Characterization of Nano-Cellulose Based Composites For Biomedical Applications

The number of orthopedic surgeries performed globally has steadily increased over the past decade due to the standardization of procedures as well as technological advancements. During this time orthopedic devices have been composed predominantly of metals, such as Titanium, Vanadium, Molybdenum, and Stainless steel, as well as their alloys, due to the high strength and durability of these materials. However, metals may, in fact, be suboptimal for orthopedic devices. For example, metals exhibit Young’s modulus much greater than the surrounding bone, inducing localized stress-shielding promoting cortical atrophy, which can lead to osteoporosis. In recent years polymers have been successfully explored as a potential substitute for metals in non-load bearing locations. Some of these polymers were designed to be bio-absorbable overtime. Unfortunately, this chemical breakdown results in local acidification, which can leach into proximal bone, causing demineralization and weakening of the surrounding bone, along with increased degradation of the implant itself. In other cases, the resorbed device leaves a “mushy” non-calcified mass that is never fully regrown as a structural bone. Our proposed solution is a cellulose nanofibril (CNF) composite-based platform material for non-load bearing surgical devices (plates, pins, screws). This composite has the potential to be safely bio-resorbable while providing sufficient stiffness during initial implantation, eventually softening overtime promoting the natural formation of strong bone. The method of making composites is adaptive allowing a host of material additives to be introduced during the CNF formation process. Physical properties of the produced composites were analyzed and compared, in particular, flexural modulus, porosity, and Shore D hardness. Which are the primary physical assessments for many orthopedic devices and materials. Additionally, the use of ceramic synthesized biomimetic hydroxyapatite was used in making composites of CNF, this potentially adding a degree of osteoinduction to the CNF. Lastly, aqueous degradation of CNF was monitored and recorded in two separate tests, long period and short hour periods. Flexural modulus, water content increase, volume increase, and Shore D hardness was measured for all samples, with the material loss being monitored solely with long period testing. For use in material degradation, an apparatus was established and utilized for long term testing. Short term testing demonstrated initially drier specimens resistances to water gain, volume gain, and flexural decay over hydrated specimens which showed less resistance to all three parameters. Long term trials displayed the materials longevity in an aqueous solution with minimal material loss, however, specimens had high flexural decay, water gain, and volume gain after 24 hours. Recent results and recommendations are presented

A ring as a model of the main belt in planetary ephemerides

We assess the ability of a solid ring to model a global perturbation induced by several thousands of main-belt asteroids. The ring is first studied in an analytical framework that provides an estimate of all the ring's parameters excepting mass. In the second part, numerically estimated perturbations on the Earth-Mars, Earth-Venus, and Earth-Mercury distances induced by various subsets of the main-belt population are compared with perturbations induced by a ring. To account for large uncertainties in the asteroid masses, we obtain results from Monte Carlo experiments based on asteroid masses randomly generated according to available data and the statistical asteroid model. The radius of the ring is analytically estimated at 2.8 AU. A systematic comparison of the ring with subsets of the main belt shows that, after removing the 300 most perturbing asteroids, the total main-belt perturbation of the Earth-Mars distance reaches on average 246 m on the 1969-2010 time interval. A ring with appropriate mass is able to reduce this effect to 38 m. We show that, by removing from the main belt ~240 asteroids that are not necessarily the most perturbing ones, the corresponding total perturbation reaches on average 472 m, but the ring is able to reduce it down to a few meters, thus accounting for more than 99% of the total effect.Comment: 18 pages, accepted in A&

The Influence of Processing and Additives on Cellulose Nanofiber Properties for Orthopedic Application

