ORCID Profile
0000-0003-3761-7517
Current Organisation
Nanyang Technological University
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Publisher: American Chemical Society (ACS)
Date: 19-10-2009
DOI: 10.1021/JP9076883
Publisher: American Chemical Society (ACS)
Date: 02-05-2012
DOI: 10.1021/JP211562P
Publisher: Springer Science and Business Media LLC
Date: 13-01-2009
Publisher: Wiley
Date: 07-02-2017
Abstract: A facile method has been developed for the large-scale synthesis of random copolypeptides composed of multiple (i.e., cationic, hydrophobic, and hydrophilic) amino acids and their relative ratios have been optimized for broad-spectrum antibacterial effect. The copolypeptides obtained have measured compositions close to the design ratios in spite of the differing reactivities of the different amino acids. An optimized random copolypeptide of lysine, leucine, and serine (denoted as KLS-3) mimicking the composition of LL-37 host defense peptide gives broad spectrum antibacterial activity against clinically relevant Gram-negative and Gram-positive bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa (PAO1) with minimum inhibitory concentrations (MICs) of 32-64 μg mL
Publisher: Wiley
Date: 28-12-2009
Abstract: Heparin sodium salt is investigated as a dispersant for dispersing single-walled carbon nanotubes (SWNTs). Photoluminescence excitation (PLE) spectroscopy is used for identification and abundance estimation of the chiral species. It is found that heparin sodium salt preferentially disperses larger-diameter Hipco SWNTs. When used to disperse CoMoCAT nanotube s les, heparin has a strong preference for (8,4) tubes, which have larger diameter than the predominant (6,5) in pristine CoMoCAT s les. PLE intensity due to (8,4) tubes increases from 7% to 60% of the total after threefold extractions. Computer modeling verifies that the complex of (8,4) SWNTs and heparin has the lowest binding energy amongst the four semiconducting species present in CoMoCAT. Network field-effect transistors are successfully made with CoMoCAT/heparin and CoMoCAT/sodium dodecylbenzene sulfonate (SDBS)-heparin (x3), confirming the easy removability of heparin.
Publisher: Elsevier BV
Date: 06-2004
Publisher: American Chemical Society (ACS)
Date: 19-04-2008
DOI: 10.1021/JP711039S
Publisher: American Chemical Society (ACS)
Date: 31-10-2006
DOI: 10.1021/LA061334Q
Abstract: As-fabricated deep reactive ion etched (DRIE) silicon mold with very high aspect ratio (>10) feature patterns is unsuitable for poly(dimethylsiloxane) (PDMS) replication because of the strong interaction between the Si surface and the replica and the corrugated mold sidewalls. The silicon mold can be conveniently passivated via plasma polymerization of octafluorocyclobutane (C4F8), which is also employed in the DRIE process itself, to enable the mold to be used repeatedly. To optimize the passivation conditions, we have undertaken a Box-Behnken experimental design on the basis of three passivation process parameters (plasma power, C4F8 flow rate, and deposition time). The measured responses were fluorinated film thickness, demolding status/success, demolding force, and fluorine/carbon ratio on the fifth replica surface. The optimal passivation process conditions were predicted to be an input power of 195 W, a C4F8 flow rate of 57 sccm, and a deposition time of 364 s these were verified experimentally to have high accuracy. Demolding success requires medium-deposited film thickness (66-91 nm), and the thickness of the deposited films correlated strongly with deposition time. At moderate to high ranges, increased plasma power or gas flow rate promoted polymerization over reactive etching of the film. It was also found that small quantities of the fluorinated surface were transferred from the Si mold to the PDMS at each replication, entailing progressive wear of the fluorinated layer.
