on calcium retention. We strengthened the silica network by covalent crosslinking with trimethylolpropane ethoxylate which was functionalized with 3-(triethoxysilyl)propyl isocyanate. Those scaffolds showed compressive yield strengths of up to 12.7 MPa and compressive moduli between 18 and 288 MPa. Energy dispersive X-ray spectroscopy showed that a crosslinker content of 60% in the hybrids resulted in a homogeneous calcium distribution in the glass, in contrast to 40% where we found a layer of CaCl2 on the scaffold surface. The materials exhibited bioactivity in simulated body fluid which was monitored by scanning electron microscopy and X-ray powder diffraction.

Indirect rapid prototyping of sol-gel hybrid glass scaffolds for bone regeneration – Effects of organic crosslinker valence, content and molecular weight on mechanical properties

We present a series of organic/inorganic hybrid sol-gel derived glasses, made from a tetraethoxysilane-derived silica sol (100% SiO2) and oligovalent organic crosslinkers functionalized with 3-isocyanatopropyltriethoxysilane. The material was susceptible to heat sterilization. The hybrids were processed into pore-interconnected scaffolds by an indirect rapid prototyping method, described here for the first time for sol-gel glass materials. A large panel of polyethylene oxide-derived 2- to 4-armed crosslinkers of molecular weights ranging between 170 and 8000Da were incorporated and their effect on scaffold mechanical properties was investigated. By multiple linear regression, ‘organic content’ and the ‘content of ethylene oxide units in the hybrid’ were identified as the main factors that determined compressive strength and modulus, respectively. In general, 3- and 4-armed crosslinkers performed better than linear molecules. Compression tests and cell culture experiments with osteoblast-like SaOS-2 cells showed that macroporous scaffolds can be produced with compressive strengths of up to 33±2MPa and with a pore structure that allows cells to grow deep into the scaffolds and form mineral deposits. Compressive moduli between 27±7MPa and 568±98MPa were obtained depending on the hybrid composition and problems associated with the inherent brittleness of sol-gel glass materials could be overcome. SaOS-2 cells showed cytocompatibility on hybrid glass scaffolds and mineral accumulation started as early as day 7. On day 14, we also found mineral accumulation on control hybrid glass scaffolds without cells, indicating a positive effect of the hybrid glass on mineral accumulation. Statement of SignificanceWe produced a hybrid sol-gel glass material with significantly improved mechanical properties towards an application in bone regeneration and processed the material into macroporous scaffolds of controlled architecture by indirect rapid prototyping. We were able to produce macroporous materials of relevant porosity and pore size with compressive moduli, covering the range reported for cancellous bone while an even higher compressive strength was maintained. By multiple linear regression, we identified crosslinker parameters, namely organic content and the content of ethylene oxide units in the hybrids that predominantly determined the mechanics of the hybrid materials. The scaffolds proved to be cytocompatible and induced mineralization in SaOS-2 cells. This provides new insight on the critical parameters for the design of the organic components of covalent hybrid sol-gel glasses.

Photoluminescence of sol–gel synthesized transparent amorphous semiconducting La- and Sm-codoped organo-silicate hybrid material

Transparent amorphous La- and Sm-codoped organo-silicate hybrid material has been synthesized, in the form of bulk samples, via an alkoxide sol–gel route. Thermal behavior of the synthesized glass was investigated using differential thermal analysis (DTA) coupled with thermogravimetry (TGA). From the DTA–TGA analyses, it can be concluded that the organic component constitutes ≈8 weight percentage (wt%) of the as prepared La–Sm-codoped hybrid organo-silica glass; and this glass contains about 29.3 wt% structural water in the form of hydroxyl groups. Fourier transform infrared (FTIR) and ultraviolet–visible-near infrared (UV–Vis-NIR) spectroscopic analyses were used for structural characterization of the prepared hybrid sol–gel glass. FTIR spectroscopy reveals functional groups, specifically –CH2, C–O–C, Si–O–C and Si–O–Si bonds, which indicate the formation of covalent bonds between the organic and inorganic components in the produced polyethylene glycol (PEG)-silica hybrid network. UV–Vis-NIR spectrum exhibits well defined absorption bands of Sm3+ ions due to 4f–4f transitions. The evolution of Sm3+ photoluminescence in the La–Sm-codoped hybrid organo-silica glass was investigated at room temperature. A strong reddish-orange emission, attributed to the 4G5/2 → 6H7/2 electronic transition in 4f5 configuration of Sm3+ ions, was observed. Semiconducting characteristics have been also reported and interpreted.

