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Who is Who in Polymer Science: T
No full textTantayanon, Supawan Tirrell, David A Tirrell, Matthew Tsuji, Masaki Tsvetanov, Christ
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Rational Design of Peptide-Modified Nanoparticles for use as Targeted Diagnostic and Drug Delivery Vehicles
Full text linkThe use of functionalized nanoparticles in biomedical applications has become increasingly more prevalent due to the promise of using these materials as drug delivery and molecular imaging agents. Targeting of specific markers of disease in vivo can be achieved using functionalization techniques, such as modification of nanoparticles with short peptides. Peptides exhibit definite advantages versus whole proteins and their small size leads to the ability to display tens to hundreds of targeting peptides per nanoparticle. In this work, peptide functionalization is utilized to target both model drug delivery and imaging nanoparticles to a site of interest displayed by atherosclerotic plaques. The biocompatibility and clearance of these particles is considered as well as the overall targeting ability. After first establishing the biocompatibility and safety of these materials, the ability to create targeted nanoparticles as well as modular, multifunctional materials through the combination of multiple targeting peptides was assessed.Two strategies are employed in this work: the formation of self-assembled peptide amphiphile micelles as potential drug delivery agents and peptide-functionalized iron oxide particles for use as molecular imaging agents. The establishment of biocompatibility was first determined using a peptide amphiphile micelle, DSPE-PEG2000-CREKA, which specifically binds to fibrin, a marker of late stage atherosclerotic plaques. Both CREKA and PEG micelles were evaluated in vivo using a standard atherosclerosis mouse model, ApoE -/-. Atherosclerosis is characterized by the presentation of multiple markers of the disease and progresses from early to late stage through the growth and development of lipid-core plaques. Injection of small, spherical Cy7-labeled micelles allowed for in vivo near-infrared imaging. These studies showed that these micelles are cleared both through the liver and spleen, therefore involving the reticuloendothelial and renal system, respectively. Biocompatibility was observed through histological staining of excised tissues, determining the level of apoptosis in the liver and spleen, as well as testing for liver function. By varying the mole percent of Cy7 in the micelles, it was possible to determine an optimal regime for both whole body and ex vivo near-infrared imaging.Utilizing the results of the biocompatibility and biodistribution of peptide amphiphile micelle study, it was found that 10 mole percent Cy7 was optimal for future in vivo studies. While late stage targeting of atherosclerotic plaques had previously been demonstrated, the ability to actively target earlier stages of plaque formation with a model drug delivery vehicle formed from peptide amphiphile micelles had yet to be observed. Therefore, the formation of early stage targeting was achieved through DSPE-PEG2000-VCAM micelles labeled with a near-infrared dye, Cy7. These micelles were designed to be small enough to extend their in vivo circulation time and also be spherical. The intended target, vascular cell adhesion molecule-1, or VCAM-1, is expressed by endothelial cells that line the developing plaque, making it a great target for intravenously injected particles. VCAM-1 targeting micelles were shown to accumulate in the cardiovascular system in early stage mice. Immunohistochemistry showed that VCAM-1 expression overlapped with Cy7 dye in the aortic tree, providing evidence for active targeting of early and mid-stage atherosclerotic plaques. While active targeting was shown with VCAM-1 micelles, one of the central advantages to self-assembled micelles is the inherent ability to incorporate multifunctionality through mixing of different amphiphiles. To determine the ability to mix different peptide amphiphiles, micelles composed of DSPE-PEG2000-CREKA and DSPE-PEG2000-VCAM as well as diC16CREKA and diC16VCAM were analyzed. Both systems showed some degree of mixing via transmission electron microscopy and Förster resonance energy transfer and the region over which one population of mixed micelles were formed was established. Mixing was additionally determined using two amphiphiles that individually formed different geometry micelles, either spherical or cylindrical particles. The ability to incorporate otherwise cylindrical-forming amphiphiles into spherical micelles was demonstrated.Each peptide amphiphile study focused on the formation of platform technologies for drug delivery. A strategy to form a targeted contrast agent was additionally developed via peptide functionalization of iron oxide particles. Using CREKA-functionalized contrast agents developed for magnetic particle imaging (MPI), the ability to bind to fibrin ex vivo was shown to be dependent on the amount of fibrin present, demonstrating the specific binding of these nanoparticles. The in vivo biodistribution additionally showed clearance through the liver, as is expected for iron oxide nanoparticles.In each study presented, both the function of the peptide-functionalized nanoparticle system was established as well as the in vivo biodistribution. As the field of targeted delivery agents progresses, the design parameters as well as the establishment of biodistribution, safety, and targeting ability set forth in this work will be a guide for future studies using peptide-functionalized and targeted nanoparticles
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Self-Assembled, Peptide Based Biomaterials for Regenerative Medicine and Drug Delivery