Current orthopedics are separated into three different classes of materials, metals, polymers, and ceramics. While these devices have had success throughout the years they are not without their faults. Metallic devices for example are usually extraordinarily stiff when compared with the surrounding bone. This difference in stiffness induces localized stress-shielding promoting cortical atrophy, which can lead to osteoporosis. Polymers while having the capacity of being biodegradable and bioabsorbable also have the potential to incite localized demineralization and weakness in surrounding bone. A result of breakdown byproducts not efficiently being evacuated from the area, which additionally acts as catalysts expediating the degradation rate. Ceramic devices while providing superior osteointegration, with a potential of being comprised from minerals analogous to naturally sourced bone, tend to be extremely brittle causing premature failure of devices. While materials currently used have their benefits, providing medical professionals with sufficient alternatives is imperative, for them to have more variety during operations. Our proposed solution is the use of a recent biopolymer of interest, cellulose nanofibrils (CNF). CNF is a biopolymer that is incredibly naturally abundant, being the base structure sourced from cellulosic materials and byproducts of many agricultural industries. CNF additionally has physical properties that make it a promising material within the orthopedic field. It is morphologically similar to collagen, can be easily chemically modified, and has tunable mechanical properties. CNF, while heavily studied by many research groups has rarely been studied in large bulk. Throughout this thesis processing and additive properties of CNF were determined, including bulk orientation, effects of composites, and crosslinking. Bulk orientation was determined through a multitude of mechanical testing and found an orientation within the large length direction of molds. Composite films were produced under different conditions and tested to view their effects. Crosslinking of CNF was conducted and viewed with an acute submersion and water absorption testing, viewing effects of crosslinker and amount (~2.5 % crosslinker). Finally, a simple computer simulation was made using CNFs now determined properties and placed under known loads experienced by specific orthopedic devices

Physical Properties of Near-Earth Asteroid 2011 MD

We report on observations of near-Earth asteroid 2011 MD with the Spitzer Space Telescope. We have spent 19.9 h of observing time with channel 2 (4.5 {\mu}m) of the Infrared Array Camera and detected the target within the 2{\sigma} positional uncertainty ellipse. Using an asteroid thermophysical model and a model of nongravitational forces acting upon the object we constrain the physical properties of 2011 MD, based on the measured flux density and available astrometry data. We estimate 2011 MD to be 6 (+4/-2) m in diameter with a geometric albedo of 0.3 (+0.4/-0.2) (uncertainties are 1{\sigma}). We find the asteroid's most probable bulk density to be 1.1 (+0.7/-0.5) g cm^{-3}, which implies a total mass of (50-350) t and a macroporosity of >=65%, assuming a material bulk density typical of non-primitive meteorite materials. A high degree of macroporosity suggests 2011 MD to be a rubble-pile asteroid, the rotation of which is more likely to be retrograde than prograde.Comment: 20 pages, 4 figure

Infrared Lightcurves of Near Earth Objects

We present lightcurves and derive periods and amplitudes for a subset of 38 near earth objects (NEOs) observed at 4.5 microns with the IRAC camera on the the Spitzer Space Telescope, many of them having no previously reported rotation periods. This subset was chosen from about 1800 IRAC NEO observations as having obvious periodicity and significant amplitude. For objects where the period observed did not sample the full rotational period, we derived lower limits to these parameters based on sinusoidal fits. Lightcurve durations ranged from 42 to 544 minutes, with derived periods from 16 to 400 minutes. We discuss the effects of lightcurve variations on the thermal modeling used to derive diameters and albedos from Spitzer photometry. We find that both diameters and albedos derived from the lightcurve maxima and minima agree with our previously published results, even for extreme objects, showing the conservative nature of the thermal model uncertainties. We also evaluate the NEO rotation rates, sizes, and their cohesive strengths.Comment: 16 pages, 4 figures, 3 tables, to appear in the Astrophysical Journal Supplement Serie

Constraints on the perturbed mutual motion in Didymos due to impact-induced deformation of its primary after the DART impact

Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA Double Asteroid Redirection Test (DART), part of the Asteroid Impact & Deflection Assessment (AIDA) mission concept. In this mission, the DART spacecraft is planned to impact the secondary body of Didymos, perturbing mutual dynamics of the system. The primary body is currently rotating at a spin period close to the spin barrier of asteroids, and materials ejected from the secondary due to the DART impact are likely to reach the primary. These conditions may cause the primary to reshape, due to landslides, or internal deformation, changing the permanent gravity field. Here, we propose that if shape deformation of the primary occurs, the mutual orbit of the system would be perturbed due to a change in the gravity field. We use a numerical simulation technique based on the full two-body problem to investigate the shape effect on the mutual dynamics in Didymos after the DART impact. The results show that under constant volume, shape deformation induces strong perturbation in the mutual motion. We find that the deformation process always causes the orbital period of the system to become shorter. If surface layers with a thickness greater than ~0.4 m on the poles of the primary move down to the equatorial region due to the DART impact, a change in the orbital period of the system and in the spin period of the primary will be detected by ground-based measurement.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA