Kestur U.S.,Purdue University |
Ivanesivic I.,SSCI |
Alonzo D.E.,Purdue University |
Taylor L.S.,Purdue University
The goal of this study was to investigate the influence of particle size on the crystallization kinetics of amorphous felodipine powders stored below the glass transition temperature and to evaluate the role of surface versus bulk crystallization in influencing the kinetic profiles. Raman spectroscopy and powder X-ray diffraction (PXRD) were used to monitor the crystallization of different particle size fractions of pure amorphous felodipine and solid dispersions containing 3. wt.% polyvinyl pyrrolidone (PVP) stored at 30 °C. Optical microscopy was employed to measure surface and bulk growth rates of felodipine in the absence and presence of PVP at 30 °C. Solution concentration-time profiles for the various samples prior to and after crystallization were also measured. The growth rate of felodipine from the free surface was approximately 6 times faster than from the bulk in the absence of any polymer while the ratio of the surface to bulk growth rates was 23 in the presence of PVP. The overall crystallization rates of the amorphous powders were particle size dependent with smaller particle sizes crystallizing faster. Felodipine alone crystallized within 7. days. However, in the presence of the polymer, crystallization was retarded and was incomplete after 6. months. For the polymer-containing powders, biphasic crystallization was observed whereby rapid initial crystallization (50-60. days) was followed by a much slower rate over the remaining time period. The initial rapid crystallization was attributed to a faster rate of surface growth. The dissolution profiles of the different powders were dependent on both the particle size and the extent of crystallinity, with the level of supersaturation achieved varying considerably. © 2012 Elsevier B.V. Source
Park Y.,Korea University |
Boerrigter S.X.M.,SSCI |
Yeon J.,Korea University |
Lee S.H.,Korea University |
And 2 more authors.
Crystal Growth and Design
We obtained a new metastable packing polymorph of donepezil, designated as form K, epitaxially grown on substrate crystals of the more stable form F, or solely by increasing the concentration. Although donepezil is a highly flexible molecule, crystal structure analysis reveals that the molecular conformation is the same as in form F. Donepezil does not form hydrogen bonds and has no appreciable electrostatic interactions. Form K crystals can be grown homogeneously, i.e., without seeds, from highly supersaturated solutions (S > 12). However, in the presence of form F substrates, formation of form K is observed at supersaturations as low as S ≈ 2-3. This suggests form F can serve as a template to form K crystals. Both polymorphs share a structurally identical, common feature of inversion-related molecules. The structures differ in their translational symmetry; hence, they are packing polymorphs. By superimposing the structures and conducting structure fragment calculation using the XPac program, a common two-dimensional plane was identified, the (010) of form F and the (01-1) of form K. Using calculated BFDH morphologies, oriented in their superimposed orientations, epitaxial growth of form K on form F substrate crystals can be made plausible to occur in this fashion. The template effect can thus be understood as resulting from the 2D lattice match, resulting in a low interfacial energy, therefore leading to a low nucleation barrier. © 2016 American Chemical Society. Source
Tan J.S.,Eli Lilly and Company |
Boerrigter S.X.M.,SSCI |
Boerrigter S.X.M.,Purdue University |
Scaringe R.P.,Purdue University |
And 2 more authors.
Journal of Computational Chemistry
The present work details the development of a core-shell model for the purposes of obtaining potential-derived point charges from the ab initio molecular electrostatic potential. In contrast to atomic point charge models, the core-shell model decomposes all atoms into a core with static charge located at a fixed atomic position and a shell with variable charge and position. The optimization of shell charges and positions is discussed. The core-shell model was found to significantly improve description of the ab initio electrostatic potential when compared to potential-derived net atomic point charge models as well as distributed multipoles with contributions up to atomic quadrupole moments. The core-shell model was found to produce similar results as the Weller-Williams lone-pair model and differences in the implementation of the models are discussed. Copyright © 2011 Wiley Periodicals, Inc. Source
Perloff M.,SSCI |
Krishnan R.,Argon ST |
Ramanathan R.,BBN |
Gutierrez C.,SSCI |
And 3 more authors.
Proceedings - IEEE Military Communications Conference MILCOM
As evolving operational requirements compel forces to deploy sparsely there is a need to extend tactical network protocols to operate in the Disruption-Tolerant Networking (DTN) regime, in which frequent disconnections are the norm. In airborne networks 1, connectivity may be temporarily lost when aircraft use different paths to avoid terrain or other obstacles or to perform assigned tasks. In sensor networks, nodes may become disconnected due to environmental conditions or power management, but the set of proximal nodes may not change much over time, despite the intermittent connections. These scenarios challenge standard link-state approaches, both in terms of the protocol overhead and the ability to maintain awareness of all network nodes. Routing protocols designed for the DTN regime, however, cause extreme packet replication to cover the most general conditions conceivable, making them impractical in realistic scenarios. We propose modifications to a widely used link-state protocol, OSPF, to deal with significantly more disruptions to connectivity while retaining the efficiencies of the shortest path approach. We describe simple changes that enable OSPF to find routes through a stable set of neighbors that are only intermittently available. Our use of OSPF minimizes changes and implementation costs, and takes advantage of reliable, proven methods for network state information delivery. We have implemented DTN-extended OSPF within the quagga IP routing framework. Our simulation tests show that, with our modifications, OSPF maintains a global view of the entire network, including both reachable and unreachable nodes, so that messages can be held and routed to temporarily unavailable nodes. Using simulations as well as real packet traffic over an emulated network, we compare DTN-extended OSPF with standard OSPF, and we show that our methods permit messages to be delivered to remote nodes in cases where standard OSPF would discard most or all packets. We conclude by outlining additional methods to reduce DTN overhead traffic and increase DTN-extended OSPF's reliability. © 2011 IEEE. Source
Matousek P.,Cobalt Light Systems |
Thorley F.,SSCI |
Chen P.,SSCI |
Hargreaves M.,SSCI |
And 4 more authors.
Spectroscopy (Santa Monica)
Some of the emerging Raman techniques for rapid noninvasive characterization of pharmaceutical samples and containers are discussed. In pharmaceutical settings, Spatially Offset Raman Spectroscopy (SORS) is beneficial in interrogating the contents of opaque plastic containers, colored glass bottles, and fouled reactor windows. In transmission Raman spectroscopy, the laser beam is incident on the sample at one side and the Raman signal collected from the opposite side. Conventional Raman spectroscopy suffered from severe subsampling and could only yield the Raman signature of the surface layer in either sample orientation. A study was performed using transmission Raman spectroscopy on white capsules containing production-relevant formulations prepared in a laboratory. The results show that it is also possible to characterize the polymorphic content through the low wave number region of the Raman spectrum typically dominated by crystal lattice phonon mode vibrations. Source