Harry R. Allcock

Pennsylvania State University, University Park, Maryland, United States

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Publications (406)652.16 Total impact

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    ABSTRACT: Nanoparticles of complex architectures can have unique properties. Self-assembly of spherical nanocrystals is a high yielding route to such systems. In this study, we report the self-assembly of a polymer and nanocrystals into aggregates, where the location of the nanocrystals can be controlled to be either at the surface or in the core. These nanospheres, when surface decorated with nanocrystals, resemble disco balls thus the term nano-disco balls. We studied the mechanism of this surface loading phenomenon and found it to be Ca2+ dependent. We also investigated whether excess phospholipids could prevent nanocrystal adherence. We found surface loading to occur with a variety of nanocrystal types including iron oxide nanoparticles, quantum dots, and nanophosphors, as well as sizes (10-30 nm) and shapes. Additionally, surface loading occurred over a range of polymer molecular weights (~30-3,000 kDa) and phospholipid carbon tail length. We also show that nanocrystals remain diagnostically active after loading onto the polymer nanospheres i.e providing contrast in the case of magnetic resonance imaging for iron oxide nanoparticles and fluorescence for quantum dots. Last, we demonstrated that a fluorescently labeled protein model drug can be delivered by surface loaded nanospheres. We present a platform for contrast media delivery, with the unusual feature that the payload can be controllably localized to the core or the surface.
    ACS Nano 09/2014; 8(9):9143-9153. · 12.03 Impact Factor
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    ABSTRACT: An increased focus exists on the development of materials that might serve as ligament or tendon tissue engineering scaffolds. Requirements for a suitable candidate polymer include biodegradability, biocompatibility, and elasticity. In an attempt to meet these requirements novel citronellol-containing polyphosphazenes were synthesized, characterized, and crosslinked to generate elastomers. Citronellol was chosen as a side group due to its anti-inflammatory properties in addition to the presence of a double bond in its structure to permit polymer crosslinking. Alanine ethyl ester was chosen as a co-substituent to tune hydrolysis rates without severely affecting the glass transition temperatures of the final polymers. Hydrolysis of the uncrosslinked polymers in the form of films in deionized water at 37 °C showed between ∼8 and 16% mass loss and between a ∼28 and 88% molecular weight decline over 12 weeks. Polymers were also crosslinked using ultraviolet radiation for increasing amounts of time. Preliminary mechanical testing of the homo-citronellol polymer indicated increasing modulus and decreasing tensile strength with increased crosslink density. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014
    Journal of Polymer Science Part A Polymer Chemistry 05/2014; · 3.54 Impact Factor
  • Zhicheng Tian, Chen Chen, Harry R. Allcock
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    ABSTRACT: The design and assembly of new organophosphazene polymeric materials based on supramolecular “host–guest” interactions was accomplished by linkage of supramolecular coupling units to either the main-chain terminus or the side-chains of the parent phosphazene polymer. Noncovalent interactions at the main chain terminus were used to produce amphiphilic palm-tree like pseudoblock copolymers via host–guest interactions between an adamantane end-functionalized polyphosphazene and a 4-armed β-cyclodextrin (β-CD) initiated poly[poly(ethylene glycol) methyl ether methacylate] branched-star type polymer. Moreover, noncovalent interactions involving polymer side-chains were achieved between polyphosphazenes with β-CD pendant units and other polyphosphazene molecules with adamantyl moieties on the side-chains. These new organo–phosphazene structures based on noncovalent “host–guest” interactions generate new opportunities for the macromolecular modification of polyphosphazenes. The resultant materials demonstrated useful properties including self-aggregation, supramolecular gelation, and stimulus-responsive behavior.
    01/2014; 47(3).
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    ABSTRACT: Cross-linking in metallo-polyphosphazenes can be prevented by attaching two bidentate donors in a geminal fashion to the phosphorus atoms in the polymer chain.
    Inorganic Chemistry Communications 01/2014; · 2.02 Impact Factor
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    ABSTRACT: Phosphazene polymers with N-linked and O-linked amino acid side groups are of biomedical interest, especially for their ability to bioerode under physiological conditions. Polyphosphazenes containing serine and threonine substituents, which contain two different functional sites for attachment to a polyphosphazene backbone (N- and O-terminus), were synthesized using an improved technique, and their hydrolysis behavior was investigated. In aqueous media the solid polymers yield hydrolysis products phosphate, ammonia, the amino acid, and ethanol and have the potential to be used in several different biomedical applications ultimately determined by their hydrolysis behavior. A hydrolysis study in deionized water revealed that all the polymers are hydrolytically sensitive, regardless of the type of linkage to the polyphosphazene backbone, although the hydrolysis rates may be different. Polymers with amino acid ester side groups linked through the N-terminus underwent solid phase hydrolysis between 16 and 60 % within a 6-week period. This is the fastest reported solid state hydrolysis of any amino acid ester substituted polyphosphazene. The mechanism of hydrolysis is by bulk erosion as monitored by environmental scanning electron microscopy. Polymers with the amino acid units linked through the O-terminus are soluble in water; thus their solid state erosion profile in aqueous media could not be determined. However, 31P NMR spectroscopy confirmed their hydrolytic sensitivity in aqueous solution and the formation of phosphorus-containing oligomeric species, the concentrations of which increased during the 6-week hydrolysis period. Complete hydrolysis did not occur within 6 weeks. The O-linked species are possible starting points for bioerodible hydrogel formation.
    Journal of Inorganic and Organometallic Polymers and Materials 01/2014; 24(1). · 1.17 Impact Factor
  • Jessica L. Nichol, Ian T. Hotham, Harry R. Allcock
    Polymer Degradation and Stability 01/2014; 109:92–96. · 2.77 Impact Factor
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    ABSTRACT: A homologous series of fire-retardant oligoalkyleneoxy-phosphates was synthesized for evaluation as electrolyte media for dye sensitized solar cells (DSSCs) and lithium-batteries. Unoptimized DSSC electrolyte formulations for DSSCs achieved ionic conductivities as high as 5.71×10-3 S∙cm-1, and DSSC test-cell efficiencies up to 3.6% as well as Voc, Jsc and ff up to 0.81 V, 8.03 mA∙cm-2 and 0.69 respectively. Poly(bis-(2-(2-methoxyethoxy)ethoxy)phosphazene) based Li+ conducting gel-electrolytes plasticized with the best performing phosphate had conductivities as high as 9.9×10-4 S∙cm-1 at 30 °C. All the compounds have boiling points higher than 197 °C. The results show that the viscosity, glass transition temperatures, and conductivity of the compounds are dependent mainly on the length of the longest alkyleneoxy chain.
    ACS Applied Materials & Interfaces 12/2013; · 5.90 Impact Factor
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    ABSTRACT: Mixed-substituent fluoroalkoxyphosphazene polymers bearing ∼15% 1H,1H,2H,2H-perfluorooctan-1-oxy or 1H,1H,2H,2H-perfluorodecan-1-oxy side groups together with trifluoroethoxy cosubstituent groups were synthesized. The low reactivity of the long-chain fluoroalkoxides and their limited solubility in organic solvents prevented higher levels of substitution. Moreover, the sodium alkoxides with two methylene residues adjacent to the oxygen proved to be unstable in solution due to elimination of NaF and precipitation of side products, and this limited the time available for chlorine replacement reactions. The resulting cosubstituent polymers were characterized by proton nuclear magnetic resonance (1H-NMR), 31P-NMR, 19F-NMR, gel-permeation chromatography, and differential scanning calorimetry. Unlike homo- or mixed-substituent fluoroalkoxyphosphazene polymers, such as [NP(OCH2CF3)2]n (a microcrystalline thermoplastic, Tg ∼ −63°C, Tm ∼ 242°C) or [NP(OCH2CF3)(OCH2(CF2)xCF2H)]n (PN-F, a rubbery elastomer, Tg ∼ −60°C, but no detectable Tm), the new polymers are gums (Tg ∼ −50°C, but no detectable Tm) with molecular weights in the 105 g/mol rather than the 106 g/mol range. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers
    Polymer Engineering and Science 08/2013; · 1.24 Impact Factor