Publisher: Elsevier BV
Date: 04-2014
DOI: 10.1016/J.BIOMATERIALS.2014.01.081
Abstract: Large-scale expansion of highly functional adult human mesenchymal stem cells (aMSCs) remains technologically challenging as aMSCs lose self renewal capacity and multipotency during traditional long-term culture and their quality/quantity declines with donor age and disease. Identification of culture conditions enabling prolonged expansion and rejuvenation would have dramatic impact in regenerative medicine. aMSC-derived decellularized extracellular matrix (ECM) has been shown to provide such microenvironment which promotes MSC self renewal and "stemness". Since previous studies have demonstrated superior proliferation and osteogenic potential of human fetal MSCs (fMSCs), we hypothesize that their ECM may promote expansion of clinically relevant aMSCs. We demonstrated that aMSCs were more proliferative (∼ 1.6 ×) on fMSC-derived ECM than aMSC-derived ECMs and traditional tissue culture wares (TCPS). These aMSCs were smaller and more uniform in size (median ± interquartile range: 15.5 ± 4.1 μm versus 17.2 ± 5.0 μm and 15.5 ± 4.1 μm for aMSC ECM and TCPS respectively), exhibited the necessary biomarker signatures, and stained positive for osteogenic, adipogenic and chondrogenic expressions indications that they maintained multipotency during culture. Furthermore, fMSC ECM improved the proliferation (∼ 2.2 ×), size (19.6 ± 11.9 μm vs 30.2 ± 14.5 μm) and differentiation potential in late-passaged aMSCs compared to TCPS. In conclusion, we have established fMSC ECM as a promising cell culture platform for ex vivo expansion of aMSCs.
Publisher: American Chemical Society (ACS)
Date: 09-02-2016
DOI: 10.1021/ACS.BIOMAC.5B01766
Abstract: The development of novel reagents and antibiotics for combating multidrug resistance bacteria has received significant attention in recent years. In this study, new antimicrobial star polymers (14-26 nm in diameter) that consist of mixtures of polylysine and glycopolymer arms were developed and were shown to possess antimicrobial efficacy toward Gram positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) (with MIC values as low as 16 μg mL(-1)) while being non-hemolytic (HC50 > 10,000 μg mL(-1)) and exhibit excellent mammalian cell biocompatibility. Structure function analysis indicated that the antimicrobial activity and mammalian cell biocompatibility of the star nanoparticles could be optimized by modifying the molar ratio of polylysine to glycopolymers arms. The technology described herein thus represents an innovative approach that could be used to fight deadly infectious diseases.
Publisher: Elsevier BV
Date: 07-2005
DOI: 10.1016/J.COLSURFB.2005.04.009
Abstract: O-Carboxymethylchitosan (OCMCS) is a kind of biocompatible derivatives of chitosan whose water solubility is strongly dependent on the degree of carboxymethylation. The OCMCS with 100 carboxymethyl groups and 75 amino groups per 100 anhydroglucosamine units of OCMCS was synthesized by the reaction of chitosan and monochloroacetic. When OCMCS was dissolved in water, its solution was neutral and OCMCS behaved like a weak polyanionic polyeclectrolyte because most of carboxylic groups were not dissociated in neutral aqueous solution. The aggregation behavior of OCMCS in aqueous solution was studied by surface tensiometry, steady-state fluorescence spectroscopy and viscometry. The critical aggregation concentration (cac) of OCMCS was determined to be between 0.042 mg/ml and 0.050 mg/ml. The possible aggregation mechanism of OCMCS in water was elucidated.
Publisher: American Chemical Society (ACS)
Date: 29-06-2011
DOI: 10.1021/LA201230K
Abstract: Graphene oxide (GO) can be viewed as an hiphilic soft material, which form thin films at organic solvent-water interfaces. However, organic solvent evaporation provides little driving force, which results in slow GO transfer in aqueous phase, thus dawdling GO film formation processes for various potential applications. We present an ethanol-assisted self-assembly method for the quick formation of GO or GO-based composite thin films with tunable composition, transmittance, and surface resistivity at pentane-water interface. The thickness of pure GO and reduced GO (rGO) films ranging from ~1 nm to more than 10 nm can be controlled by the concentration of GO in bulk solution. The transmittance of rGO films can be tuned from 72% to 97% at 550 nm while the surface resistivity changes from 8.3 to 464.6 kΩ sq(-1). Ethanol is essential for achieving quick formation of GO thin films. When ethanol is injected into GO aqueous dispersion, it serves as a nonsolvent, compromising the stability of GO and providing driving force to allow GO sheets aggregate at the water-pentane interface. On the other hand, neither the evaporation of pentane nor the mixing between ethanol and water provides sufficient driving forces to allow noteworthy amount of GO sheets to migrate from the bulk aqueous phase to the interface. This method can also be extended to prepare GO-based composites thin films with tunable composition, such as GO/single walled carbon nanotube (SWCNT) composite thin films investigated in this work. Reduced GO/SWCNT composite films show much lower surface resistivity compared to pure rGO thin films. This ethanol-assisted self-assembly method opens opportunities to design and fabricate new functional GO-based hybrid materials for various potential applications.
