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Magnetic and Mechanical Properties of Ultrasoft Magnetorheological Elastomers
Get PDFMagnetorheological 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
The interplay between cognitive and affective risks in predicting COVID-19 precautions: a longitudinal representative study of Americans
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Cognitive risk figures prominently in models predicting health behaviors, but affective risk is also important. We examined the interplay between cognitive risk (personal likelihood of COVID-19 infection or death) and affective risk (worry about COVID-19) in predicting COVID-19 precautionary behaviors. We also examined how outbreak severity bias (overestimation of the severity of COVID-19 in one’s community) predicted these outcomes. Design
In a representative sample of U.S. adults (N = 738; Mage = 46.8; 52% women; 78% white), participants who had not had COVID-19 took two online surveys two weeks apart in April 2020. Main outcome measures
We assessed cognitive risk, affective risk, and outbreak severity bias at baseline and at follow-up two precaution variables: prevention behaviors (e.g. social distancing) and behavioral willingness (e.g. vaccinations). Results
Overall, affective risk better predicted precautions than cognitive risk. Moreover, overestimating the severity of the outbreak predicted more affective risk (but not cognitive risk) and in turn more precautions. Additional analyses showed that when affective risk was lower (as opposed to higher) greater cognitive risk and outbreak severity bias both predicted more precautions. Conclusion
These findings illustrate the importance of affective risk and outbreak severity bias in understanding COVID-19 precautionary behavior
The Campbell Collaboration’s systematic review of school-based anti-bullying interventions does not meet mandatory methodological standards
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Many published reviews do not meet the widely accepted PRISMA standards for systematic reviews and meta-analysis. Campbell Collaboration and Cochrane reviews are expected to meet even more rigorous standards, but their adherence to these standards is uneven. For example, a newly updated Campbell systematic review of school-based anti-bullying interventions does not appear to meet many of the Campbell Collaboration’s mandatory methodological standards. Issues
In this commentary, we document methodological problems in the Campbell Collaboration\u27s new school-based anti-bullying interventions review, including (1) unexplained deviations from the protocol; (2) inadequate documentation of search strategies; (3) inconsistent reports on the number of included studies; (4) undocumented risk of bias ratings; (5) assessments of selective outcome reporting bias that are not transparent, not replicable, and appear to systematically underestimate risk of bias; (6) unreliable assessments of risk of publication bias; (7) use of a composite scale that conflates distinct risks of bias; and (8) failure to consider issues related to the strength of the evidence and risks of bias in interpreting results and drawing conclusions. Readers who are unaware of these problems may place more confidence in this review than is warranted. Campbell Collaboration editors declined to publish our comments and declined to issue a public statement of concern about this review. Conclusions
Systematic reviews are expected to use transparent methods and follow relevant methodological standards. Readers should be concerned when these expectations are not met, because transparency and rigor enhance the trustworthiness of results and conclusions. In the tradition of Donald T. Campbell, there is need for more public debate about the methods and conclusions of systematic reviews, and greater clarity regarding applications of (and adherence to) published standards for systematic reviews
Persistent opto-ferroelectric responses in molecular ferroelectrics
Get PDFPersistent 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
Sex differences in cognitive flexibility are driven by the estrous cycle and stress-dependent
Get PDFStress is associated with psychiatric disorders such as post-traumatic stress disorder, major depressive disorder, anxiety disorders, and panic disorders. Women are more likely to be diagnosed with these stress-related psychiatric disorders than men. A key phenotype in stress-related psychiatric disorders is impairment in cognitive flexibility, which is the ability to develop new strategies to respond to different patterns in the environment. Because gonadal hormones can contribute to sex differences in response to stress, it is important to consider where females are in their cycle when exposed to stress and cognitive flexibility testing. Moreover, identifying neural correlates involved in cognitive flexibility could not only build our understanding of the biological mechanisms behind this crucial skill but also leads to more targeted treatments for psychiatric disorders. Although previous studies have separately examined sex differences in cognitive flexibility, stress effects on cognitive flexibility, and the effect of gonadal hormones on cognitive flexibility, many of the findings were inconsistent, and the role of the estrous cycle in stress-induced impacts on cognitive flexibility is still unknown. This study explored potential sex differences in cognitive flexibility using an operant strategy shifting-paradigm after either control conditions or restraint stress in freely cycling female and male rats (with estrous cycle tracking in the female rats). In addition, we examined potential neural correlates for any sex differences observed. In short, we found that stress impaired certain aspects of cognitive flexibility and that there were sex differences in cognitive flexibility that were driven by the estrous cycle. Specifically, stress increased latency to first press and trials to criterion in particular tasks. The female rats demonstrated more omissions and perseverative errors than the male rats; the sex differences were mostly driven by proestrus female rats. Interestingly, the number of orexinergic neurons was higher in proestrus female rats than in the male rats under control conditions. Moreover, orexin neural count was positively correlated with number of perseverative errors made in cognitive flexibility testing. In sum, there are sex differences in cognitive flexibility that are driven by the estrous cycle and are stress-dependent, and orexin neurons may underlie some of the sex differences observed
Mineral displays as embodiments of geologic thought and colonial invisibility
Get PDFMineral displays embodied how we think about minerals: as symbols of social status, scholarly tools, theological objects, and instruments of education. Mineral displays are also representations of how we don’t think about minerals: as human products embedded in wider human histories. This paper reviews the historical themes in mineral display, from the cabinets of curiosity of the Renaissance to modern museums, and articulates a major narrative that has been omitted from mineral display traditions: the human processes that bring mineral specimens from the ground to the display case, particularly Western colonialism and labour. Historically, mineral displays have been used to provoke thought about mineral formation and wider Earth processes; here, too, mineral displays can be used to provoke thought about the human processes that created modern Geology
Reductions of Some Two-Dimensional Crystalline Representations via Kisin Modules
Get PDFWe determine rational Kisin modules associated with 2-dimensional, irreducible, crystalline representations of Gal(Q¯p/Qp) role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-variant-caps: inherit; font-stretch: inherit; line-height: normal; vertical-align: baseline; display: inline; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative; \u3eGal(ℚ⎯⎯⎯⎯⎯p/ℚp) of Hodge–Tate weights 0,k−1 role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-variant-caps: inherit; font-stretch: inherit; line-height: normal; vertical-align: baseline; display: inline; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative; \u3e0,k−1. If the slope is larger than ⌊k−1p⌋ role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-variant-caps: inherit; font-stretch: inherit; line-height: normal; vertical-align: baseline; display: inline; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative; \u3e⌊k−1p⌋, we further identify an integral Kisin module, which we use to calculate the semisimple reduction of the Galois representation. In that range, we find that the reduction is constant, thereby improving on a theorem of Berger, Li, and Zhu
