S Khoo - Academia.edu (original) (raw)

Papers by S Khoo

The contractile force developed by fibroblasts has been studied by measuring the macroscopic cont... more The contractile force developed by fibroblasts has been studied by measuring the macroscopic contraction of porous collagen-GAG matrices over time. We have identified the microscopic deformations developed by individual fibroblasts which lead to the observed macroscopic matrix contraction. Observation of live cells attached to the matrix revealed that matrix deformation occurred as a result of cell elongation. The time dependence of the increase in average fibroblast aspect ratio over time corresponded with macroscopic matrix contraction, further linking cell elongation and matrix contraction. The time dependence of average fibroblast aspect ratio and macroscopic matrix contraction was found to be the result of the stochastic nature of cell elongation initiation and of the time required for cells to reach a final morphology (2-4 h). The proposed micromechanics associated with observed buckling or bending of individual struts of the matrix by cells may, in part, explain the observation of a force plateau during macroscopic contraction. These findings indicate that the macroscopic matrix contraction measured immediately following cell attachment is related to the extracellular force necessary to support cell elongation, and that macroscopic time dependence is not directly related to microscopic deformation events.

Nanomedicine, Jun 1, 2016

We aim to develop transparent UV-blocking photochromic soft contact lenses via polymerization of ... more We aim to develop transparent UV-blocking photochromic soft contact lenses via polymerization of a bicontinuous nanoemulsion. Materials & methods: Transparent nanostructured polymers were prepared by incorporating a polymerizable surfactant and thermal initiator together with water, monomers, UV blockers and photochromic dyes. The polymers were characterized using oxygen permeometer, tensile tester, electron microscope, UV spectrophotometer, corneal cell culture and testing in rabbits. Results: The polymers have good oxygen permeability, water content, stiffness, strength and UV-blocking ability comparable to commercial UV-blocking soft contact lenses. Their response to UV light is comparable to photochromic spectacle lenses, particularly in reverse transition from colored to colorless state. They are nontoxic and nonleaching. Conclusion: Our photochromic UV-blocking contact lenses provide a novel alternative to photochromic spectacles.

Nature Nanotechnology, Sep 28, 2008

Biomaterials, Feb 1, 2004

Highly porous, type I collagen-chondroitin-6-sulfate (collagen-GAG) scaffolds, produced by freeze... more Highly porous, type I collagen-chondroitin-6-sulfate (collagen-GAG) scaffolds, produced by freeze-drying techniques, have proven to be of value as implants to facilitate the regeneration of certain tissues. The objective of this project was to evaluate changes in the microstructure and mechanical properties of selected collagen-GAG scaffolds as they degrade in an in vitro model system. Environmental scanning electron microscopy and video imaging demonstrated that collagenase degradation caused strut erosion through the creation of 1-3 mm diameter micropits within a 2-h period, leading to eventual removal of strut material and strut breakage. Loss of microstructural topography may have been due to gelatinization when collagen was cleaved by collagenase. Chondroitinase degradation of GAG resulted in swelling of the struts, causing the pores to become smaller and rounder. The compressive modulus of the collagen-GAG matrix decreased when degraded by collagenase, but remained unchanged when degraded by chondroitinase. Carbodiimide-cross-linked matrices were found to have a higher cross-link density, a higher compressive stiffness and a greater resistance to collagenase and chondroitinase, compared to non-cross-linked controls and matrices that were cross-linked by the dehydrothermal process. This investigation provides information that can be used to design collagen-GAG scaffolds with desired compressive stiffness and degradation rate to collagenase and chondroitinase.