Publisher: World Scientific Pub Co Pte Lt
Date: 08-2005
DOI: 10.1142/S0219581X05003462
Abstract: UV embossing for polymeric micro-patterning thin film is an emerging replication technique. This paper investigates UV curable multifunctional acrylates pre-polymer resin patterned by a micro-structured mold and subsequently cured by UV irradiation. To further enhance this duplication method for high aspect ratio production, demolding must be reliable and repeatable without damage to the embossing or mold. Previously, it has been reported that UV embossed patterns for aspect ratios as high as 14 have been achieved experimentally. Finite element analyses for patterns with aspect ratios of 5 using parallel demolding between two parallel plates have also been reported. However, the parallel demolding method may not be suitable for large area patterns as forces generated were high. As such, an alternative demolding method, namely peel demolding, for micro-patterns with an aspect ratio of 14 was investigated and key parameters identified. The parameters governing the demolding process were the peel angle, the pre-crack condition, shrinkage, interface fracture toughness, tensile strength and modulus of polymer. A pre-crack between the polymer and mold was introduced before peel demolding. Numerical analyses in terms of Cohesive Zone Modeling (CZM) were used to simulate the demolding process. Shrinkage caused by UV exposure was represented by thermal strain effects and the fully cured polymer was peeled off using displacement control. The ultimate tensile strength (U.T.S) of the cured polymer was used as a failure criterion. The stresses involved were crucial for determining clean demolding. As peeling progressed, stresses experienced in the polymer matrix increased rapidly in the region ahead of the crack with little or no stress at the cracked region. When stresses experienced by the polymer were below the U.T.S, demolding was deemed to be successful.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 02-2006
Publisher: American Chemical Society (ACS)
Date: 25-01-2005
DOI: 10.1021/LA035124E
Abstract: UV embossing is a replication method whereby an UV-curable polymer is pressed against a patterned mold and cured with UV irradiation, resulting in a patterned polymeric substrate. High aspect ratio UV embossing will find erse applications in tissue engineering, micro-optics, display technologies, and sensors. Demolding of an UV-embossed polymer pattern with aspect ratio of 5 from the mold has previously been demonstrated experimentally. In this paper, parameters that affect the demolding process have been identified and investigated. They include cross-linking shrinkage during curing by UV irradiation, modulus of cured polymer, interfacial fracture strength and toughness, and loading method during demolding. Shrinkage is an important parameter, and an optimum level of shrinkage to avoid breakage of the embossing during demolding was found to exist. This optimum level is that at which the maximum stress (sigma(1)max) experienced by the polymer during demolding is minimized. The micromechanics of demolding was found to be different for shrinkage values lower or larger than the optimum value.
