Carbon nanofiber-filled conductive silicone elastomers as soft, dry bioelectronic interfaces

<div><p>Soft and pliable conductive polymer composites hold promise for application as bioelectronic interfaces such as for electroencephalography (EEG). In clinical, laboratory, and real-world EEG there is a desire for dry, soft, and comfortable interfaces to the scalp that are capable of relaying the μV-level scalp potentials to signal processing electronics. A key challenge is that most material approaches are sensitive to deformation-induced shifts in electrical impedance associated with decreased signal-to-noise ratio. This is a particular concern in real-world environments where human motion is present. The entire set of brain information outside of tightly controlled laboratory or clinical settings are currently unobtainable due to this challenge. Here we explore the performance of an elastomeric material solution purposefully designed for dry, soft, comfortable scalp contact electrodes for EEG that is specifically targeted to have flat electrical impedance response to deformation to enable utilization in real world environments. A conductive carbon nanofiber filled polydimethylsiloxane (CNF-PDMS) elastomer was evaluated at three fill ratios (3, 4 and 7 volume percent). Electromechanical testing data is presented showing the influence of large compressive deformations on electrical impedance as well as the impact of filler loading on the elastomer stiffness. To evaluate usability for EEG, pre-recorded human EEG signals were replayed through the contact electrodes subjected to quasi-static compressive strains between zero and 35%. These tests show that conductive filler ratios well above the electrical percolation threshold are desirable in order to maximize signal-to-noise ratio and signal correlation with an ideal baseline. Increasing fill ratios yield increasingly flat electrical impedance response to large applied compressive deformations with a trade in increased material stiffness, and with nominal electrical impedance tunable over greater than 4 orders of magnitude. EEG performance was independent of filler loading above 4 vol % CNF (< 10³ ohms).</p></div

Single frequency (10Hz) electrical impedance performance over a single strain cycle (increasing/decreasing) for electrode filler loadings of 3, 4 and 7 vol % carbon nanofiber in PDMS.

<p>Single frequency (10Hz) electrical impedance performance over a single strain cycle (increasing/decreasing) for electrode filler loadings of 3, 4 and 7 vol % carbon nanofiber in PDMS.</p

Rheological Characterization of Next-Generation Ballistic Witness Materials for Body Armor Testing

Roma Plastilina No. 1 (RP1), an artist modeling clay that has been used as a ballistic clay, is essential for evaluation and certification in standards-based ballistic resistance testing of body armor. It serves as a ballistic witness material (BWM) behind the armor, where the magnitude of the plastic deformation in the clay after a ballistic impact is the figure of merit (known as &ldquo;backface signature&rdquo;). RP1 is known to exhibit complex thermomechanical behavior that requires temperature conditioning and frequent performance-based evaluations to verify that its deformation response satisfies requirements. A less complex BWM formulation that allows for room-temperature storage and use as well as a more consistent thermomechanical behavior than RP1 is desired, but a validation based only on ballistic performance would be extensive and expensive to accommodate the different ballistic threats. A framework of lab-scale metrologies for measuring the effects of strain, strain rate, and temperature dependence on mechanical properties are needed to guide BWM development. The current work deals with rheological characterization of a candidate BWM, i.e., silicone composite backing material (SCBM), to understand the fundamental structure&ndash;property relationships in comparison to those of RP1. Small-amplitude oscillatory shear frequency sweep experiments were performed at temperatures that ranged from 20 &deg;C to 50 &deg;C to map elastic and damping contributions in the linear elastic regime. Large amplitude oscillatory shear (LAOS) experiments were conducted in the non-linear region and the material response was analyzed in the form of Lissajous curve representations with the values of perfect plastic dissipation ratio reported to identify the degree of plasticity. The results show that the SCBM exhibits dynamic properties that are similar in magnitude to those of temperature-conditioned RP1, but with minimal temperature sensitivity and weaker frequency dependence than RP1. Both SCBM and RP1 are identified as elastoviscoplastic materials, which is particularly important for accurate determination of backface signature in body armor evaluation. The mechanical properties of SCBM show some degree of aging and work history effects. The results from this work demonstrate that the rheological properties of SCBM, at small and large strains, are similar to RP1 with substantial improvements in BWM performance requirements in terms of temperature sensitivity and thixotropy

Spectral power for conditions of eyes-open and eyes-closed for a sample subject using CNF-PDMS and standard Ag-AgCl electrodes.

<p>Note close correspondence with the typical peak in the alpha (8–14 Hz) band during eyes-closed conditions for both CNF-PDMS (black) and Ag-AgCl (red).</p

SEM images of representative cryo-fracture surfaces for a) 3 vol %, b) 4 vol %, and c) 7 vol % CNF loadings.

<p>SEM images of representative cryo-fracture surfaces for a) 3 vol %, b) 4 vol %, and c) 7 vol % CNF loadings.</p

Resistance as a function of CNF concentration in the CNF-PDMS composites.

<p>Resistance as a function of CNF concentration in the CNF-PDMS composites.</p

Compressive stress-strain curves for electrode filler loadings of 3, 4 and 7 vol % carbon nanofiber in PDMS.

<p>Compressive stress-strain curves for electrode filler loadings of 3, 4 and 7 vol % carbon nanofiber in PDMS.</p

Sample time series of data recorded from paired 4% CNF-PDMS (top) and standard Ag-AgCl (bottom) from a sample human subject.

<p>Note high similarity in the fluctuation over time.</p