Magnetic and Mechanical Properties of Ultrasoft Magnetorheological Elastomers

Magnetorheological elastomers (MREs), composite materials consisting of magnetic particles embedded in a non-magnetic elastomeric matrix, can reversibly modulate their mechanical and magnetic properties through tuning the applied magnetic field H. Recently, ultrasoft MREs have received tremendous attention due to their great potential in biomedical applications. However, the effects of the polymer stiffness and magnetic particle concentration on the magnetic and mechanical properties of ultrasoft MREs still need to be better understood. In this dissertation, the author presents a comprehensive investigation of the magnetic and mechanical properties of ultrasoft MREs as well as their biomedical applications. The effect of polymer stiffness on magnetization reversal of MREs has been investigated using a combination of magnetometry measurements and computational modeling. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A two-dipole model that incorporates magneto-mechanical coupling not only confirms that micron-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses. Measurements of the moduli and surface roughness of ultrasoft MREs at various H’s reveal a sensitive dependence on the magnetic particle concentration and H. As increases from 0 to 23%, ultrasoft MREs at =95.5 kA/m (1200 Oe) show an increase of ≈41×,11×,and 11× in their shear storage, Young’s modulus, and surface roughness, respectively. The moduli and surface roughness can be fit to quadratic functions of and H. The presented magnetic and mechanical properties of ultrasoft MREs provides the framework for applying the MREs as dynamic platforms in biomedical engineering. Ultrasoft MREs have been applied to investigate the response of cells to 2D and 3D dynamic mechanical stimuli. Furthermore, the field-dependent particle motion observed in ultrasoft MREs has inspired an application for creating 3D heterogeneous cellular gradients. This work was performed under the guidance of the author’s thesis advisor, Professor Xuemei Cheng

College Student Success: Using Predictive Modeling and Actionable Intelligence with a Faculty Centered Information Portal to Improve Student Academic Performance

This research project examines new models and approaches to student learning and success by concentrating on the first-year experience of beginning freshmen at Valdosta State University utilizing data from 2008-2014. With a fall freshman class ranging from 1,500 to 2,500 new students, the sample size is large enough to produce a much smaller confidence interval/sampling error, yet small enough to work with individual departments and faculty to implement and monitor the effect of changes employed through the use of predictive metrics and active intervention. The predictive metrics developed for this model use three specific indicators: (1) standardized test scores from the SAT or ACT, (2) high school grade point average and (3) where the student’s high school ranks in relation to the other high schools in the state of Georgia. The purpose of this research is to develop and defend the answer in response to the research question: Can predictive modeling be used to create actionable student intelligence to improve the grades in key English and math classes resulting in higher retention rates of traditional first-year students? The findings from this research demonstrate that predictive modeling can be very effective in identifying at-risk student populations. These models provide timely insight into students’ needs for additional support to be successful academically. There were five important clusters of results: (1) the pass/fail rates based upon the 1-4 rankings for high school rank, GPA, and SAT, with these data points proving to be very useful in predicting DFW rates, (2) the multivariate regression analysis also showed that these variables are statistically significant, (3) for math the difference of means test for the changes over time once the placement index was put in place improved the pass rate in math courses, (4) the analysis of financial grouping and employment index showed that these variables also impact student success, (5) student success improved with faculty that utilized the portal vs. faculty that did not utilize the portal. This research is very closely aligned with the “Complete College America” movement.Chapter I: STATE OF HIGHER EDUCATION 1 Background 1 Scope of Study 3 Significance of Problem 4 Research Question 7 Overview of the Dissertation 8 Chapter II: A STUDY OF COLLEGE STUDENT SUCCESS 11 The Impact of Financial Aid 25 Standardized Indicators 29 Chapter III: METHODOLOGY 39 Predictive Modeling 39 Faculty Portal 49 Chapter IV: RESULTS 55 Introduction 55 Predictive Modeling 55 Taking Math One Step Further 72 Financial Aid Predictive Models 84 Faculty Portal, Early Alert, and Interventions 88 Chapter V: PREDICTIVE ANALYTICS AND THE FUTURE OF STUDENT SUCCESS 92 Research Question and Key Findings 92 Lessons Learned 101 High-Impact Practices 102 Additional Avenues of Research and Study 107 Cautionary Warning and Conclusion 109 REFERENCES 111 APPENDIX A: Institutional Review Board Exemption 117 APPENDIX B: Complete Data Sheet 1190 -LaPlant, JamesSavoie, Michael P.Yehl, Robert (Sherman)Richards, Connie L.D.P.A.Public Administratio