Publisher: American Chemical Society (ACS)
Date: 12-04-2011
DOI: 10.1021/JZ200417B
Publisher: American Chemical Society (ACS)
Date: 19-11-2010
DOI: 10.1021/JP106398K
Publisher: American Chemical Society (ACS)
Date: 08-11-2008
DOI: 10.1021/JP807133J
Publisher: American Chemical Society (ACS)
Date: 04-03-2011
DOI: 10.1021/JP107865M
Publisher: Royal Society of Chemistry (RSC)
Date: 2011
DOI: 10.1039/C1EE01479J
Publisher: American Chemical Society (ACS)
Date: 15-04-2003
DOI: 10.1021/LA026967T
Publisher: Springer Science and Business Media LLC
Date: 05-2018
DOI: 10.1038/S41467-018-04172-1
Abstract: Microfluidic biochips hold great potential for liquid analysis in biomedical research and clinical diagnosis. However, the lack of integrated on-chip liquid mixing, bioseparation and signal transduction presents a major challenge in achieving rapid, ultrasensitive bioanalysis in simple microfluidic configurations. Here we report magnetic nanochain integrated microfluidic chip built upon the synergistic functions of the nanochains as nanoscale stir bars for rapid liquid mixing and as capturing agents for specific bioseparation. The use of magnetic nanochains enables a simple planar design of the microchip consisting of flat channels free of common built-in components, such as liquid mixers and surface-anchored sensing elements. The microfluidic assay, using surface-enhanced Raman scattering nanoprobes for signal transduction, allows for streamlined parallel analysis of multiple specimens with greatly improved assay kinetics and delivers ultrasensitive identification and quantification of a panel of cancer protein biomarkers and bacterial species in 1 μl of body fluids within 8 min.
Publisher: American Chemical Society (ACS)
Date: 13-10-2017
Abstract: Cationic antimicrobial peptides (AMPs) and polymers are active against many multidrug-resistant (MDR) bacteria, but only a limited number of these compounds are in clinical use due to their unselective toxicity. The typical strategy for achieving selective antibacterial efficacy with low mammalian cell toxicity is through balancing the ratio of cationicity to hydrophobicity. Herein, we report a cationic nanoparticle self-assembled from chitosan-graft-oligolysine (CSM5-K5) chains with ultralow molecular weight (1450 Da) that selectively kills bacteria. Further, hydrogen bonding rather than the typical hydrophobic interaction causes the polymer chains to be aggregated together in water into small nanoparticles (with about 37 nm hydrodynamic radius) to concentrate the cationic charge of the lysine. When complexed with bacterial membrane, these cationic nanoparticles synergistically cluster anionic membrane lipids and produce a greater membrane perturbation and antibacterial effect than would be achievable by the same quantity of charge if dispersed in in idual copolymer molecules in solution. The small zeta potential (+15 mV) and lack of hydrophobicity of the nanoparticles impedes the insertion of the copolymer into the cell bilayer to improve biocompatibility. In vivo study (using a murine excisional wound model) shows that CSM5-K5 suppresses the growth of methicillin-resistant Staphylococcus aureus (MRSA) bacteria by 4.0 orders of magnitude, an efficacy comparable to that of the last resort MRSA antibiotic vancomycin it is also noninflammatory with little/no activation of neutrophils (CD11b and Ly6G immune cells). This study demonstrates a promising new class of cationic polymers-short cationic peptidopolysaccharides-that effectively attack MDR bacteria due to the synergistic clustering of, rather than insertion into, bacterial anionic lipids by the concentrated polymers in the resulting hydrogen-bonding-stabilized cationic nanoparticles.
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C7RA11823F
Abstract: COEs are emerging antimicrobials to combat drug resistant infections and to which bacteria develop only limited resistance.
Publisher: Wiley
Date: 19-03-2010
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C4LC01416B
Abstract: Micromolded hydrogels lose non-fouling properties enabling cell adhesion and migration on the gel (left). We develop a method to regenerate the cell resistance of the gels resulting in robust resistance to cell adhesion (right).
