Publications by authors named "Bert Klumperman"

36 Publications

Facile Route to Targeted, Biodegradable Polymeric Prodrugs for the Delivery of Combination Therapy for Malaria.

ACS Biomater Sci Eng 2020 11 7;6(11):6217-6227. Epub 2020 Oct 7.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

A facile synthetic methodology has been developed to prepare multifaceted polymeric prodrugs that are targeted, biodegradable, and nontoxic, and used for the delivery of combination therapy. This is the first instance of the delivery of the WHO recommended antimalarial combination of lumefantrine (LUM, drug 1) and artemether (AM, drug 2) a polymeric prodrug. To achieve this, reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization of -vinylpyrrolidone (NVP) was conducted using a hydroxy-functional RAFT agent, and the resulting polymer was used as the macroinitiator in the ring-opening polymerization (ROP) of α-allylvalerolactone (AVL) to synthesize the biodegradable block copolymer of poly(-vinylpyrrolidone) and poly(α-allylvalerolactone) (PVP--PAVL). The ω-end thiol group of PVP was protected using 2,2'-pyridyldisulfide prior to the ROP, and was conveniently used to bioconjugate a peptidic targeting ligand. To attach LUM, the allyl groups of PVP--PAVL underwent oxidation to introduce carboxylic acid groups, which were then esterified with ethylene glycol vinyl ether. Finally, LUM was conjugated to the block copolymer an acid-labile acetal linkage in a "click"-type reaction, and AM was entrapped within the hydrophobic core of the self-assembled aggregates to render biodegradable multidrug-loaded micelles with targeting ability for combination therapy.
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http://dx.doi.org/10.1021/acsbiomaterials.0c01234DOI Listing
November 2020

Influence of DIBMA Polymer Length on Lipid Nanodisc Formation and Membrane Protein Extraction.

Biomacromolecules 2021 Feb 29;22(2):763-772. Epub 2020 Dec 29.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7602, South Africa.

Polymer-based lipid nanoparticles like styrene-maleic acid lipid particles have revolutionized the study of membrane proteins. More recently, alternative polymers such as poly(diisobutylene--maleic acid) (DIBMA) have been used in this field. DIBMA is commonly synthesized via conventional radical copolymerization. In order to study the influence of its chain length on lipid nanodisc formation and membrane protein extraction, we synthesized DIBMA with molar masses varying from 1.2-12 kDa via RAFT-mediated polymerization. For molar masses in the range of 3-7 kDa, the rate of lipid nanodisc formation was the highest and similar to those of poly(styrene-maleic acid) (SMA) and commercially available DIBMA. ZipA solubilization efficiency was significantly higher than for commercially available DIBMA and similar to SMA (circa 75%). Furthermore, RAFT-made DIBMA with a molar mass of 1.2-3.9 kDa showed a much cleaner separation on SDS-PAGE, without the smearing that is typically seen for SMA and commercially available DIBMA.
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http://dx.doi.org/10.1021/acs.biomac.0c01538DOI Listing
February 2021

Chemical Identity of Poly(-vinylpyrrolidone) End Groups Impact Shape Evolution During the Synthesis of Ag Nanostructures.

J Am Chem Soc 2021 Jan 21;143(1):184-195. Epub 2020 Dec 21.

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Ag nanocubes (AgNCs) are predominantly synthesized by the polyol method, where the solvent (ethylene glycol) is considered the reducing agent and poly(-vinylpyrrolidone) (PVP) the shape-directing agent. An experimental phase diagram for the formation of Ag nanocubes as a function of PVP monomer concentration () and molecular weight () demonstrated end groups of PVP impact the final Ag product. Measured rates of the initial Ag reduction at different PVP and confirmed the reducing effect originates from end-groups. PVP with well-defined aldehyde and hydroxyl end groups lead to the formation of Ag nanocubes and nanowires respectively, indicating the faster reducing agent formed kinetically preferred nanowires. We demonstrate PVP end-groups induce initial reduction of Ag to form seeds followed by autocatalytic reduction of Ag by ethylene glycol (and not solvent oxidation products) to form Ag nanostructures. The current study enabled a quantitative description of the role of PVP in nanoparticle shape-control and demonstrates a unique opportunity to design nanostructures by combining nanoparticle synthesis with polymer design to introduce specific physicochemical properties.
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http://dx.doi.org/10.1021/jacs.0c08528DOI Listing
January 2021

Poly(-vinylpyrrolidone) Antimalaria Conjugates of Membrane-Disruptive Peptides.