Functional nanostructured surfaces in hybrid sol–gel glass in large area for antireflective and super-hydrophobic purposes

We demonstrate the modification of substrate surfaces by the fabrication of thin layer sub-wavelength nanostructures in sol–gel glass in large area on their surfaces. Compared with the traditional methods, the sol–gel method eliminates the etching step and therefore becomes more convenient. It is found that 90% of the height of nanostructures in the master can be transferred into the hybrid SiO2–TiO2 sol–gel glass in our experiment, while the horizontal size of the nanostructures can be replicated precisely. The optical characterization results show that the light reflectivity can be reduced by about 1% over a wide waveband ranging from 400 nm to 800 nm for ARS with a height of 32.4 nm. In addition, it is found that the contact angle of the substrate surface with sol–gel nanostructures has been significantly improved from about 47° to 108°. Therefore, it can be concluded that with a thin layer sol–gel glass sub-wavelength nanostructures, the substrate surface can be effectively modified to have the function of both anti-reflectivity and super hydrophobicity, which can find applications in the area of photovoltaic concentrating lenses for enhancing light transmission, antifogging purposes and others.

Lithographic fabrication of diffractive optical elements in hybrid sol-gel glass on 3-D curved surfaces

We demonstrate the lithographic fabrication of diffractive optical elements (DOEs) in hybrid SiO2/TiO2 sol-gel glass on 3-D curved surfaces. The concentric circular gratings with periods of 20 μm, 10 μm and 5 μm have been fabricated in sol-gel glass on concave lens by laser direct writing successfully. Continuous 3-Dimensional surface relief with a height of 435 nm, 110 nm and 50 nm has been obtained for the period of 20 μm, 10 μm and 5 μm respectively. The optical test results of the fabricated DOE shows only a little bit deviation from the theoretical calculated results which can be explained by 3-D curved surface effect. We believe this technology can be an effective method to fabricate DOEs with even more complex surface profile on 3-D curved surfaces in terms of its simplicity and cost-effectiveness.

Studies of the pumping effect on the nanoporous microstructure of disordered mesoporous silica materials prepared by calcinations of PMMA-silica hybrids

Abstract We present the first comparative studies of pumping effect (i.e., exhaustive vacuum evacuation) on the as-prepared nanoporous microstructure silica to elucidate the effect of different preparative procedure on a series of obtained disordered mesoporous silica materials. The as-prepared hybrid materials (PMMA-silica) through the pumping treatment exhibited higher thermal decomposition temperature ( T d ) and density ( D ) than these system without pumping treatment based on the studies of thermogravimetric analysis and density measurements and compared with the nanoporous materials by calcinations of PMMA-silica materials was found to exhibit a significant higher value of surface area ( A BET ) based on N 2 adsorption–desorption isotherm. Furthermore, a marked decrease in A BET and V BJH of as-prepared mesoporous silica are observed as the feeding content of TEOS is increased. The morphological images of as-prepared hybrid sol–gel glasses and their corresponding disordered mesoporous silica materials were also studied by scanning electron microscopy and transmission electron microscopy.

Low-cost lithographic fabrication of relief patterns in a SiO2-TiO2 hybrid Sol-Gel glass on curved surfaces

We describe a low-cost method of fabricating optical patterns in a hybrid sol-gel glass on curved surfaces by means of soft lithography. The ion etching step, which is presently difficult to realize on curved surfaces, is avoided in this process. Using the proposed soft lithography technique, it is possible to transfer high-quality patterns to a hybrid sol-gel glass on curved surfaces. This technique offers a low-cost and simple method for obtaining optical glass structures on such surfaces. The use of a sol-gel glass (replacing an epoxy) layer greatly prolongs the life of replica gratings, especially in the vacuum ultraviolet applications.

