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Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains. Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy. A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques. The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction. In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress.

The residual effect of thermally and mechanically loaded polyurea samples was investigated in this study using terahertz time-domain spectroscopy (THz-TDS). Samples of different thicknesses were submerged in liquid nitrogen and allowed to reach cryogenic isothermal conditions while another set of samples were extracted from quasi-statically loaded strips. All samples were interrogated using THz-TDS since terahertz waves exhibit non-ionizing, nondestructive interactions with polymers. The time-domain terahertz signals were used to extract the optical and electrical properties as a function of sample thickness and loading conditions. The residual effect was prominent in the mechanically loaded samples compared to a nearly negligible presence in thermally loaded polyurea. On average, the results of the thermally loaded samples were subtle when compared to the virgin samples, whereas samples that were mechanically stretched showed a considerable difference in the characteristics of the time-domain signals. For example, the peak amplitude in the time-domain signal of the stretched thick sample showed a 9% difference from that of the virgin sample, whereas the thermally loaded sample saw only a 4.9% difference. Spectral analysis reported the frequency-dependent, complex refractive index of virgin and loaded polyurea as a function of thickness and spectral peaks associated with fundamental vibrational modes of the polyurea structure. The disappearance of three spectral peaks, 0.56 THz, 0.76 THz, and 0.95 THz, elucidated the residual effect of the mechanically loaded samples. In general, terahertz spectroscopy was shown to be a promising tool for future in situ and in operando investigations of field-dependent polymer responses.

Neural prosthetics, which are increasingly being considered for the dual functionalities of recording and stimulation, are implanted in a corrosive biochemical environment that requires them to possess superior electrical and electrochemical stability and performance. These probes are required to withstand these operating conditions through billions of cycles of pulses of electrical stimulations and also maintain electrochemical sensitivity for potential applications in voltammetry. In this research, microelectrodes made of two material systems; namely, platinum and glassy carbon, supported on a flexible substrate are fabricated and investigated for correlation between process parameters and the electrochemical efficacy of the neural interfaces, particularly charge storage capacity and corrosion rate. Using scanning electron and atomic force microscopies, the correlation between process parameters, surface morphology and topography in both platinum and glassy carbon were investigated. The results demonstrate that changes in surface topography and the rate of corrosion are correlated to variations in the process parameters. Furthermore, the results indicate a relationship between surface roughness and corrosion rate, in which the increase or decrease of the former corresponds to a similar change in the latter.

Polyurea has been integrated in various applications ranging from moisture and chemical resistant coatings to improving the impact mitigation of structures. These applications imply that extended exposure to the sun is imminent. Ultraviolet (UV) radiation with energies higher than the activation energy of polyurea can be detrimental to the mechanical and physical properties based on changes in its segmental microstructure. In this paper, the evolution of the phase-segregated microstructure of polyurea was evaluated as a function of extended ultraviolet exposure at an accelerated factor of 4.28 of artificial weathering, where one year of natural weathering was accomplished in less than 15 weeks. Several sets of exposed samples were removed from the exposure environment at various times within 15 weeks and were characterized using atomic force microscopy and thermogravimetric analysis. First, analysis of the phase morphology using the atomic force microscope demonstrated the segmental evolution of the polyurea surface, where the hard segments had agglomerated, and the soft segments appeared to etch away as ultraviolet radiation increased. The concentration of the hard segments increased from 12% within 3 weeks of exposure to 23% at the 15th week of exposure. Second, gravimetric analysis of the pyrolysis of polyurea samples related the exposure duration and penetration depth of the degraded layer. The thermogravimetric results showed a reduction in the weight percent of the soft segment, i.e., the degradation of the soft segment occurring at ~300°C, with increasing UV exposure duration. At 15 weeks, the weight loss at ~300°C was 22% compared to the ~80% of pyrolyzed virgin polyurea. The gravimetric reporting of the hard segments was in good agreement with phase data from the atomic force micrographs.

Emerging polymeric foams exhibiting unique microstructure of microspherical shells with reinforcing dense microspheres creates a new opportunity for impact-tolerant foam paddings in sport gears applications. This paper describes the static response of reinforced microcell consisting of an outer spherical shell and uniformly distributed microspheres while quantifying the stiffening effect. The distribution of the microspheres is illustrated using the Fourier series, allowing tuning of the reinforcing strategy. Expressions of the external and internal works are derived, whereas the Ritz energy method is adopted to calculate the deformations due to a compressive load distributed over a range of areas. Emphasis is given to the effect of the geometrical attributes of the microcell and the reinforcing microspheres on the resulting deformation response and stiffening effect. The framework is used to investigate the response of several case studies to elucidate the effects of relative radii ratio, reinforcement density, microcell wall thickness, and loading configurations on the stiffness. A new normalized strain energy parameter is introduced to simplify and accelerate the analysis while providing insights on the underpinnings of the observed buckling response. The results strongly suggest the viability of the newly discovered foam microstructure in managing static loads while providing an opportunity to strategically tune the mechanical response using the analytical framework presented herein.