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  • Zhicheng Tian, Chen Chen, Harry R. Allcock
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    ABSTRACT: Biodegradable poly(organophosphazenes) containing side chains of various oligo(ethylene glycol) methyl ethers (mPEGs) and glycine ethyl ester units were synthesized and characterized. Novel supramolecular-structured hydrogel systems based on the inclusion complex between the mPEG grafted polyphosphazenes and α-cyclodextrin were prepared in aqueous media. The gelation time depended on the length of the mPEG side chains, the molar ratio between mPEG repeat units and α-cyclodextrin, and the concentration of the polymeric gel precursors. The rheological measurements of the supramolecular hydrogels indicate a fast gelation process and flowable character under a large strain. The hydrogel systems demonstrate unique structure-related reversible gel–sol transition properties at a certain temperature due to the reversible supramolecular assembly. The formation of a channel-type inclusion complex induced gelation mechanism was studied by DSC, TGA, 13C CP/MAS NMR, and X-ray diffraction techniques. The strong potential of the system for injectable drug delivery applications was explored with the use of bovine serum albumin as a model protein for in vitro release studies. All the supramolecular hydrogels studied showed disintegration by dethreading of the α-cyclodextrin. Polymers with longer poly(ethylene glycol) side chains had better stability and slower protein release profiles. The molecular weights of the polymers were monitored by GPC to show the biodegradability of the hydrogel system.
    Macromolecules 04/2013; 46(7):2715-2724. · 5.93 Impact Factor
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    ABSTRACT: A series of novel polyphosphazene ionomers with short chain poly(ethylene oxide) (PEO) moieties, bound ammonium cations, and free iodide anions were previously synthesized. Ion dynamics during anion conduction of the ionomers were studied by dielectric relaxation spectroscopy (DRS). These polyphosphazenes provide interesting conductive materials to study because of their low glass transition temperature, high segmental mobility, and high ion content. Analysis of DRS results provides static dielectric constant, conducting ion mobility, and conducting ion content for the materials. An increase in the length of the alkyl group extending from the polymer-bound ammonium cation increases conductivity and conducting ion concentration due to new steric interactions weakening ion-ion associations that restrict segmental mobility. By placing ether oxygens in the short alkyl group a large increase in conductivity and a decrease in the glass transition temperature is observed due to strong associations between the cation and ether oxygen lone pairs.
    03/2013;
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    ABSTRACT: Novel polyphosphazenes containing the fluoroquinolone antibiotic substituents, ciprofloxacin or norfloxacin, were synthesized and characterized. Nano/microfibers of several selected polymers were prepared by an electrospinning technique. The sensitivity to hydrolysis, pH behavior, and antibiotic release profile of all the polymers as films and the selected polymers as nano/microfibers were investigated. The hydrolysis release behavior was further studied by an in vitro antibacterial test against E. coli. Protection and deprotection reactions of ciprofloxacin and norfloxacin were carried out before and after polymer synthesis to prevent cross-linking of multi-functional reagents with polymers. No more than 25 mol% of antibiotics can be introduced to poly(dichlorophosphazene) which was synthesized by a thermal ring opening polymerization of hexachlorocyclotriphosphazene. Various amino acid ethyl esters (glycine, alanine, and phenylalanine) were linked to the polymers as co-substituents to increase the solubility as well as to control the rate of antibiotic release. Depending on the polymer compositions, 5–23% weight loss and 4–30% antibiotic release were observed in a six week hydrolysis study at 37 °C. The corresponding nano/microfibers showed a much faster degradation and antibiotics release due to a substantially larger surface area. The in vitro antibacterial tests showed an antibacterial effect as long as ciprofloxacin or norfloxacin was released from the polymers. Meanwhile, all the polymers gave a near-neutral hydrolysis environment with the pH ranging from 5.9 to 6.8.
    Polym. Chem. 02/2013; 4(6).
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    ABSTRACT: This report describes the surface morphology and wetting behavior of one aryl and three fluorous polyphosphazenes: poly[bis(2,2,2-trifluoroethoxy)phosphazene], PTFEP, 1, poly[bis(para-fluorophenoxy)phosphazene] 2, poly[bis(meta-trifluoromethylphenoxy)phosphazene] 3, and poly(diphenoxyphosphazene) 4. Thermal analysis confirms high thermal stability for these polyphosphazenes. With phenyl side chains, 2–4 are particularly stable with T5% >400 °C. Surface morphology via tapping mode atomic force microscopy (TM-AFM) and dynamic contact angles (DMA) are correlated with composition and crystallinity estimated by differential scanning calorimetry (DSC). The resulting structure property relationships define trends in processability, hydrophobicity and oleophobicity. Compared to 1, 3, and 4 poly[bis(para-fluorophenoxy)phosphazene] 2 is noteworthy by virtue of a high melting point (179 °C), high crystallinity (ΔHf, 30.2 J/g), and a moderate molecular weight. The remaining polyphosphazenes in this group have Tm <100 °C. Heats of fusion (J/g) are in the order 1 (29.4) ≈ 2 (30.2) > 3 (17) >> 4 (4.0). High molecular weights for 3 and 4 (>103 kDa) would make melt processability problematic. Thus 1 or 2 has a favorable combination of properties, with 2 favored for many applications due to a higher Tm. In summary, for performance and processing, high Tm, and resistance to water and hydrocarbons poly[bis(para-fluorophenoxy)phosphazene], 2 is attractive. This finding suggests other para-substituted phenyl phosphazenes are worth further study.
    Polymer. 02/2013; 54(3):1123–1129.
  • Xiao Liu, Zhicheng Tian, Chen Chen, Harry R. Allcock
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    ABSTRACT: UV-cleavable star polymers composed of a well-defined 6-arm amphiphilic block copolymer and a UV-cleavable core with photolabile o-nitrobenzyl groups have been synthesized and characterized. The core of the star polymer is a cyclotriphosphazene, which can biodegrade to phosphate and ammonium ion. The resultant unimolecular micelles can be dissociated in a controlled manner by UV irradiation. The inner lipophilic poly(methyl methacrylate) (PMMA) and the outer hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) were grafted by atom transfer radical polymerization (ATRP), leading to the formation of star-PMMA-PPEGMA with different compositions. The effects of various factors, such as molecular weight, solution concentration, solvents (THF, toluene, and water) and monomers (MMA, PEGMA, and 2-(dimethylamino)ethyl methacrylate (DMA)) on the photodegradation rate of the star polymers were studied by gel permeation chromatography (GPC). The micellar behavior of star-PMMA-PPEGMA was unambiguously demonstrated by dynamic light scattering (DLS), fluorescence techniques and transmission electron microscopy (TEM). The formation of nanoparticles (star-AGG) from the aggregation of the two star-PMMA-PPEGMA polymer micelles in aqueous solution was detected by DLS with hydrodynamic radii of 86 and 111 nm. The critical aggregation concentration (CAC) of star-AGG-2 from star-PMMA179-PPEGMA89-2 was 0.0026 g L−1 and 0.022 g L−1 before and after UV-irradiation indicating the reduced stability of the polymer micellar structures after UV-irradiation due to the detachment of the amphiphilic arms from the cyclotriphosphazene core. As a result, spontaneous dissociation of cleaved micelles can be induced by the dilution effect in the human body for stimulus-controlled drug release.
    Polym. Chem. 01/2013; 4(4):1115-1125.