Removal of ghost images by using tilted element optical systems with polynomial surfaces for aberration compensation

A novel solution to problematic ghost images is implemented by using tilted lens elements with polynomial surfaces. Tilting the lens surfaces sends reflections out of the imaging path. The nonrotationally symmetric polynomial surfaces correct aberrations caused by tilts. The complex lens surfaces are fabricated by using gray-scale lithographic patterning of hybrid solgel glass.

Photopatterning hybrid sol–gel glass materials prepared from ethylene tellurate and alkoxysilane

Photopatterning of sol–gels is a versatile technique that has been used to prepare optical components and devices of silica and titania. Telluria (TeO 2) has a higher third-order non-linearity than silica and titania and offers advantages in optical devices. Ethylene tellurate was used as a high purity precursor in the preparation of nanoparticulate tellurium dioxide powders and glassy films by sol–gel processes. Powder and film samples were characterized by XRD, SEM and FTIR. The refractive index of the sol–gel derived telluria film was 1.67. [3-(Methacryloloxypropyl)]trimethoxysilane (MPTMS) was incorporated into ethylene tellurate sol–gels in order to allow photopatterning by polymerization and cross-linking of methacrylate functionality. UV exposure of films with a photomask was used to produce line patterns. FTIR measurements showed the disappearance of acrylate double bonds in MPTMS, indicating the polymerization and condensation of the films.

Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO2–TiO2 sol-gel glass

This paper describes a low-cost soft-lithography method for building large-numerical-aperture microlenses in hybrid sol-gel glass. The fabrication processes comprise three steps, namely fabrication of large numerical aperture microlens array in photoresist as a master, replication of the master in poly-dimethylsiloxane (PDMS) as elastomeric molds and reproduction of the PDMS replica onto the hybrid sol-gel glass as an end structure. Comparing with the direct UV fabrication in the photosensitive sol-gel glass, the proposed method provides a unique advantage in terms of fabrication freedom in shape and thickness of micro-optical elements in the sol-gel glass. This eliminates the difficulties in direct photolithography in sol-gel glass for arbitrary shapes and great thickness due to ever increasing demands.

Study of the luminescence of tris(2-thenoyltrifluoroacetonato)lanthanide(III) complexes covalently linked to 1,10-phenanthroline-functionalized hybrid sol–gel glasses

The solubility and uniform distribution of lanthanide complexes in sol–gel glasses can be improved by covalently linking the complexes to the sol–gel matrix. In this study, several lanthanide β -diketonate complexes (Ln=Nd, Sm, Eu, Tb, Er, Yb) were immobilized on a 1,10-phenanthroline functionalized sol–gel glass. For the europium(III) complex, a sol–gel material of diethoxydimethylsilane (DEDMS) with polymer-like properties was derived. For the other lanthanide complexes, the sol–gel glass was prepared by using a matrix of tetramethoxysilane (TMOS) and DEDMS. Both systems were prepared under neutral reaction conditions. High-resolution emission and excitation spectra were recorded. The luminescence lifetimes were measured.

Photopatterning of hybrid sol–gel glasses: generation of volume phase gratings under visible light

Free-radical photocurable hybrid sol–gel was used as a matrix precursor for the creation of crack-free holographic materials filling 100 μm thick glass cells. It combines the advantages of a glass-like material obtained under mild conditions and a spatially controlled photopatterning allowing the generation of optical elements with a few steps number. Hydrolysis and condensation reactions were first performed and investigated by 29Si NMR and FTIR spectroscopies. The presented results concern the generation of 1000 lines/mm gratings created using interference pattern at 514 nm. Diffraction efficiencies up to 90% are reached. The strategy used to avoid the cracks propagation is also described.

Sample-inverted reflow technique for fabrication of a revolved-hyperboloid microlens array in hybrid solgel glass.