Publisher: American Chemical Society (ACS)
Date: 07-2009
DOI: 10.1021/JP904431U
Publisher: American Chemical Society (ACS)
Date: 23-02-2008
DOI: 10.1021/CM703567G
Publisher: American Chemical Society (ACS)
Date: 29-09-2010
DOI: 10.1021/CM102071M
Publisher: AIP Publishing
Date: 14-11-2005
DOI: 10.1063/1.2132075
Abstract: We have demonstrated the functionalization of single-wall carbon nanotubes (SWNTs) by argon (Ar) plasma-assisted ultraviolet (UV) grafting of 1-vinylimidazole (VZ). The Ar plasma treatment generates defect sites at the tube ends and sidewalls, which act as the active sites for the subsequent UV grafting of VZ monomer. Atomic force microscopy analyses indicate that the original nanotube bundles exfoliate to in idual tubes after the VZ grafting. By control of the deposited energy of Ar plasma treatment (200W) and treatment time (5min), no visible chopping of the functionalized SWNT was observed. This method may be extended to other vinyl monomers and offers another erse way of sidewall functionalization of SWNT.
Publisher: American Chemical Society (ACS)
Date: 25-11-2008
DOI: 10.1021/CM8017677
Publisher: Wiley
Date: 11-01-2008
Publisher: American Chemical Society (ACS)
Date: 21-11-2011
DOI: 10.1021/NN2038725
Abstract: The currently applied disinfection methods during water treatment provide effective solutions to kill pathogens, but also generate harmful byproducts, which are required to be treated with additional efforts. In this work, an alternative and safer water disinfection system consisting of silver nanoparticle/multiwalled carbon nanotubes (Ag/MWNTs) coated on a polyacrylonitrile (PAN) hollow fiber membrane, Ag/MWNTs/PAN, has been developed. Silver nanoparticles of controlled sizes were coated on polyethylene glycol-grafted MWNTs. Ag/MWNTs were then covalently coated on the external surface of a chemically modified PAN hollow fiber membrane to act as a disinfection barrier. A continuous filtration test using E. coli containing feedwater was conducted for the pristine PAN and Ag/MWNTs/PAN composite membranes. The Ag/MWNT coating significantly enhanced the antimicrobial activities and antifouling properties of the membrane against E. coli. Under the continuous filtration mode using E. coli feedwater, the relative flux drop over Ag/MWNTs/PAN was 6%, which was significantly lower than that over the pristine PAN (55%) at 20 h of filtration. The presence of the Ag/MWNT disinfection layer effectively inhibited the growth of bacteria in the filtration module and prevented the formation of biofilm on the surface of the membrane. Such distinctive antimicrobial properties of the composite membrane is attributed to the proper dispersion of silver nanoparticles on the external surface of the membrane, leading to direct contact with bacterium cells.
Publisher: American Chemical Society (ACS)
Date: 19-07-2008
DOI: 10.1021/JP805434D
Publisher: American Chemical Society (ACS)
Date: 24-03-2010
DOI: 10.1021/JP910916A
Publisher: Royal Society of Chemistry (RSC)
Date: 2011
DOI: 10.1039/C1NR10024F
Abstract: Despite unique and useful properties of multi-walled carbon nanotubes (MWNTs) such as high strength and a low synthesis cost, their weak antimicrobial property h ers their use as an antimicrobial material. Herein, we demonstrate that the immobilization of nisin, a natural and inexpensive antimicrobial peptide, with poly(ethylene glycol) (PEG(1000)) as a linker significantly enhanced the antimicrobial and anti-biofilm properties of MWNTs. The MWNT-nisin composite showed up to 7-fold higher antimicrobial property than pristine MWNTs against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Bacillus subtilis. Moreover, the MWNT-nisin composite had a dramatically improved capability to prevent biofilm formation both on a deposited film and in suspension. In particular, the MWNT-nisin deposit film exhibited a 100-fold higher anti-biofilm property than the MWNT deposit film. Further, it has been shown that PEG and nisin are covalently attached to MWNTs with excellent stability against leaching. We envision that our novel MWNT-nisin composite can serve as an effective and economical antimicrobial material.