Biomacromolecules 2020 12 6;21(12):5053-5066. Epub 2020 Nov 6.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

The concepts of polymer-peptide conjugation and self-assembly were applied to antimicrobial peptides (AMPs) in the development of a targeted antimalaria drug delivery construct. This study describes the synthesis of α-acetal, ω-xanthate heterotelechelic poly(-vinylpyrrolidone) (PVP) via reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization, followed by postpolymerization deprotection to yield α-aldehyde, ω-thiol heterotelechelic PVP. A specific targeting peptide, GSRSKGT, for -infected erythrocytes was used to sparsely decorate the α-chain ends via reductive amination while cyclic decapeptides from the tyrocidine group were conjugated to the ω-chain end via thiol-ene Michael addition. The resultant constructs were self-assembled into micellar nanoaggregates whose sizes and morphologies were determined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The in vitro activity and selectivity of the conjugates were evaluated against intraerythrocytic parasites.
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http://dx.doi.org/10.1021/acs.biomac.0c01202DOI Listing
December 2020

Iterative RAFT-Mediated Copolymerization of Styrene and Maleic Anhydride toward Sequence- and Length-Controlled Copolymers and Their Applications for Solubilizing Lipid Membranes.

Biomacromolecules 2020 08 30;21(8):3287-3300. Epub 2020 Jul 30.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

The use of poly(styrene--maleic acid) (SMA) for the solubilization of lipid membranes and membrane proteins is becoming more widespread, and with this, the need increases to better understand the chemical properties of the copolymer and how these translate into membrane solubilization properties. SMA comes in many different flavors that include the ratio of styrene to maleic acid, comonomer sequence distribution, average chain length, dispersity, and potential chemical modifications. In this work, the synthesis and membrane active properties are described for 2:1 (periodic) SMA copolymers with varying from ∼1.4 to 6 kDa. The copolymers were obtained via an iterative RAFT-mediated radical polymerization. Characterization of these polymers showed that they represent a well-defined series in terms of chain length and overall composition ( ∼ 0.33), but that there is heterogeneity in comonomer sequence distribution ( ∼ 0.50) and some dispersity in chain length (1.1 < < 1.6), particularly for the larger copolymers. Investigation of the interaction of these polymers with phosphatidylcholine lipid self-assemblies showed that all copolymers inserted equally effectively into lipid monolayers, independent of the copolymer length. Nonetheless, smaller polymers were more effective at solubilizing lipid bilayers into nanodiscs, possibly because longer polymers are more prone to become intertwined with each other, thereby hampering their solubilization efficiency. Nanodisc sizes were independent of the copolymer length. However, nanodiscs formed with larger copolymers were found to undergo slower lipid exchange, indicating a higher stability. The results highlight the usefulness of having well-defined copolymers for systematic studies.
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http://dx.doi.org/10.1021/acs.biomac.0c00736DOI Listing
August 2020

Polymeric siRNA gene delivery - transfection efficiency versus cytotoxicity.

J Control Release 2019 12 2;316:263-291. Epub 2019 Nov 2.

Stellenbosch University, Department of Chemistry and Polymer Science, Private Bag X1, Matieland 7602, South Africa. Electronic address:

Within the field of gene therapy, there is a considerable need for the development of non-viral vectors that are able to compete with the efficiency obtained by viral vectors, while maintaining a good toxicity profile and not inducing an immune response within the body. While there have been many reports of possible polymeric delivery systems, few of these systems have been successful in the clinical setting due to toxicity, systemic instability or gene regulation inefficiency, predominantly due to poor endosomal escape and cytoplasmic release. The objective of this review is to provide an overview of previously published polymeric non-coding RNA and, to a lesser degree, oligo-DNA delivery systems with emphasis on their positive and negative attributes, in order to provide insight in the numerous hurdles that still limit the success of gene therapy.
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http://dx.doi.org/10.1016/j.jconrel.2019.10.046DOI Listing
December 2019

Degradation of Proteins and Starch by Combined Immobilization of Protease, α-Amylase and β-Galactosidase on a Single Electrospun Nanofibrous Membrane.

Molecules 2019 Jan 31;24(3). Epub 2019 Jan 31.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

Two commercially available enzymes, Dextrozyme (α-amylase) and Esperase (protease), were covalently immobilized on non-woven electrospun poly(styrene--maleic anhydride) nanofiber mats with partial retention of their catalytic activity. Immobilization was achieved for the enzymes on their own as well as in different combinations with an additional enzyme, β-galactosidase, on the same non-woven nanofiber mat. This experiment yielded a universal method for immobilizing different combinations of enzymes with nanofibrous mats containing maleic anhydride (MAnh) residues in the polymer backbone.
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http://dx.doi.org/10.3390/molecules24030508DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6384644PMC
January 2019

Coordinated autophagy modulation overcomes glioblastoma chemoresistance through disruption of mitochondrial bioenergetics.

Sci Rep 2018 07 9;8(1):10348. Epub 2018 Jul 9.

Department of Physiological Sciences, Faculty of Science, University of Stellenbosch, Stellenbosch, 7600, South Africa.