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    ABSTRACT: New polyphosphazenes were designed, synthesized, and characterized to determine their potential as scaffolds for ligament and tendon tissue engineering. The carboxylic acid moiety of the amino acids L-alanine and L-phenylalanine were protected with alkyl esters with increasing chain length from 5 to 8 carbon atoms. This combined the hydrolytic sensitivity of the amino acid ester polyphosphazenes with the elastomeric characteristics induced by the long chain alkoxy polyphosphazenes. Test side group substitution reactions were performed on the cyclic small molecule model, hexachlorocyclotriphosphazene (NPCl2)3, to determine if steric hindrance would inhibit the degree of chlorine replacement by the amino acid ester units. Counterpart polymers were then synthesized by replacement of the chlorine atoms in poly(dichlorophosphazene) (NPCl2)n by the same amino acid esters. The glass transition temperatures of the polymers decreased with increasing alkyl ester chain length, ranging from 11.6 to −24.2 °C. Polymer hydrolysis was studied for solid samples in deionized water at physiological temperature for 12 weeks. The starting pH was 6.3 and the final pH ranged between 5.2 and 6.8. Polymer film mass decreased between 8.7 and 26 percent during the 12 week period, while the molecular weights decreased 57 to 99 percent.
    Polym. Chem. 01/2013; 4(3):600-606.
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    ABSTRACT: The synthesis and characterization of novel poly (CTFE-g-oligoEO) graft copolymers [chlorotrifluoroethylene (CTFE) and ethylene oxide (EO)] are presented. First, vinyl ether monomers bearing oligo(EO) were prepared by transe-therification of x-hydroxyoligo(EO) with ethyl vinyl ether catalyzed by a palladium complex in 70–84% yields. Two vinyl ethers of different molecular weights (three and 10 EO units) were thus obtained. Then, radical copolymerization of the above vinyl ethers with CTFE led to alternating poly(CTFE-alt-VE) copolymers that bore oligo(OE) side chains in satisfactory yields (65%). These original poly(CTFE-g-oligoEO) graft copolymers were characterized by 1 H, 19 F, and 13 C NMR spec-troscopy.
    Journal of Polymer Science Part A Polymer Chemistry 01/2013; 51(4):977. · 3.54 Impact Factor
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    ABSTRACT: Hard tissues are made of nanoapatite crystallites grown on a nanofibrous collagen matrix. The mechanical interlocking and the chemical bonding between both phases provide the unique properties of hard tissues. A biodegradable alanine-substituted polyphosphazene nanofibrous scaffold was prepared by an electrospinning technique. Scaffolds were loaded with precursors that form Ca-deficient hydroxyapatite upon hydrolysis in aqueous media. Composite scaffolds containing 30, 60, and 90 wt% were subjected to hydrolysis in a phosphate buffer solution for up to 10 days, and was followed by pH measurements, x-ray diffraction and scanning electron microscopy. Results showed a delayed conversion of the precursors into Ca-deficient apatite, which was proven to be attributed to the encapsulation of the precursors within the polymer nanofibrous scaffold and the slow introduction of water of hydrolysis to the precursors. This was accompanied by an increasing swelling of the nanofibers. An overall buffering effect took place within the system as a result of the degradation of the polymeric nanofibers, maintaining pH of the media within physiologic pH values.
    Ceramics International. 01/2013; 39(1):519–528.
  • Chen Chen, Xiao Liu, Zhicheng Tian, Harry R. Allcock
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    ABSTRACT: High polymeric organophosphazenes have been synthesized with trichloroethoxy side groups either as the sole substituents or as cosubstituents with trifluoroethoxy, phenoxy, or methoxyethoxyethoxy groups. Initially, small-molecule model compound studies were carried out between sodium trichloroethoxide and hexachlorocyclotriphosphazene at ambient temperature to yield the fully substituted product hexakis(trichloroethoxy)cyclotriphosphazene as a prelude to the synthesis of high polymeric trichloroethoxy homo- and cosubstituted phosphazenes. The cyclic trimeric and polymeric species were characterized by 1H and 31P NMR, GPC, and DSC techniques. Physical property comparisons were made with the long-established elastomers that contain both trifluoroethoxy and longer chain fluoroalkoxy side groups by the use of DSC, TGA, and limited oxygen index tests. The introduction of trichloroethoxy side groups further improves the resistance of these polymers to combustion.
