The effects of PEG‐based surface modification of PDMS microchannels on long‐term hemocompatibility

The current study demonstrates the first surface modification for poly(dimethylsiloxane) (PDMS) microfluidic networks that displays a long shelf life as well as extended hemocompatibility. Uncoated PDMS microchannel networks rapidly adsorb high levels of fibrinogen in blood contacting applications. Fibrinogen adsorption initiates platelet activation, and causes a rapid increase in pressure across microchannel networks, rendering them useless for long term applications. Here, we describe the modification of sealed PDMS microchannels using an oxygen plasma pretreatment and poly(ethylene glycol) grafting approach. We present results regarding the testing of the coated microchannels after extended periods of aging and blood exposure. Our PEG‐grafted channels showed significantly reduced fibrinogen adsorption and platelet adhesion up to 28 days after application, highlighting the stability and functionality of the coating over time. Our coated microchannel networks also displayed a significant reduction in the coagulation response under whole blood flow. Further, pressure across coated microchannel networks took over 16 times longer to double than the uncoated controls. Collectively, our data implies the potential for a coating platform for microfluidic devices in many blood‐contacting applications. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 4195–4205, 2014.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109295/1/jbma35090.pd

Mechanically adaptive nanocomposites for neural interfacing

The recording of neural signals with microelectrodes that are implanted into the cortex of the brain is potentially useful for a range of clinical applications. However, the widespread use of such neural interfaces has so far been stifled because existing intracortical electrode systems rarely allow for consistent long-term recording of neural activity. This limitation is usually attributed to scar formation and neuron death near the surface of the implanted electrode. It has been proposed that the mechanical property mismatch between existing electrode materials and the brain tissue is a significant contributor to these events. To alleviate this problem, we utilized the architecture of the sea cucumber dermis as a blueprint to engineer a new class of mechanically adaptive materials as substrates for "smart” intracortical electrodes. We demonstrated that these originally rigid polymer nanocomposites soften considerably upon exposure to emulated physiological and in vivo conditions. The adaptive nature of these bioinspired materials makes them useful as a basis for electrodes that are sufficiently stiff to be easily implanted and subsequently soften to better match the stiffness of the brain. Initial histological evaluations suggest that mechanically adaptive neural prosthetics can more rapidly stabilize neural cell populations at the device interface than rigid systems, which bodes well for improving the functionality of intracortical device

Behavioral paradigm for the evaluation of stimulation-evoked somatosensory perception thresholds in rats

Intracortical microstimulation (ICMS) of the somatosensory cortex via penetrating microelectrode arrays (MEAs) can evoke cutaneous and proprioceptive sensations for restoration of perception in individuals with spinal cord injuries. However, ICMS current amplitudes needed to evoke these sensory percepts tend to change over time following implantation. Animal models have been used to investigate the mechanisms by which these changes occur and aid in the development of new engineering strategies to mitigate such changes. Non-human primates are commonly the animal of choice for investigating ICMS, but ethical concerns exist regarding their use. Rodents are a preferred animal model due to their availability, affordability, and ease of handling, but there are limited choices of behavioral tasks for investigating ICMS. In this study, we investigated the application of an innovative behavioral go/no-go paradigm capable of estimating ICMS-evoked sensory perception thresholds in freely moving rats. We divided animals into two groups, one receiving ICMS and a control group receiving auditory tones. Then, we trained the animals to nose-poke – a well-established behavioral task for rats – following either a suprathreshold ICMS current-controlled pulse train or frequency-controlled auditory tone. Animals received a sugar pellet reward when nose-poking correctly. When nose-poking incorrectly, animals received a mild air puff. After animals became proficient in this task, as defined by accuracy, precision, and other performance metrics, they continued to the next phase for perception threshold detection, where we varied the ICMS amplitude using a modified staircase method. Finally, we used non-linear regression to estimate perception thresholds. Results indicated that our behavioral protocol could estimate ICMS perception thresholds based on ~95% accuracy of rat nose-poke responses to the conditioned stimulus. This behavioral paradigm provides a robust methodology for evaluating stimulation-evoked somatosensory percepts in rats comparable to the evaluation of auditory percepts. In future studies, this validated methodology can be used to study the performance of novel MEA device technologies on ICMS-evoked perception threshold stability using freely moving rats or to investigate information processing principles in neural circuits related to sensory perception discrimination

Status Epilepticus due to Intraperitoneal Injection of Vehicle Containing Propylene Glycol in Sprague Dawley Rats

Published reports of status epilepticus due to intraperitoneal injection containing propylene glycol in rats are sparse. In fact, there are no reports specifying a maximum safe dose of propylene glycol through intraperitoneal administration. We report here a case of unexpected seizures in Sprague Dawley rats after receiving an intraperitoneal injection containing propylene glycol. Nine-week-old, 225–250 gram male rats were reported to experience tremor progressing to seizures within minutes after given injections of resveratrol (30 mg/kg) dissolved in a 40 : 60 propylene glycol/corn oil vehicle solution by direct intraperitoneal (IP) slow bolus injection or via a preplaced intraperitoneal catheter. The World Health Organization suggests a maximum dose of 25 mg/kg/day of propylene glycol taken orally and no more than 25 mg/dL in blood serum, whereas the animals used in our study got a calculated maximum 0.52 g/kg (25 times lower dose). Blood tests from the seizing rat support a diagnosis of hemolysis and lactic acidosis which may have led to the seizures, all of which appeared to be a consequence of the propylene glycol administration. These findings are consistent with oral and intravenous administration of propylene glycol toxicity as previously reported in other species, including humans. To our knowledge, this report represents the first published case of status epilepticus due to an IP injection containing propylene glycol