Glioblastoma Multiforme (GBM) is known to be one of the most malignant and aggressive forms of brain cancer due to its resistance to chemotherapy. Recently, GBM was found to not only utilise both oxidative phosphorylation (OXPHOS) and aerobic glycolysis, but also depend on the bulk protein degradation system known as macroautophagy to uphold proliferation. Although autophagy modulators hold great potential as adjuvants to chemotherapy, the degree of upregulation or inhibition necessary to achieve cell death sensitisation remains unknown. Therefore, this study aimed to determine the degree of autophagy modulation necessary to impair mitochondrial bioenergetics to the extent of promoting cell death onset. It was shown that coordinated upregulation of autophagy followed by its inhibition prior to chemotherapy decreased electron transfer system (ETS) and oxidative phosphorylation (OXPHOS) capacity, impaired mitochondrial fission and fusion dynamics and enhanced apoptotic cell death onset in terms of cleaved caspase 3 and cleaved PARP expression. Therefore, coordinated autophagy modulation may present a favourable avenue for improved chemotherapeutic intervention in the future.
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http://dx.doi.org/10.1038/s41598-018-28590-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6037778PMC
July 2018

Synthesis and Cell Interaction of Statistical l-Arginine-Glycine-l-Aspartic Acid Terpolypeptides.

Biomacromolecules 2018 07 23;19(7):3058-3066. Epub 2018 May 23.

Copolymerizations and terpolymerizations of N-carboxyanhydrides (NCAs) of glycine (Gly), N-carbobenzyloxy-l-ornithine (Z-Orn), and β-benzyl-l-aspartate (Bz-Asp) were investigated. In situ H NMR spectroscopy was used to monitor individual comonomer consumptions during binary and ternary copolymerizations. The six relevant reactivity ratios were determined from copolymerizations of the NCAs of amino acids via nonlinear least-squares curve fitting. The reactivity ratios were subsequently used to maximize the occurrence of the Asp-Gly-Orn ( DGR') sequence in the terpolymers. Terpolymers with variable probability of occurrence of DGR' were prepared in the lab. Subsequently, the ornithine residues on the terpolymers were converted to l-arginine (R) residues via guanidination reaction after removal of the protecting groups. The resulting DGR terpolymers translate to traditional peptides and proteins with variable RGD content, due to the convention in nomenclature that peptides are depicted from N- to C-terminus, whereas the NCA ring-opening polymerization is conducted from C- to N-terminus. The l-arginine containing terpolymers were evaluated for cell interaction, where it was found that neuronal cells display enhanced adhesion and process formation when plated in the presence of statistical DGR terpolymers.
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http://dx.doi.org/10.1021/acs.biomac.8b00620DOI Listing
July 2018

Influence of Poly(styrene- co-maleic acid) Copolymer Structure on the Properties and Self-Assembly of SMALP Nanodiscs.

Biomacromolecules 2018 03 16;19(3):761-772. Epub 2018 Jan 16.

Diamond Light Source , Harwell Science and Innovation Campus , Didcot OX11 ODE , United Kingdom.

Polymer stabilized nanodiscs are self-assembled structures composed of a polymer belt that wraps around a segment of lipid bilayer, and as such are capable of encapsulating membrane proteins directly from the cell membrane. To date, most studies on these nanodiscs have used poly(styrene- co-maleic acid) (SMA) with the term SMA-lipid particles (SMALPs) coined to describe them. In this study, we have determined the physical and thermodynamic properties of such nanodiscs made with two different SMA copolymers. These include a widely used and commercially available statistical poly(styrene- co-maleic acid) copolymer (coSMA) and a reversible addition-fragmentation chain transfer synthesized copolymer with narrow molecular weight distribution and alternating styrene and maleic acid groups with a polystyrene tail, (altSMA). We define phase diagrams for each polymer, and show that, regardless of polymer topological structure, self-assembly is driven by the free energy change associated with the polymers. We also show that nanodisc size is polymer dependent, but can be modified by varying polymer concentration. The thermal stability of each nanodisc type is similar, and both can effectively solubilize proteins from the E. coli membrane. These data show the potential for the development of different SMA polymers with controllable properties to produce nanodiscs that can be optimized for specific applications and will enable more optimized and widespread use of the SMA-based nanodiscs in membrane protein research.
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http://dx.doi.org/10.1021/acs.biomac.7b01539DOI Listing
March 2018

Synthesis, Characterization, and Evaluation of Cytotoxicity of Poly(3-methylene-2-pyrrolidone).

Biomacromolecules 2016 05 25;17(5):1795-800. Epub 2016 Apr 25.

Department of Chemistry and Polymer Science, Stellenbosch University , Private Bag X1, Matieland 7602, South Africa.