We propose a novel fabrication method, which is referred to as the sample-inverted reflow technique, to fabricate a refractive microlens array (MLA) with a revolved-hyperboloid profile in a solgel material. The fabricated solgel MLA demonstrates an excellent smooth profile with a fabrication error much less than the difference between the revolved hyperboloid and the spherical surface. In an application of coupling a laser diode (LD) to a single-mode fiber (SMF), we propose a two-MLA coupling scheme in which two revolved-hyperboloid MLAs are used between the LD and the SMF. In this configuration the coupling efficiency achieves 81.7% (-0.88 dB).

Localized self-volume growth in hybrid sol-gel glass induced by ultraviolet radiation with a gray-scale mask.

UV-induced self-volume growth of the hybrid sol-gel glass was investigated for fabrication of an arbitrary-surface relief structure with a user-defined gray scale mask. The gray scale mask was generated through exposure of a high-energy beam-sensitive glass under an electron beam. Saw-tooth gratings of different periods were fabricated by a simple UV exposure through the gray-scale mask. Experimental results showed that the smaller the period is, the shallower the depth obtained owing to the self-volume growth effect is.

Incorporation of a highly luminescent semiconductor quantum dot in ZrO2–SiO2hybrid sol–gel glass film

In this paper we describe a method for transferring semiconductor quantum dots, produced in non-polar solvents by an organometallic approach, into sol–gel matrices. ZrO2–SiO2 hybrid sol–gel glass films have been homogeneously doped with different semiconductor quantum dots (CdSe, CdSe@CdS and CdSe@ZnS). Both the absorption and the emission properties of the semiconductor nanocrystals are only slightly affected by the incorporation into the sol–gel matrix. The doped films showed sufficiently high refractive index for the realization of planar waveguides.

Realization of refractive microoptics through grayscale lithographic patterning of photosensitive hybrid glass

Refractive microlenses with more than 50 microm sag are fabricated using grayscale lithography. Mechanical assembly features are made simultaneously alongside the microlenses to facilitate high precision assembly of miniature optical systems. The microlens elements are formed using lithographic patterning of photosensitive hybrid sol-gel glass requiring no etch transfer to the substrate material. Grayscale lithography enables the straightforward patterning of aspheric lenses and arbitrary surfaces within the material depth. Lessons learned in the design of a grayscale photomask are described. Characterization of the fabricated lens elements is reported including lens shape, surface quality, and image quality of a complete assembled imaging system.

Fabrication of multilevel structures in self-development photosensitive hybrid sol-gel glass by a gray scale mask

To extend the application of self-development sol-gel glass, we report a simple method to fabricate arbitrary multilevel structures in self-development photosensitive hybrid sol-gel glass by using a high-energy beam-sensitive gray scale mask under UV radiation. Characterization results showed that the maximum depth of deformation induced by the UV light can be more than 1.3 μm, which makes it possible to fabricate diffractive optical elements with a 2π phase delay for visible light. As an example, the self-development sol-gel glass was used for fabrication of blazed gratings with excellent surface smoothness.

UV induced controllable volume growth in hybrid sol-gel glass for fabrication of a refractive microlens by use of a grayscale mask

We report a simple method for forming surface corrugation of microlenses based on a volume growth effect of sol-gel glass. The volume growth effect was fully calibrated with a high-energy beam-sensitive grayscale mask, where the amount of growth was determined as a function of electron-beam-induced optical densities in the mask. This technique provides an excellent surface smoothness compared with other development based fabrication techniques. For example, a refractive microlens with a diameter of 140 microm and a sag height of 2.13 microm was demonstrated.

Volume growth initiated by point-to-point ultraviolet-laser direct writing in hybrid solgel glass for three-dimensional microfabrication.

Volume-growth solgel glass acting as self-developing material is used for three-dimensional (3D) fabrication of micro-optical elements. The solgel glass is exposed to a focused laser beam (lambda = 325 nm), and surface corrugation is created as a result of surface expansion in the exposed area. The profile depth is related to the laser beam intensity, which is controlled by an acousto-optic modulator, which makes it possible to fabricate 3D structures in the material. Preliminary results show that the controllable volume-growth effect will be of considerable use for production of 3D micro-optical elements with excellent surface smoothness in solgel materials.