This paper reports the unique microstructure of polyurea foams that combines the advantages of open and closed cell polymeric foams, which were synthesized through a self-foaming process. The latter was the result of aggressive mechanical mixing of diamine curative, isocyanate, and deionized water at ambient conditions, which can be adjusted on-demand to produce variable density polyurea foam. The spherical, semi-closed microcellular structure has large perforations on the cell surface resulting from the concurrent expansion of neighboring cells and small holes at the bottom surface of the cells. This resulted in a partially perforated microcellular structure of polyurea foam. As a byproduct of the manufacturing process, polyurea microspheres nucleate and deposit on the inner cell walls of the foam, acting as a reinforcement. Since cell walls and the microspheres are made of polyurea, the resulting reinforcement effect overcomes the fundamental interfacial issue of different adjacent materials. The partially perforated, self-reinforced polyurea foam is compared to the performance of traditional counterparts in biomechanical impact scenarios. An analytical model was developed to explicate the stiffening effect associated with the reinforcing microspheres. The model results indicate that the reinforced microcell exhibited, on average, ~30% higher stiffness than its barren counterpart.

The applicability of additive manufacturing (AM) continues to expand because of research and development efforts in industrial, academic, and governmental institutions. The lure of additive manufacturing lies in the quick time-to-market and ability to produce parts and components with high degrees of topographical complexity empowered by the layer-by-layer production approach. One challenge that is attracting a significant amount of attention is improving the multi-functionality of additively manufactured parts, as enabling multi-functionality will result in transitioning AM to a broader application domain. The objective of this paper is to report novel developments that improve the functionality of polymer-based parts by adding electrical conductivity and fluid management to the existing load-bearing capabilities. A space structure was 3D-printed using acrylonitrile butadiene styrene (ABS) with embedded internal channels throughout the entire structure and then sealed using an acetone-diluted epoxy. The inner surfaces of the embedded channels of the sealed structure were then metallized using an electroless silver-coating process; these processes were found to be robust and independent of the inner diameter and length of the structure. The electromechanical performance of the structure was verified by applying mechanical loading while monitoring the change in electrical resistivity. The latter was found to remain nearly constant up to the point of ultimate mechanical failure. Finite element modeling was used to identify the areas of structural weaknesses and assist in elucidating the failure modes. The results were found to be in good agreement with the experimental data.

Computational studies can supplement existing ultrahigh strain rate experimental techniques in the absence of invasive full-field measurement and visualization. In this study, a computational model is employed to elucidate various phenomena accompanying the generation, propagation, and interaction of multimode shock waves in a viscoelastic material. Specifically, a 4 mm diameter polyurea plug with a thickness of 0.5 mm was modeled as a linear viscoelastic solid, where the relaxation behavior of the shear modulus was described using a Prony series while the bulk modulus was assumed to be linear elastic based on the Poisson's ratio of polyurea. The results are presented in three case studies, where a different type of shock wave was emphasized in each case while focusing on the regions at the leading and trailing edges of the shock wavefront. Generally, the wavefront interacted with the accompanying and reflected waves, resulting in compromising the purity of the sought-after loading condition, especially during the return trip of the wave upon approaching the free surface. In Case Study I, the propagation of laser-induced pressure wave remained pure during the forward trip towards the free surface but was compromised by the accompanying shear wave and side spherical patterned pressure waves. Case Study II simulated the generation of surface waves by incorporating a ring-shaped loading site, where the release of a surface displacement was found to be focused and amplified at the central point. In the final case study, Case Study III, the applied shear wave at ultrahigh strain rate generated secondary pressure and horizontal shear waves at the edges of the loading site, which complicated the loading scenario but provided new insight into the interaction of laser-generated shock waves within the solid. The results can be used to improve the analysis of experimental data to quantify the accompanying deformation and failure mechanisms of polymers subjected to hypervelocity impacts.

Fiber-reinforced polymer matrix composites continue to attract scientific and industrial interest since they offer superior strength-, stiffness-, and toughness-to-weight ratios. The research herein characterizes two sets of E-Glass/Epoxy composite skins: stressed and unstressed. The stressed samples were previously installed in an underground power distribution vault and were exposed to fire while the unstressed composite skins were newly fabricated and never-deployed samples. The mechanical, morphological, and elemental composition of the samples were methodically studied using a dynamic mechanical analyzer, a scanning electron microscope (SEM), and an x-ray diffractometer, respectively. Sandwich composite panels consisting of E-glass/Epoxy skin and balsa wood core were originally received, and the balsa wood was removed before any further investigations. Skin-only specimens with dimensions of ~12.5 mm wide, ~70 mm long, and ~6 mm thick were tested in a Dynamic Mechanical Analyzer in a dual-cantilever beam configuration at 5 Hz and 10 Hz from room temperature to 210 °C. Micrographic analysis using the SEM indicated a slight change in morphology due to the fire event but confirmed the effectiveness of the fire-retardant agents in quickly suppressing the fire. Accompanying Fourier transform infrared and energy dispersive X-ray spectroscopy studies corroborated the mechanical and morphological results. Finally, X-ray diffraction showed that the fire event consumed the surface level fire-retardant and the structural attributes of the E-Glass/Epoxy remained mainly intact. The results suggest the panels can continue field deployment, even after short fire incident.

Polymeric foams are ubiquitous in body armors for civilian and military applications due to their functional and economic advantage. This class of materials offers good impact mitigation properties without the weight penalty associated with other classes of materials. Generally, foams inherit the mechanical properties of their base material while the performance can be defined by the cellular structure. In this study, new polyurea foams were manufactured and subjected to low energy dynamic impacts to experimentally validate the time-dependent response of polyurea foams based on linear viscoelastic modeling. The time-dependent properties of bulk polyurea were scaled down by accounting for the change in density and microstructure of the polyurea foam. Drop-weight testing of approximately 7 J in a fully-instrumented drop weight testing machine was conducted on newly-fabricated polyurea foams of two nominal densities, namely, 227 kg/m3 and 355 kg/m3. Simultaneous to the impact event, high-speed photography was used to capture the deformation and recovery response of the samples. The performance was then compared with a promising off-the-shelf, closed-cell foam material used to protect against biomechanical impacts. The polyurea foams proved to mitigate the impact more effectively than the benchmark foam without the addition of a severe weight penalty. The finite element simulation performed well in predicting the amplitude and the rise time of the dynamic impact testing of the two foams; the numerical results were found to be in close agreement with the experimental data.

The present study focuses on the microstructural characterization of polyurea after extended UV exposure using a mechanics-based, atomic force microscope (AFM) investigation. Extended exposure to UV radiation resulted in photo-degradation with subsequent photo-oxidation reactions causing an increase in the surface roughness associated with crack nucleation. The crack opening and length were found to increase considerably with the rise in UV exposure duration. The crack growth was modeled using linear elastic fracture mechanics (LEFM) since the mechanical properties of the surface degraded layer were found to be within the limits of applicability of the theory. Force-distance measurements performed on the surface of the samples revealed a significant increase in the local modulus after 4.5 weeks of exposure. The local modulus was found to reach a steady limit after 7.5 weeks of UV radiation. The adhesive tip-sample forces resolved from the AFM force-displacement measurements indicated a drastic decrease after 3 weeks of exposure due to embrittlement of photo-oxidized exposed surface. Radiation effect; Microstructure; Polymers; LEFM

One of the fundamental investigations on neural interfaces for stimulation of damaged and innervated areas of the brain is focused on microelectrode arrays (MEAs). Most notably are micromachined devices of platinum electrodes insulated between polyimide, a flexible and patternable polymer.(1) Electrochemical impedance and cyclic voltammetry (CV) testings have shown that metal electrodes degrade and delaminate after only 2500 cycles whereas glassy carbon electrodes are robust enough to withstand 250,000 cycles. (2) Atomic force microscopy (AFM) images, as shown in figures 1(a) and 1(b), were obtained to compare the two types of electrodes and analysis has shown that glassy carbon electrodes have a much smoother surface topography than that of platinum electrodes. Other studies have confirmed that increased surface roughness decreases circuit yield. (3) We submit that if the metal deposition process is optimized to produce a less granular surface of the platinum electrodes, then the electrochemical performance of these MEAs should improve. In order to test this concept, devices will be fabricated under four different sputtering conditions where two parameters will be modified: the vacuum pressure and the distance between the target and substrate. The control MEAs will be fabricated at a pressure of 0.01 mbar (~7.5 mTorr) and a distance of ~10 cm. The next two sets of devices will have the same distance as the control but the pressure will be reduced by 25% and 50%. For the last condition the vacuum will be kept at 0.01 mbar and the distance will decrease to 5 cm. After which the surface morphology of the different platinum films will be studied using AFM and the circuit resistance will be tested electrochemically in a three electrode cell. References: 1. Kassegne, S., Vomero, M., Gavuglio, R., Hirabayashi, M., Özyilmaz, E., Nguyen, S., Rodriguez, J., Özyilmaz, E., van Niekerk, P., Khosla, A., “Electrical Impedance, Electrochemistry, Mechanical Stiffness, and Hardness Tunability in Glassy Carbon MEMS mECoG Electrodes”, J. of Microelectronics Engineering, Volume 133, Pages 36–44, 2015. 2. Vomero, M., Goshi, R., Dryg, I., Maxfield, T., Castagnola, E., Maggiolini, E., Ciarpella, F., Ordonex, J., Gueli, C., “ Glassy Carbon Based Microelectrode Array Technology for Use in Long-Term Neural Recording and Stimulation with Superior Electrical and Electrochemical Properties”, (in process), 2015. 3. Logeeswaran, V., Chan, M., Bayam, Y. , Saif Islam, M., Horsley, D., Li, X., Wu, W., Wang, S., and Williams, R., “Ultra-smooth Metal Surfaces Generated by Pressure-Induced Surface Deformation Figure 1

Neuroplasticity, the ability of the brain to form new neural pathways and synaptic connections, is gaining renewed interest for therapeutic purposes in stroke or spinal cord injury patients. Earlier work in this area in animal models has shown that behavior modulation was associated with the release of serotonin that subsequently leads to changes in gene expression and new neural growth [1]. A separate work has also demonstrated that using serotonin 1A agonists in rats with respiratory dysfunction due spinal cord injury results in better respiratory function [2]. Taken together, these studies indicate that neurotransmitters such as serotonin are closely linked to neuroplasticity, suggesting that changes in levels of these neurotransmitters can be correlated to synaptic changes. Currently, there is growing interest in the use of electrical stimulation as a means for inducing neuroplasticity in stroke and spinal cord injury patients [3]. However, while the data suggests that plasticity could be induced by electrical stimulation, neither the mechanism nor the protocols of stimulation are understood. We submit that a possible path to understanding these mechanisms involves simultaneous real-time monitoring of neurotransmitters such as serotonin electrochemically and electrical signals during stimulation and finding a correlation between these electrical signals and neurotransmitters. In cases where detection of several key electroactive neurotransmitters like serotonin and dopamine is sought, fast scan cyclic voltammetry (FSCV) with its high temporal resolution is the preferred electrochemical method of obtaining real time data [4]. Now, to be able to simultaneously monitor these electrochemical signals along with electrical signals, new approaches in neural probe material and architecture selection are required, as almost all neural probes currently in use are capable of recording only electrical or chemical signals, but not both. Thus, implementation of electrochemical monitoring techniques in animal models requires an integrated electrode array with a size suitable for an animal containing a counter electrode, working electrodes and a reference electrode. Furthermore, electrode materials capable of electrochemically detecting the presence of neurotransmitters without undergoing non-reversible reactions are sought. To meet this need, we investigate and report on the use of glassy carbon, a form of carbon that has been demonstrated to possess ideal characteristics, such as a wide electrochemical window with no irreversible reactions as a coupled electrical and electrochemical sensing platform. In this particular study, we introduce a 12-channel glassy carbon (GC) microelectrode array fabricated on a flexible substrate. The specific design introduced here consists of electrodes supported on a polyimide substrate with penetrating probes at their edges that can penetrate up to 1.25 mm into an animal's cortex or spinal cord. Each probe contains a glassy carbon electrode that can be used to detect serotonin, stimulate a neuron, or read an electrophysiological signal. Our preliminary results show that serotonin can be detected using a polyimide-glassy carbon device down to at least 1 uM in vitro by stimulating through one portion of the device and reading through another (Figure 1). Furthermore, we have shown that our electrodes can read electrophysiological data in-vivo. Figure 2b shows the Fourier Transform of a few seconds of electrical brain signals from a song bird without stimulus. In this graph low frequency alpha waves are most prominent. Figure 2c, the stimulus graph, on the other hand, shows that the beta and gamma waves became more prominent under external, audio stimulation. We believe that the ability to couple this kind of electrical signal detection with electrochemical sensing through FSCV (fast-scan CV) in a single device for in-vivostudies give promise for the disambiguation behind the mechanisms involved in neuroplasticity. Figure 1: Background subtracted cyclic voltagramms for various concentrations of serotonin Figure2: In vivo ECoG signals recorded with carbon electrodes. (a) Time lapse of the audio stimulus (b) Amplitude spectra of the animal during quiet time (c) Amplitude spectra of the signal during the stimulus phase. [1] Kandel, Eric R. Science 294, no. 5544 (2001): 1030-1038. [2] Choi, Howard, Wei-Lee Liao, Kimberly M. Newton, Renna C. Onario, Allyson M. King, Federico C. Desilets, Eric J. Woodard et al. The Journal of neuroscience 25, no. 18 (2005): 4550-4559. [3] Dimyan, Michael A., and Leonardo G. Cohen. Nature Reviews Neurology 7, no. 2 (2011): 76-85. [4] Lama, Rinchen D., Karl Charlson, Arun Anantharam, and Parastoo Hashemi. Analytical chemistry 84, no. 19 (2012): 8096-8101. Figure 1