    Macromolecules 11/2012; 45(22):9085-9091. · 5.93 Impact Factor
  • Xiao Liu, Jonathan P. Breon, Chen Chen, Harry R. Allcock
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    ABSTRACT: Side group exchange reactions have been studied for linear high polymeric organophosphazenes, [N═P(OR2)]n (n 15 000). Specifically, the exchange behavior of polymers was examined where OR = OCH2CF3, OCH2CF2CF2CF2CF2H, OCH2Cl3, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide. No aryloxy group replacement by trifluoroethoxy was detected, probably due to the well-protected reactive sites of the polyphosphazenes shielded by aryloxy side groups. For the exchange behavior of [N═P(OCH2CF3)2]n and [N═P(OCH2CF2CF2CF2CF2H)2]n with NaOCH2CF2CF2CF2CF2H and NaOCH2CF3, partial substituent exchange was achieved for both reactions. Furthermore, under conditions with excess nucleophile these side group exchange reactions are followed by reactions that introduce sodium–oxo groups attached to phosphorus in place of organic substituents, and this is a mechanism for subsequent hydrolysis and molecular weight decline in the presence of water. Thus, the relative instability of high polymeric [N═P(OCH2CF3)2]n in the presence of excess NaOCH2CF3 emerged as an explanation for problems with the synthesis of [NP(OCH2CF3)2]n and [NP(OCH2CF3)x(OCH2(CF2)mCF2H)2–x]n, polymers that have attracted wide fundamental and practical interest. However, substituent exchange reactions do appear to be an alternative synthetic approach for the synthesis of some polyphosphazenes, especially mixed-substituted elastomers and the recently reported trichloroethoxy/trifluoroethoxy-containing cosubstituted polyphosphazenes.
    Macromolecules 11/2012; 45(22):9100-9109. · 5.93 Impact Factor
  • Xiao Liu, Jonathan P Breon, Chen Chen, Harry R Allcock
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    ABSTRACT: Side-group-exchange reactions have been studied for short-chain linear oligomeric phosphazenes, (RO)(4)P[N═P(OR(2))](n)OR (n = 6, 10, 20, and 40) as models for the corresponding linear high polymers (n ∼ 15000). Specifically, the exchange behavior of oligomers where OR = OCH(2)CF(3), OC(6)H(5), OC(6)H(4)CHO-p, OC(6)H(4)CN-p, and OC(6)H(4)NO(2)-p with sodium trifluoroethoxide was examined. The ease of aryloxy group replacement by trifluoroethoxy increased with the electron-withdrawing character in the order OR = OC(6)H(5) ≪ OC(6)H(4)CHO-p < OC(6)H(4)CN-p < OC(6)H(4)NO(2)-p, but the reaction was efficient only if the phosphazene contained no more than 20 repeating units. However, attempts to force slower reactions by the use of excess sodium trifluoroethoxide induced secondary reactions at the trifluoroethoxy units already introduced to produce CF(3)CH(2)OCH(2)CF(3) and generate -O(-)Na(+) units in their place. The longest chain model underwent side-group-exchange reactions preferentially at the end units. These results are significant for the synthesis of phosphazene high polymers with fluoroalkoxy and aryloxy side groups.
    Inorganic Chemistry 10/2012; · 4.59 Impact Factor

Publication Stats

3k Citations
652.16 Total Impact Points

Institutions

  • 1981–2014
    • Pennsylvania State University
      • Department of Chemistry
      University Park, Maryland, United States
  • 2003–2012
    • University of Virginia
      • • Department of Chemistry
      • • Department of Orthopaedic Surgery
      • • Department of Chemical Engineering
      Charlottesville, Virginia, United States
  • 2009–2011
    • SASTRA University
      • Centre for Nanotechnology & Advanced Biomaterials [CeNTAB]
      Thanjāvūr, State of Tamil Nadu, India
  • 2008–2010
    • University of Connecticut
      • Department of Orthopaedic Surgery
      Mansfield City, CT, United States
    • William Penn University
      University Park, Florida, United States
  • 2002
    • Drexel University
      • Department of Chemical and Biological Engineering
      Philadelphia, PA, United States
  • 1998
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, MA, United States
    • Morris Brown College
      Atlanta, Georgia, United States
  • 1991
    • The University of Calgary
      • Department of Chemistry
      Calgary, Alberta, Canada