The homo- and copolymerization of 3-methylene-2-pyrrolidone (3M2P) is introduced. 3M2P is readily polymerized via conventional free radical polymerization, and two reversible deactivation radical polymerization methods including reversible addition-fragmentation (chain) transfer and single-electron-transfer living radical polymerization. Poly(3M2P) has a high thermal stability and a very high glass transition temperature. Poly(3M2P) does not dissolve in most common organic solvents, but it has a high aqueous solubility. Cytotoxicity tests reveal that it is nontoxic to cells, even up to concentrations of 1 mg/mL. This adds poly(3M2P) to the family of water-soluble and biocompatible pyrrolidone-based vinyl polymers.
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http://dx.doi.org/10.1021/acs.biomac.6b00210DOI Listing
May 2016

Determination of the shell growth direction during the formation of silica microcapsules by confocal fluorescence microscopy.

J Mater Chem B 2015 Oct 10;3(39):7745-7751. Epub 2015 Sep 10.

Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

A novel procedure was developed to determine the direction of silica growth during the formation of a silica shell around aqueous microdroplets in water-in-oil Pickering emulsions. Two fluorescently labeled silica precursors were added consecutively and the resulting microcapsules were visualized via confocal fluorescence microscopy, allowing the conclusion that the locus of reaction moves in the positive radial direction, i.e. from the inside to the outside. A consequence of the growth direction is that water has to diffuse through the shell to participate in the reaction on the outer surface of the shell.
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http://dx.doi.org/10.1039/c5tb01232eDOI Listing
October 2015

Compartmentalization of bacteria in microcapsules.

Chem Commun (Camb) 2014 Dec 29;50(97):15427-30. Epub 2014 Oct 29.

Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, P.O. Box 513, 5600 MB Eindhoven University of Technology, The Netherlands.

Lactobacillus plantarum strain 423 was encapsulated in hollow poly(organosiloxane) microcapsules by templating water-in-oil Pickering emulsion droplets via the interfacial reaction of alkylchlorosilanes. The bacteria were suspended in growth medium or buffer to protect the cells against pH changes during the interfacial reactions with alkylchlorosilanes. The results of this work open up novel avenues for the encapsulation of microbial cells.
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http://dx.doi.org/10.1039/c4cc04901bDOI Listing
December 2014

Formation of hybrid poly(styrene-co-maleic anhydride)-silica microcapsules.

J Mater Chem B 2014 Aug 23;2(30):4826-4835. Epub 2014 Jun 23.

Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.

In this contribution we report the synthesis of hybrid poly(styrene-co-maleic anhydride)-SiO microcapsules by cross-linking of the stabilizing particles of an inverse Pickering emulsion droplet at the interface. This was achieved by the ring-opening aminolysis reaction of the maleic anhydride residue of poly(St-co-MAh) with amine-functionalized silica particles that stabilized the Pickering emulsion. The cross-linking reaction is clearly shown by labeling the polymer with a green dye and the silica particles with a red dye, followed by confocal fluorescence microscopy analysis. Because poly(St-co-MAh) is a versatile polymer that can react with different other polymers, this opens the possibility of producing microcapsules with versatile properties. Encapsulation of delicate matter, e.g., live cells or enzymes, might be suitable as a result of the straightforward synthesis method.
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http://dx.doi.org/10.1039/c4tb00473fDOI Listing
August 2014

Deformation of the water/oil interface during the adsorption of sterically stabilized particles.

Langmuir 2014 Jul 18;30(25):7327-33. Epub 2014 Jun 18.

Laboratory of Polymer Chemistry and ‡Laboratory of Chemical Reactor Engineering, University of Technology Eindhoven , P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

The adsorption of sterically stabilized colloids at water/oil (w/o) interfaces is studied experimentally by the formation of Pickering emulsions. Specifically, the effect of the steric stabilizer with respect to the rate of particle adsorption is investigated. Uniform, micrometer-sized poly(methyl methacrylate) (pMMA) particles, which are sterically stabilized with poly(isobutylene) (pIB), are used. The pIB concentration on the particle surface (ΓPIB) is controlled during the synthesis by adjusting the pIB/monomer ratio. Pickering emulsions are formed directly by the addition of water to the nonaqueous pMMA dispersions and subsequent emulsification. A strong dependence of the rate of particle adsorption on ΓPIB is found. The rate constant k for particle adsorption decays exponentially with ΓPIB, which suggest the use of a Boltzmann factor to model the experimentally found rate constants. The experimental results can be explained when the activation barrier for particle adsorption EA is of the same order as the average kinetic energy EK of a particle colliding with an emulsion droplet, which is equivalent to 10(5) kBT. Interestingly, this makes EA several orders of magnitude greater than the steric interaction with another particle. A possible mechanism that can lead to such a significant repulsive force is the inhibited drainage of solvent between the particle and o/w interface. Deformation of the o/w interface then occurs, when the solvent does not have time to drain, which results in a dramatic increase in the interfacial energy. This study identified the relevance of drainage in the formation of Pickering emulsions.
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http://dx.doi.org/10.1021/la501334pDOI Listing
July 2014

pH-dependent adhesion of mycobacteria to surface-modified polymer nanofibers.

J Mater Chem B 2013 Dec 6;1(48):6608-6618. Epub 2013 Nov 6.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, 7602 Matieland, South Africa.

Children across the world are greatly affected by tuberculosis (TB) due to high morbidity and mortality. It is important to diagnose TB in children timeously and accurately in order to provide effective treatment. In this study we aimed to test the hypothesis that a modified polymer could be developed to capture Mycobacterium tuberculosis (M. tuberculosis), the pathogen that causes TB, under different pH conditions, mimicking different clinical specimens. The modified polymer was electrospun into nanofibrous mats thereby ensuring optimal surface areas. Affinity studies were conducted on these modified polymer nanofibers with Mycobacterium bovis bacillus Calmette-Guérin (BCG) and verified with M. tuberculosis to evaluate these nanofibrous surfaces as mycobacterium-capturing platforms. The results indicate that BCG and M. tuberculosis were successfully captured under different pH conditions depending on the affinity ligand. Concentration and time studies showed that binding efficiency is dependent on the incubation time and the concentration of mycobacteria. The affinity studies also reveal that the nanofibrous-capturing polymer should not be too hydrophobic in character as this causes poor wetting of the modified nanofibers, thus preventing close contact with the mycobacteria and a reduction in the capture effectivity of the polymer nanofibers. The detection of M. tuberculosis using microscopy is simplified by the tendency of the mycobacteria to aggregate on the hydrophobic surface of the modified nanofibers. As a result of this aggregation, fluorescence and light microscopy are regarded as feasible detection methods to image M. tuberculosis on the surface of the modified nanofibers, even at relatively low concentration. In this study a polymer was therefore successfully modified in such a way that it acquired an affinity for M. tuberculosis and enabled the capture of this organism onto the modified polymer nanofibrous surface.
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http://dx.doi.org/10.1039/c3tb21393eDOI Listing
December 2013

Templated hierarchical self-assembly of poly(p-aryltriazole) foldamers.

Angew Chem Int Ed Engl 2013 Oct 3;52(42):11040-4. Epub 2013 Sep 3.

Stellenbosch University, Department of Chemistry and Polymer Science, Private Bag X1, Matieland 7602 (South Africa) http://www.klumperman-group.net/

A biomimetic approach has been used for the templated self-assembly of a helical poly(para-aryltriazole) foldamer. The solvophobic folding process yields helices that further self-assemble into long nanotubes (see picture; scale bar: 100 nm). Constructs of controlled length and chirality can be generated by applying a poly(γ-benzyl-l-glutamate) scaffold at the appropriate assembly conditions, mimicking tobacco mosaic virus self-assembly.
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http://dx.doi.org/10.1002/anie.201303135DOI Listing
October 2013

Poly(methyl methacrylate)-silica microcapsules synthesized by templating Pickering emulsion droplets.

J Mater Chem B 2013 May 3;1(18):2394-2406. Epub 2013 Apr 3.

Eindhoven University of Technology, Chemical Engineering and Department of Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

In this contribution, we present the silica microencapsulation of hydrophilic compounds by templating Pickering emulsion droplets without contamination of the dispersed phase by either the catalyst or the silica precursor. This is accomplished by the use of an amphiphilic catalyst, which situates around the Pickering emulsion droplets and directs the reaction to the interface. Both the silica precursor and the amphiphilic catalyst are soluble in the oil phase and therefore initially do not reside in the hydrophilic microcapsule templates. The thickness of the capsules can be tuned by adjusting the amount of precursor. Thus, the permeability of the capsules can in principle be controlled. The possibility of tuning the permeability holds promise for a variety of applications of the microcapsules. Because of the straightforward synthesis method and minimized mixing of the core with contaminants, the technique is potentially suitable for the encapsulation of delicate matter including live organisms, drugs, enzymes or bacteria.
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http://dx.doi.org/10.1039/c3tb20175aDOI Listing
May 2013

Furanone-containing poly(vinyl alcohol) nanofibers for cell-adhesion inhibition.

Water Res 2013 Mar 1;47(3):1049-59. Epub 2012 Dec 1.

Division of Polymer Science, University of Stellenbosch, Stellenbosch, South Africa.

The 3(2H) furanone derivative 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF) was investigated for its antimicrobial and cell-adhesion inhibition properties against Klebsiella pneumoniae Xen 39, Staphylococcus aureus Xen 36, Escherichia coli Xen 14, Pseudomonas aeruginosa Xen 5 and Salmonella typhimurium Xen 26. Nanofibers electrospun from solution blends of DMHF and poly(vinyl alcohol) (PVA) were tested for their ability to inhibit surface-attachment of bacteria. Antimicrobial and adhesion inhibition activity was determined via the plate counting technique. To quantify viable but non-culturable cells and to validate the plate counting results, bioluminescence and fluorescence studies were carried out. Nanofiber production was upscaled using the bubble electrospinning technique. To ascertain that no DMHF leached into filtered water, samples of water filtered through the nanofibrous mats were analyzed using gas chromatography coupled with mass spectrometry (GC-MS). Scanning electron microscopy (SEM) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) were used to characterize the electrospun nanofibers.
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http://dx.doi.org/10.1016/j.watres.2012.11.012DOI Listing
March 2013

Poly(N-vinylpyrrolidone)-block-poly(vinyl acetate) as a drug delivery vehicle for hydrophobic drugs.

Biomacromolecules 2012 Dec 14;13(12):4109-17. Epub 2012 Nov 14.

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

Poly((N-vinylpyrrolidone)-block-poly(vinyl acetate)) (PVP-b-PVAc) block copolymers of varying molecular weight and hydrophobic block lengths were synthesized via controlled radical polymerization and investigated as carriers for the solubilization of highly hydrophobic riminophenazine compounds. These compounds have recently been shown to exhibit a strong activity against a variety of cancer types. PVP-b-PVAc self-assembles into polymer vesicles in aqueous media, and the dialysis method was used to load the water-insoluble drug (clofazimine) into these polymer vesicles. The polymer vesicles were characterized by 1H NMR spectroscopy to confirm vesicle formation and the incorporation of the anticancer drugs into the polymer vesicles. Dynamic light scattering was used to determine the particle size and particle size distribution of the drug-loaded vesicles as well as the stability of the vesicles under physiological conditions. The size of the polymer vesicles did not increase upon loading with clofazimine, and the particle size of 180-200 nm and the narrow particle size distribution were maintained. The morphology of the vesicles was examined by transmission electron microscopy. The polymer vesicles had a relatively high drug loading capacity of 20 wt %. In vitro cytotoxicity studies of PVP-b-PVAc and drug-loaded PVP-b-PVAc were performed against MDA-MB-231 multidrug-resistant breast epithelial cancer cells and MCF12A nontumorigenic breast epithelial cells. In vitro experiments demonstrated that the PVP-b-PVAc drug carrier showed no cytotoxicity, which confirms the biocompatibility of the PVP-b-PVAc drug carrier. The results indicate that the present PVP-b-PVAc block copolymer could be a potential candidate as a drug carrier for hydrophobic drugs.
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http://dx.doi.org/10.1021/bm301410dDOI Listing
December 2012

Structure of colloidosomes with tunable particle density: simulation versus experiment.

Phys Rev E Stat Nonlin Soft Matter Phys 2012 Jun 6;85(6 Pt 1):061404. Epub 2012 Jun 6.

National Institute for Theoretical Physics and Institute of Theoretical Physics, University of Stellenbosch, Stellenbosch 7600, South Africa.

Colloidosomes are created in the laboratory from a Pickering emulsion of water droplets in oil. The colloidosomes have approximately the same diameter and by choosing (hairy) particles of different diameters it is possible to control the particle density on the droplets. The experiment is performed at room temperature. The radial distribution function of the assembly of (primary) particles on the water droplet is measured in the laboratory and in a computer experiment of a fluid model of particles with pairwise interactions on the surface of a sphere.
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http://dx.doi.org/10.1103/PhysRevE.85.061404DOI Listing
June 2012

Immobilized furanone derivatives as inhibitors for adhesion of bacteria on modified poly(styrene-co-maleic anhydride).

Biomacromolecules 2012 Oct 19;13(10):3138-50. Epub 2012 Sep 19.

Division of Polymer Science, University of Stellenbosch , Stellenbosch, South Africa.

The ability of brominated furanones and other furanone compounds with 2(3H) and 2(5H) cores to inhibit bacterial adhesion of surfaces as well deactivate (destroy) them has been previously reported. The furanone derivatives 4-(2-(2-aminoethoxy)-2,5-dimethyl-3(2H)-furanone and 5-(2-(2-aminoethoxy)-ethoxy)methyl)-2(5H)-furanone were synthesized in our laboratory. These furanone derivatives were then covalently immobilized onto poly(styrene-co-maleic anhydride) (SMA) and electrospun to fabricate nonwoven nanofibrous mats with antimicrobial and cell-adhesion inhibition properties. The electrospun nanofibrous mats were tested for their ability to inhibit cell attachment by strains of bacteria commonly found in water ( Klebsiella pneumoniae Xen 39, Staphylococcus aureus Xen 36, Escherichia coli Xen 14, Pseudomonas aeruginosa Xen 5, and Salmonella tymphimurium Xen 26). Proton nuclear magnetic resonance spectroscopy ((1)H NMR), electrospray mass spectroscopy (ES-MS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were used to confirm the structures of the synthesized furanones as well as their successful immobilization on SMA. To ascertain that the immobilized furanone compounds do not leach into filtered water, samples of water, filtered through the nanofibrous mats were analyzed using gas chromatography coupled with mass spectroscopy (GC-MS). The morphology of the electrospun nanofibers was characterized using scanning electron microscopy (SEM).
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http://dx.doi.org/10.1021/bm300932uDOI Listing
October 2012

Antimicrobial fibers: therapeutic possibilities and recent advances.