Magneto-Driven Gradients of Diamagnetic Objects for Engineering Complex Tissues

Engineering complex tissues represents an extraordinary challenge and, to date, there have been few strategies developed that can easily recapitulate native‐like cell and biofactor gradients in 3D materials. This is true despite the fact that mimicry of these gradients may be essential for the functionality of engineered graft tissues. Here, a non‐traditional magnetics‐based approach is developed to predictably position naturally diamagnetic objects in 3D hydrogels. Rather than magnetizing the objects within the hydrogel, the magnetic susceptibility of the surrounding hydrogel precursor solution is enhanced. In this way, a range of diamagnetic objects (e.g., polystyrene beads, drug delivery microcapsules, and living cells) are patterned in response to a brief exposure to a magnetic field. Upon photo‐crosslinking the hydrogel precursor, object positioning is maintained, and the magnetic contrast agent diffuses out of the hydrogel, supporting long‐term construct viability. This approach is applied to engineer cartilage constructs with a depth‐dependent cellularity mirroring that of native tissue. These are thought to be the first results showing that magnetically unaltered cells can be magneto‐patterned in hydrogels and cultured to generate heterogeneous tissues. This work provides a foundation for the formation of opposing magnetic‐susceptibility‐based gradients within a single continuous material

Magnetic field tuning of mechanical properties of ultrasoft PDMS-based magnetorheological elastomers for biological applications

We report tuning of the moduli and surface roughness of magnetorheological elastomers (MREs) by varying applied magnetic field. Ultrasoft MREs are fabricated using a physiologically relevant commercial polymer, SylgardTM 527, and carbonyl iron powder (CIP). We found that the shear storage modulus, Young\u27s modulus, and root-mean-square surface roughness are increased by ∼41×, ∼11×, and ∼11×, respectively, when subjected to a magnetic field strength of 95.5 kA m−1. Single fit parameter equations are presented that capture the tunability of the moduli and surface roughness as a function of CIP volume fraction and magnetic field strength. These magnetic field-induced changes in the mechanical moduli and surface roughness of MREs are key parameters for biological applications

Interfacial and Surface Magnetism in Epitaxial NiCo2O4(001)/MgAl2O4 Films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry, x-ray magnetic circular dichroism (XMCD) based on x-ray absorption spectroscopy, and spin-polarized inverse photoemission spectroscopy (SPIPES) with various thickness down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion

Interfacial and surface magnetism in epitaxial NiCo\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e(001)/MgAl\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion

The effect of polymer stiffness on magnetization reversal of magnetorheological elastomers

Ultrasoft magnetorheological elastomers (MREs) offer convenient real-time magnetic field control of mechanical properties that provides a means to mimic mechanical cues and regulators of cells in vitro. Here, we systematically investigate the effect of polymer stiffness on magnetization reversal of MREs using a combination of magnetometry measurements and computational modeling. Poly-dimethylsiloxane- based MREs with Young’s moduli that range over two orders of magnitude were synthesized using commercial polymers SylgardTM 527, Sylgard 184, and carbonyl iron powder. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A simple two-dipole model that incorporates magneto-mechanical coupling not only confirms that micrometer-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis of ultrasoft MREs but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses

YYY Dynamic Tuning of Viscoelastic Hydrogels with Carbonyl Iron Microparticles Reveals the Rapid Response of Cells to Three-Dimensional Substrate Mechanics

Current methods to dynamically tune three-dimensional hydrogel mechanics require specific chemistries and substrates that make modest, slow, and often irreversible changes in their mechanical properties, exclude the use of protein-based scaffolds, or alter the hydrogel microstructure and pore size. Here, we rapidly and reversibly alter the mechanical properties of hydrogels consisting of extracellular matrix proteins and proteoglycans by adding carbonyl iron microparticles (MPs) and applying external magnetic fields. This approach drastically alters hydrogel mechanics: rheology reveals that application of a 4000 Oe magnetic field to a 5 mg/mL collagen hydrogel containing 10 wt % MPs increases the storage modulus from approximately 1.5 to 30 kPa. Cell morphology experiments show that cells embedded within these hydrogels rapidly sense the magnetically induced changes in ECM stiffness. Ca2+ transients are altered within seconds of stiffening or subsequent softening, and slower but still dynamic changes occur in YAP nuclear translocation in response to time-dependent application of a magnetic field. The near instantaneous change in hydrogel mechanics provides new insight into the effect of changing extracellular stiffness on both acute and chronic changes in diverse cell types embedded in protein-based scaffolds. Due to its flexibility, this method is broadly applicable to future studies interrogating cell mechanotransduction in three-dimensional substrates

Persistent opto-ferroelectric responses in molecular ferroelectrics

Persistent photoresponses require optical excitations to metastable states, which are rare of ionic origin due to the indirect photon-ion interaction. In this work, we explore the photoinduced metastable proton states in the proton-transfer type molecular ferroelectric croconic acid. We observe that, after the photoexcitation, the changes of structural and ferroelectric properties relax in ∼10^3s, indicating persistent photoresponses of ionic origin. In contrast, the photoconductivity relaxes within 1 s. The 10^3s timescale suggests that the ionic metastable states result from proton transfer both along and out of the hydrogen bonds. This discovery unveils an ionic mechanism for the phototunability, which offers persistent opto-ferroelectric control for proton-transfer type molecular ferroelectrics