Full text linkA focus of the field of biomaterials is to use directed design to create new materials which replicate and enhance the intricate functions of the human body. Nature's own building blocks, peptides, are an ideal material to create self-assembling biomaterials as they are biodegradable, relatively easy to synthesize, and can be designed with a wide array of functions. In this dissertation, self-assembling peptide materials were optimized for two important medical applications: regenerative medicine and drug delivery. Peptide amphiphiles (PAs), peptides conjugated to fatty acid tails, can self-assemble into both spherical micelles and worm-like micelles. PA worm-like micelles are of particular interest for regenerative medicine applications for their ability to form viscoelastic hydrogels at high concentration. Here we created PA hydrogel systems with active formation and stabilization triggers that are amenable to in situ gelation. Two different methods of in situ gel formation in PA systems were investigated, shear force and pH. Shear-induced formation of worm-like micelles is demonstrated in the PA termed C16-W3K. Before shearing, C16-W3K PAs form spherical micelles in solution and exhibit little to no viscoelasticity. As the solution is subjected to simple shear flow with increasing shear rate, spherical micelles form elongated worm-like micelles up to microns in length. In the C16-W3K PA system, shear force induced the change not only of the micelle structure but also of the peptide secondary structure simultaneously. Worm-like micelle formation was also demonstrated using pH modulation, in the PA termed C16GSH, which was designed with a branched peptide headgroup of histidine and serine amino acids. At low pH, the histidine side chains are protonated and hydrogen bonding does not occur, creating weakly elastic hydrogels. At pH 7.4, above the pKa of the histidine imidazole group, cooperative hydrogen bonding occurs, stabilizing the self-assembled worm-like micelles and creating a strong viscoelastic hydrogel. This unique architecture of C16GSH makes it possible to create hydrogels spanning a wide range of stiffness (0.1-10 kPa). C16GSH were optimized in vitro and in vivo for the application of peripheral nerve regeneration. Peripheral nerve injury is a debilitating condition for which new bioengineering solutions are needed. One strategy to enhance regeneration inside nerve guide conduits is to fill the conduits with a hydrogel to mimic the native extracellular matrix found in peripheral nerves. C16GSH hydrogels were compared to a commercially available collagen gel, which has been previously investigated as a nerve guide filler gel. Schwann cells, a cell type important in the peripheral nerve regenerative cascade, were able to spread, proliferate and migrate better on C16GSH gels in vitro when compared to cells seeded on collagen gels. Moreover, C16GSH gels were implanted subcutaneously in a murine model and were found to be biocompatible, degrade over time, and support angiogenesis without causing inflammation or a foreign body immune response. Taken together, these results help optimize and instruct the development of a new synthetic, hydrogel as a luminal filler for conduit-mediated peripheral nerve repair.In the second half of this dissertation, peptide based complex coacervates were optimized for delivery of protein therapeutics. Complex coacervation is a liquid-liquid phase separation based on the electrostatic association of two oppositely charged polymers in aqueous solution. Coacervation results in micron sized droplets of a dense polymer-rich phase (coacervate) which is separate from the dilute polymer-poor solution phase (aqueous phase). Complex coacervates based on synthetic polypeptides have many desirable features for therapeutic protein delivery. They can be synthetically produced, can be made to be biocompatible and biodegradable, and their formation can be tuned by a wide array of parameters. In this dissertation, a method to encapsulate proteins by complex coacervation using polypeptides is explored. Protein encapsulation with a model protein system: bovine serum albumin (BSA) was demonstrated. Rheological properties were studied to determine the viscoelasticity which may have implications for cell internalization. It was demonstrated that there is tradeoff between loading efficiency and total loading. Therefore, depending on the application, high loading capacity, up to 1:3 molar ratio of protein to polypeptide, or 100% loading of the protein can be achieved, depending on the process and cost of the protein which is often high. Encapsulated BSA retained its secondary structure when encapsulated and was released under conditions of low pH due to disassembly of the coacervate. Lastly, protein loaded coacervates were shown to be non-toxic in a cell viability assay.Polypeptide complex coacervates show promise at encapsulating proteins for therapeutic delivery, but it is difficult to control their size and stability to due dynamic rearrangement and coalescence. To control the size and stability of polypeptide coacervates, the crosslinker EDC was used to create a peptide bond between the amino acid side groups of poly(L-lysine) (PLys) and poly(D/L-glutamic acid) (PGlu). By changing the ratio of PGlu to PLys colloidal stability was achieved without the need for an additional excipient. Surface charge of the particles was also controlled by this method. Final particle size was controlled by both molecular weight and concentration of the polypeptides. A span of particle diameter from to 272nm to 1.3 µm was achieved. Lastly, stability at low pH, where non-crosslinked coacervates disassemble, was demonstrated. A simple and tunable method to control particle size, such as the one presented here provides a possible solution to a major limitation in the field of drug delivery, control of particle size
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Enclosed Microfluidic Platform Realizing Enhanced Stem-cell Survival
Full text linkHere we present a microfluidic culture platform for enhancing single stem cell survival. Traditional plate culture is inadequate for large scale single cell studies because of (1) less than 40% single stem cell survival leading to (2) possible inaccuracies in the biostatistics in single cell studies from lack of sufficient data. This platform mitigates these issues by doubling overall survival rates as well as increasing the available data points by two orders of magnitude, both factors which improve statistical certainty. The platform is fabricated using widely accepted biocompatible polymers and incorporates the novel integration of an enclosed microwell array with an upstream linear gradient generator. The device can selectively trap single cells, tightly control intercell spacing, and change the chemical microenvironment around the stem cells in real-time. In this dissertation, we first characterize the performance of the platform in improving single cell survivability. Subsequently, the system was tested using two difference stem cell types: mouse embryonic stem cells (mES) and induced pluripotent stem cells (iPS). Using this platform we are able to highlight the thresholds of leukemia inhibitory factor concentrations which lead to metastable gene expression of Nanog, a key stem cell pluripotency regulator in mES cells. The enhanced survival of single cells has also enabled the statistically significant observation of Nanog+, non-proliferative, single mES cells which are previously unreported in literature
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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