Publisher: American Chemical Society (ACS)
Date: 16-02-2008
DOI: 10.1021/JP8003322
Abstract: Highly selective enrichment of (6,5) and (8,3) SWCNTs (above 85% of the semiconducting tubes) was achieved through multistep extraction by sodium dodecyl sulfate (SDS) and sodium cholate (SC) cosurfactant solution from narrowly (n,m) distributed SWCNTs produced by the catalyst Co-MCM-41. A systematic change in the chirality selectivity was observed when the weight ratio between SDS and SC varied in cosurfactant solutions, with maximum enrichment selectivity for (6,5) tubes yielded at 1:4. Furthermore, surfactants were washed away easily to produce "clean" SWCNTs. This observation sheds light on the possibility of obtaining SWCNTs with the desired (n,m) structure via an easily scalable approach. No selectivity was detected when using sodium dodecyl benzene sulfonate (SDBS)/SC cosurfactants, hence suggesting the need for further exploration of various cosurfactant combinations for more effective extraction of different (n,m) species.
Publisher: Wiley
Date: 16-08-2011
Abstract: Three isomers of chondroitin sulfate (CS), i.e., CS-A, CS-B, and CS-C, are investigated as nanotube dispersants and are found to have vastly different abilities to disperse single-walled carbon nanotubes (SWNTs) in water due to their different intramolecular interactions. Only CS-A and CS-C effectively disperse SWNTs into small bundles or in idual tubes while CS-B disperses SWNTs poorly. Computer simulation and circular dichrosim show that neat CS-A and CS-C have weak intramolecular hydrogen bonding and extended conformations in solution resulting in energetically more favorable interactions with nanotubes. CS-B has relatively strong intramolecular Coulombic interaction and more alpha-helical secondary structure in solution resulting in energetically less favorable interaction with the nanotubes. Atomic force microscopy images show helical wrappings of CS-A and CS-C around the SWNTs. Transmission electron microscopy corroborates the helical wrapping of CS-A. Different isomeric forms of a polymer can have vastly different dispersing power because of their different intramolecular interactions and conformations. The easy removability of CS-A from nanotubes is confirmed with X-ray photoelectron spectroscopy showing almost no detectable sulphur content after washing with water and by application of washed CS-A dispersed SWNTs in field-effect transistors.
Publisher: American Chemical Society (ACS)
Date: 27-02-2004
DOI: 10.1021/CM0342849
Publisher: Elsevier BV
Date: 03-2014
DOI: 10.1016/J.BIOMATERIALS.2013.12.035
Abstract: Herein we combine cell sheet technology and electrospun scaffolding to rapidly generate circumferentially aligned tubular constructs of human aortic smooth muscles cells with contractile gene expression for use as tissue engineered blood vessel media. Smooth muscle cells cultured on micropatterned and N-isopropylacrylamide-grafted (pNIPAm) polydimethylsiloxane (PDMS), a small portion of which was covered by aligned electrospun scaffolding, resulted in a single sheet of unidirectionally aligned cells. Upon cooling to room temperature, the scaffold, its adherent cells, and the remaining cell sheet detached and were collected on a mandrel to generating tubular constructs with circumferentially aligned smooth muscle cells which possess contractile gene expression and a single layer of electrospun scaffold as an analogue to a small diameter blood vessel's internal elastic lamina (IEL). This method improves cell sheet handling, results in rapid circumferential alignment of smooth muscle cells which immediately express contractile genes, and introduction of an analogue to small diameter blood vessel IEL.
Publisher: Wiley
Date: 23-07-2013
Abstract: Transparent, high water content (>65%), and cytocompatible hydrogels, which also possess super high refractive indices (RI > 1.5), are needed for ophthalmological applications. Most hydrogels can achieve either high RI or high water content but not both in the same system because water is a low RI material. Here, high water content/high RI hydrogels fabricated through elevated-temperature UV polymerization of an aqueous solution of acrylamide (AM) and methacrylamide (MAM) with tri(ethylene glycol) dimethacrylate (TEDA) crosslinker are reported. By varying the AM:MAM ratios (2:8 to 8:2) and crosslinker density (5 to 11 mol %), it is discovered that high water content (66%) AM:MAM copolymer hydrogels exhibiting anomalously high refractive indices (1.53) they are also colorless, transparent (99.4%), and cytocompatible with human keratinocytes.
Location: United States of America
No related grants have been discovered for Mary Chan-Park.