Biomaterials, Feb 1, 2002

The lack of exotherm during setting, absence of monomer and improved release of incorporated ther... more The lack of exotherm during setting, absence of monomer and improved release of incorporated therapeutic agents has resulted in the development of glass ionomer cements (GICs) for biomedical applications. In order to improve biocompatibility and biomechanically match GICs to bone, hydroxyapatite-ionomer (HAIonomer) hybrid cements were developed. Ultra-fine hydroxyapatite (HA) powders were produced using a new induction spraying technique that utilizes a radio-frequency source to spheriodize an atomized suspension containing HA crystallites. The spheriodized particulates were then held at 800 degrees C for 4 h in a carbolite furnace using a heating and cooling rate of 25 degrees C/min to obtain almost fully crystalline HA powders. The heat-treated particles were characterized and introduced into a commercial glass ionomer cement. 4 (H4), 12 (H12) and 28 (H28) vol% of fluoroalumino silicate were substituted by crystalline HA particles that were dispersed using a high-speed dispersion technique. The HAIonomer cements were subjected to hardness, compressive and diametral tensile strength testing based upon BS6039:1981. The storage time were extended to one week to investigate the effects of cement maturation on mechanical properties. Commercially available capsulated GIC (GC) and GIC at maximum powder:liquid ratio (GM) served as comparisons. Results were analyzed using factorial ANOVA/Scheffe's post-hoc tests and independent samples t-test at significance level 0.05. The effect of time on hardness was material dependent. With the exception of H12, a significant increase in hardness was observed for all materials at one week. A significant increase in compressive strength was, however, observed for H12 over time. At 1 day and 1 week, the hardness of H28 was significantly lower than for GM, H4, and H12. No significant difference in compression and diametral tensile strengths were observed between materials at both time intervals. Results show that HAIonomers is a promising material, which possess good mechanical properties. Potential uses of this new material include bone cements and performed implants for hard tissue replacement in the field of otological, oral-maxillofacial and orthopedic surgery.

Frontiers in Microbiology, Jul 25, 2022

and Rice SA (2022) Bacterial biofilm colonization and succession in tropical marine waters are si... more and Rice SA (2022) Bacterial biofilm colonization and succession in tropical marine waters are similar across different types of stone materials used in seawall construction.

Journal of Materials Science: Materials in Medicine, Jul 9, 2011

Chemotropic proteins guide neuronal projections to their final target during embryo development a... more Chemotropic proteins guide neuronal projections to their final target during embryo development and are useful to guide axons of neurons used in transplantation therapies. Site-specific delivery of the proteins however is needed for their application in the brain to avoid degradation and pleiotropic affects. In the present study we report the use of Poly (ethylene glycol)-Silica (PEG-Si) nanocomposite gel with thixotropic properties that make it injectable and suitable for delivery of the chemotropic protein semaphorin 3A. PEG-Si gel forms a functional gradient of semaphorin that enhances axon outgrowth of dopaminergic neurons from rat embryos or differentiated from stem cells in culture. It is not cytotoxic and its properties allowed its injection into the striatum without inflammatory response in the short term. Long term implantation however led to an increase in macrophages and glial cells. The inflammatory response could have resulted from non-degraded silica particles, as observed in biodegradation assays.

Biomaterials, Nov 1, 2008

We have created a porous bioresorbable nanocomposite bone scaffold that chemically, structurally ... more We have created a porous bioresorbable nanocomposite bone scaffold that chemically, structurally and mechanically matched natural bone so that it could be recognized and remodeled by natural bone. Containing collagen fibers and synthetic apatite nanocrystals, our scaffold has high strength for supporting the surrounding tissue. The foam-like scaffold has a similar microstructure as trabecular bone, with nanometer-sized and micron-sized pores. The apatitic phase of the scaffold exhibited similar chemical composition, crystalline phase and grain size as the trabecular bone apatite. The nanocomposite scaffold demonstrated excellent bioactivity for promoting cell attachment and proliferation. It was osteoconductive and successfully healed a non-union fracture in rat femur as well as a critical-sized defect in pig tibia.

Biomaterials, Feb 1, 2009

We have developed an injectable bone cement composed of nanocrystalline apatite and crosslinked h... more We have developed an injectable bone cement composed of nanocrystalline apatite and crosslinked hyaluronic acid-tyramine conjugates (HA-Tyr). This bone cement was formed via the oxidative coupling of tyramine moieties catalyzed by hydrogen peroxide (H 2 O 2) and horseradish peroxidise (HRP). The bone cement set within 60 s after H 2 O 2 and HRP were added to the apatite/HA-Tyr pastes. The mechanical strength of the apatite/HA-Tyr cement was tuned by varying the apatite loading and H 2 O 2 concentration. This rapid enzyme-mediated setting of our bone cement results in minimal heat release (DH ¼ À11.39 J/g) as compared to conventional bone cements. The crystalline phase and crystallite size (20 nm) of the apatitic phase in our bone cement matched that of trabecular bone. The storage modulus (G 0), yield stress (s y), and compressive stiffness (E c) of our bone cement prepared with different apatite loadings and H 2 O 2 concentrations were measured, and optimized at G 0 ¼ 40 MPa, s y ¼ 0.308 MPa and E c ¼ 2.270 MPa when the cement was formed with 0.4 g/ml of apatite, 0.61 units/ml of HRP and 6.8 mM of H 2 O 2. Our biocompatible bone cement also successfully healed small bone and joint defects in mice within 8 weeks.