Future Med Chem 2011 Oct;3(14):1821-47

Stellenbosch University, Department of Chemistry and Polymer Science, Private Bag X1, Matieland 7602, South Africa.

The emergence of multi-drug-resistant bacteria such as methicillin-resistant strains of Staphylococcus aureus (MRSA), vancomycin-resistant enterococci, Pseudomonas aeruginosa, Acinetobacter baumannii and extended-spectrum β-lactamase (carbapenemase)-producing Enterobacteriaceae is becoming a serious threat. New-generation antimicrobial agents need to be developed. This includes the design of novel antimicrobial compounds and drug-delivery systems. This review provides an introduction into different classes of antimicrobial materials. The main focus is on strategies for the introduction of antimicrobial properties in polymer materials. These can be roughly divided into surface modification, inclusion of antimicrobial compounds that can leach from the polymer, and the introduction of polymer-bound moieties that provide the polymer with antimicrobial properties. One of the main challenges in the development of antimicrobial polymers for the use in contact with human tissue is the concomitant demand of non-cytotoxicity. Current research is strongly focused on the latter aspect.
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http://dx.doi.org/10.4155/fmc.11.131DOI Listing
October 2011

Release of bacteriocins from nanofibers prepared with combinations of poly(d,l-lactide) (PDLLA) and poly(ethylene oxide) (PEO).

Int J Mol Sci 2011 29;12(4):2158-73. Epub 2011 Mar 29.

Department of Microbiology, University of Stellenbosch, 7602 Matieland (Stellenbosch), South Africa; E-Mail:

Plantaricin 423, produced by Lactobacillus plantarum, and bacteriocin ST4SA produced by Enterococcus mundtii, were electrospun into nanofibers prepared from different combinations of poly(d,l-lactide) (PDLLA) and poly(ethylene oxide) (PEO) dissolved in N,N-dimethylformamide (DMF). Both peptides were released from the nanofibers with a high initial burst and retained 88% of their original antimicrobial activity at 37 °C. Nanofibers have the potential to serve as carrier matrix for bacteriocins and open a new field in developing controlled antimicrobial delivery systems for various applications.
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http://dx.doi.org/10.3390/ijms12042158DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3127109PMC
January 2015

Use of a profluorophore for visualization of the rupture of capsules in self-healing coatings.

Macromol Rapid Commun 2010 Apr 29;31(7):625-8. Epub 2009 Dec 29.

Department of Chemistry and Polymer Science, Stellenbosch University Private Bag X1, Matieland 7602, South Africa.

An important category of self-healing materials relies on the release of a healing agent from a capsule upon the occurrence of damage to the material. Visualization of the release of the healing agent is difficult to accomplish. Here we show that a profluorophore can successfully be used to visualize the local release of a healing agent in a self-healing coating. A tetra-functional thiol compound encapsulated in nanocapsules or microcapsules is dispersed in a poly(methyl acrylate) film, in which the profluorophore is molecularly dispersed. A strong fluorescence signal is observed when a cut is introduced in the film. This fluorescence provides clear evidence that the capsules rupture locally during the introduction of a cut. In a more general sense, it proves that profluorophores can be very useful in materials science.
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http://dx.doi.org/10.1002/marc.200900728DOI Listing
April 2010

RAFT-Mediated Polymerization-A Story of Incompatible Data?

Macromol Rapid Commun 2010 Nov;31(21):1846-62

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

The mechanistic interpretation of kinetic anomalies in reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization is critically reviewed. The main conclusion of this exercise is that available data do not allow model discrimination between the two prevailing mechanistic schemes, i.e., the slow fragmentation model and the intermediate radical termination model. However, assessment of the rate parameters reveals that the incompatibilities may not be as large as previously reported in literature. Dedicated kinetic studies on model compounds should be performed to shed further light on the seemingly incompatible data that currently exists in literature.
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http://dx.doi.org/10.1002/marc.200900907DOI Listing
November 2010

Pickering emulsions: wetting and colloidal stability of hairy particles--a self-consistent field theory.

Langmuir 2011 Jun 3;27(11):6574-83. Epub 2011 May 3.

Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

The assembly of sterically stabilized colloids at liquid-liquid interfaces is studied with the self-consistent field (SCF) theory using the discretization scheme that was developed by Scheutjens, Fleer, and co-workers. The model is based on a poly(methyl methacrylate) (pMMA) particle with poly(isobutylene) (pIB) grafted to the surface. The stabilizing groups on the particle surface have a significant effect on the interfacial assembly and, therefore, also on the formation and properties of Pickering emulsions. The wetting behavior of the particle is altered by the presence of the stabilizing groups, which affects the equilibrium position of the particles at the interface. The stabilizing groups can also lead to an activation barrier before interfacial adsorption, analogous to the steric repulsion between two particles. These effects are numerically solved with the SCF theory. It is commonly known that flocculating conditions enhance the interfacial adsorption and yield stable Pickering emulsions, which is confirmed in this work. Additionally, it is concluded that those conditions are not an absolute requirement. There is a window of stabilizer concentrations Γ(pIB), 2.2-3.3 mg/m(2) pIB, that shows both partial wetting and colloidal stability. The activation barrier for interfacial assembly is 140-550 k(B)T and is an order of magnitude higher than the colloidal stability. The difference can be attributed to the unfavorable interaction of pIB with water and a difference in geometry (plate-sphere vs sphere-sphere). This study demonstrates the interplay and provides a quantitative comparison between the wetting behavior and the colloidal stability, and it gives a better understanding of the colloidal assembly at soft interfaces and formation of Pickering emulsions in general.
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http://dx.doi.org/10.1021/la104886uDOI Listing
June 2011

Controlling electrical percolation in multicomponent carbon nanotube dispersions.

Nat Nanotechnol 2011 Apr 10;6(6):364-9. Epub 2011 Apr 10.

Group Theory of Polymers and Soft Matter, Technische Universiteit Eindhoven, PO Box 513, 5600 MB Eindhoven, The Netherlands.

Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic percolation, depending on the ease of charge transport between the various conductive components.
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http://dx.doi.org/10.1038/nnano.2011.40DOI Listing
April 2011

Steric stabilization of Pickering emulsions for the efficient synthesis of polymeric microcapsules.

Langmuir 2010 Sep;26(18):14929-36

Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

It is commonly known that Pickering emulsions are extremely stable against coalescence and are, therefore, potentially interesting for the synthesis of new materials, such as colloidosomes, microcapsules, composite particles, foams, and so on. However, for the efficient synthesis of such materials, one also has to consider the colloidal stability against aggregation, which is often neglected. In this study, it is demonstrated that steric stabilization is provided to Pickering emulsion droplets by the adsorption of poly(styrene-block-ethylene-co-propylene) (pS-b-EP) and that it is a requirement for the efficient synthesis of polymeric microcapsules. Monodisperse polystyrene particles of 648 nm are synthesized by soap-free emulsion polymerization. A model Pickering emulsion is then formed by the addition of sodium chloride at a critical concentration of 325 mM and mixing it with either heptane or decane. Subsequently, pS-b-EP is added to the Pickering emulsion to provide steric stabilization. Size exclusion chromatography is used to prove and quantify the adsorption of pS-b-EP onto the Pickering emulsion droplets. A maximum surface coverage of 1.3 mg/m(2) is obtained after 2 h, which is approximately one-third of the adsorption on a pure pS surface. We believe that the presence of polar sulfate groups on the particle, which initially stabilized the particle in water, reduces the adsorption of pS-b-EP. Microcapsules are formed by heating the Pickering emulsion above the glass-transition temperature of the particles. Significant aggregation is observed, if no pS-b-EP is used. The adsorption of pS-b-EP provides steric stabilization to the Pickering emulsion droplets, reduces aggregation significantly, and ultimately leads to the successful and efficient synthesis of pS microcapsules.
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http://dx.doi.org/10.1021/la1025284DOI Listing
September 2010

Probing the cooperative nature of the conductive components in polystyrene/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-single-walled carbon nanotube composites.

ACS Nano 2010 Apr;4(4):2242-8

Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

The percolation threshold of single-walled carbon nanotubes (SWCNTs) introduced into polystyrene (PS) via a latex-based route has been reduced by using conductive surfactants. The use of the conductive polymeric latex, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), in conjunction with SWCNTs leads to conductive composites with loadings of both constituents below their own individual percolation thresholds. The high concentration of PEDOT:PSS in the final composites raises the concern that the composite conductivity is a result of the presence of the PEDOT:PSS alone. To elucidate the cooperative nature of the two conductive components, the contribution of the SWCNTs to the overall composite conductivity is investigated by replacing the original high-quality SWCNTs with SWCNTs of a lower quality. Percolation thresholds recorded for systems utilizing the lower quality tubes stabilized with nonconductive surfactants were over 2 wt % SWCNTs (4 times that of previously reported systems). The introduction of PEDOT:PSS was, once again, found to lower the percolation threshold (to 0.3 wt %) and to increase the ultimate conductivity up to the level of a pure PEDOT:PSS/PS blend. In the PS/PEDOT:PSS-SWCNT systems, the role of the SWCNT network is proposed to be limited to the formation of a template or scaffold on which a (more or less) continuous PEDOT:PSS layer deposits. The ultimate conductivity is therefore determined by the PEDOT:PSS alone.
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http://dx.doi.org/10.1021/nn901643hDOI Listing
April 2010