Biomaterials, 2010

Recent studies have demonstrated the effect of matrix stiffness on the phenotype and differentiat... more Recent studies have demonstrated the effect of matrix stiffness on the phenotype and differentiation pathway of mesenchymal stem cells (MSCs). MSCs differentiated into neural, myogenic or osteogenic phenotypes depending on whether they were cultured on two-dimensional (2D) substrates of elastic moduli in the lower (0.1-1 kPa), intermediate (8-17 kPa) or higher ranges (34 kPa). In this study, MSCs were cultured in thixotropic gels of varying rheological properties, and similar results were found for the three-dimensional (3D) culture as for the previous findings in 2D culture. For the 3D cell cultures in thixotropic gels, the liquefaction stress (s y), the minimum shear stress required to liquefy the gel, was used to characterize the matrix stiffness. The highest expressions of neural (ENO2), myogenic (MYOG) and osteogenic (Runx2, OC) transcription factors were obtained for gels with s y of 7, 25 and 75 Pa, respectively. Immobilization of the cell-adhesion peptide, RGD, promoted both proliferation and differentiation of MSCs, especially for the case of the stiffer gels (>75 Pa). This study demonstrated the usefulness of thixotropic gels for 3D cell culture studies, as well as the use of s y as an effective measure of matrix stiffness that could be correlated to MSC differentiation.

Ecological Engineering, Mar 1, 2022

Experimental Cell Research, Sep 1, 2001

Acta Biomaterialia, Nov 1, 2011

The extracellular matrix (ECM) provides both molecular and physical cues that affect cell phenoty... more The extracellular matrix (ECM) provides both molecular and physical cues that affect cell phenotype and function, and hence both normal and abnormal tissue development. The evolution of ECM mechanical properties with time is thus of great importance for cell culture. We have developed a thixotropic polymer-silica nanocomposite gel that may be used to characterize the ECM secretions of cells cultured in a three-dimensional environment. This thixotropic gel can be characterized by its liquefaction stress (s y), defined as the minimum shear stress required to liquefy the gel. When cells were cultivated within these gels, s y was observed to change in a predictable fashion according to the cell type and quantity of ECM secreted. A general trend was observed for different cell lines, where s y increased with increasing ECM concentration when cells were cultivated in gels of lower stiffness, and decreased with increasing ECM concentration when cells were cultivated in gels of higher stiffness. A particular gel stiffness could be identified where increasing the ECM concentration did not change the s y of the gels. We have defined the s y at this point as s ECM , which is characteristic for the ECM produced by a specific cell type cultured in the thixotropic gel.

Frontiers in Microbiology

Seawalls are important in protecting coastlines from currents, erosion, sea-level rise, and flood... more Seawalls are important in protecting coastlines from currents, erosion, sea-level rise, and flooding. They are, however, associated with reduced biodiversity, due to their steep orientation, lack of microhabitats, and the materials used in their construction. Hence, there is considerable interest in modifying seawalls to enhance the settlement and diversity of marine organisms, as microbial biofilms play a critical role facilitating algal and invertebrate colonization. We assessed how different stone materials, ranging from aluminosilicates to limestone and concrete, affect biofilm formation. Metagenomic assessment of marine microbial communities indicated no significant impact of material on microbial diversity, irrespective of the diverse surface chemistry and topography. Based on KEGG pathway analysis, surface properties appeared to influence the community composition and function during the initial stages of biofilm development, but this effect disappeared by Day 31. We conclude...

Ecological Engineering, 2022

L'invention concerne un composite de matiere de charge biologique poreuse comprenant une mati... more L'invention concerne un composite de matiere de charge biologique poreuse comprenant une matiere de charge, notamment du collagene, intercale avec une matiere de charge de type phosphate de calcium. La porosite du composite est similaire a celle des eaux naturelles et peut caracteriser une dimension de pore comprise entre quelques nanometres jusqu'a plus de 100 microns. Des ossatures preparees a partir du composite de matiere de charge biologique conviennent a des matieres de substitution d'os resorbables.

La presente invention concerne un procede de fabrication d'un gel comprenant la combinaison d... more La presente invention concerne un procede de fabrication d'un gel comprenant la combinaison d'une espece silanol comprenant au moins deux groupes silanol par molecule et une espece hydroxyle hydrophile comprenant au moins deux groupes hydroxyle par molecule. Le gel est susceptible d'etre converti en un liquide par l'application d'une force de cisaillement mecanique et le liquide est susceptible d'etre converti en gel en l'absence de ladite force de cisaillement mecanique.