Understanding the Effects of Both CD14-Mediated Innate Immunity and Device/Tissue Mechanical Mismatch in the Neuroinflammatory Response to Intracortical Microelectrodes

Intracortical microelectrodes record neuronal activity of individual neurons within the brain, which can be used to bridge communication between the biological system and computer hardware for both research and rehabilitation purposes. However, long-term consistent neural recordings are difficult to achieve, in large part due to the neuroinflammatory tissue response to the microelectrodes. Prior studies have identified many factors that may contribute to the neuroinflammatory response to intracortical microelectrodes. Unfortunately, each proposed mechanism for the prolonged neuroinflammatory response has been investigated independently, while it is clear that mechanisms can overlap and be difficult to isolate. Therefore, we aimed to determine whether the dual targeting of the innate immune response by inhibiting innate immunity pathways associated with cluster of differentiation 14 (CD14), and the mechanical mismatch could improve the neuroinflammatory response to intracortical microelectrodes. A thiol-ene probe that softens on contact with the physiological environment was used to reduce mechanical mismatch. The thiol-ene probe was both softer and larger in size than the uncoated silicon control probe. Cd14-/- mice were used to completely inhibit contribution of CD14 to the neuroinflammatory response. Contrary to the initial hypothesis, dual targeting worsened the neuroinflammatory response to intracortical probes. Therefore, probe material and CD14 deficiency were independently assessed for their effect on inflammation and neuronal density by implanting each microelectrode type in both wild-type control and Cd14-/- mice. Histology results show that 2 weeks after implantation, targeting CD14 results in higher neuronal density and decreased glial scar around the probe, whereas the thiol-ene probe results in more microglia/macrophage activation and greater blood–brain barrier (BBB) disruption around the probe. Chronic histology demonstrate no differences in the inflammatory response at 16 weeks. Over acute time points, results also suggest immunomodulatory approaches such as targeting CD14 can be utilized to decrease inflammation to intracortical microelectrodes. The results obtained in the current study highlight the importance of not only probe material, but probe size, in regard to neuroinflammation

The Role of Toll-Like Receptor 2 and 4 Innate Immunity Pathways in Intracortical Microelectrode-Induced Neuroinflammation

We have recently demonstrated that partial inhibition of the cluster of differentiation 14 (CD14) innate immunity co-receptor pathway improves the long-term performance of intracortical microelectrodes better than complete inhibition. We hypothesized that partial activation of the CD14 pathway was critical to a neuroprotective response to the injury associated with initial and sustained device implantation. Therefore, here we investigated the role of two innate immunity receptors that closely interact with CD14 in inflammatory activation. We implanted silicon planar non-recording neural probes into knockout mice lacking Toll-like receptor 2 (Tlr2−/−), knockout mice lacking Toll-like receptor 4 (Tlr4−/−), and wildtype (WT) control mice, and evaluated endpoint histology at 2 and 16 weeks after implantation. Tlr4−/− mice exhibited significantly lower BBB permeability at acute and chronic time points, but also demonstrated significantly lower neuronal survival at the chronic time point. Inhibition of the Toll-like receptor 2 (TLR2) pathway had no significant effect compared to control animals. Additionally, when investigating the maturation of the neuroinflammatory response from 2 to 16 weeks, transgenic knockout mice exhibited similar histological trends to WT controls, except that knockout mice did not exhibit changes in microglia and macrophage activation over time. Together, our results indicate that complete genetic removal of Toll-like receptor 4 (TLR4) was detrimental to the integration of intracortical neural probes, while inhibition of TLR2 had no impact within the tests performed in this study. Therefore, approaches focusing on incomplete or acute inhibition of TLR4 may still improve intracortical microelectrode integration and long term recording performance

Surface-directed assembly of fibrillar extracellular matrices

Biologically-inspired materials have emerged as promising substrates for enhanced repair in various therapeutic and regenerative medicine applications, including nervous and vascular tissues, bone, and cartilage. These strategies focus on the development of materials that integrate well-characterized domains from biomacromolecules to mimic individual functions of the extracellular matrix (ECM), including cell adhesive motifs, growth factor binding sites, and protease sensitivity. A vital property of the ECM is the fibrillar architecture arising from supramolecular assembly. For example, the fibrillar structure of fibronectin (FN) matrices modulates cell cycle progression, migration, gene expression, cell differentiation, and the assembly of other matrix proteins. Current biomaterials do not actively promote deposition and assembly of ECM. In this research, we describe the rational design and investigation of non-fouling biomimetic surfaces in which an oligopeptide sequence (FN13) from the self-assembly domain of FN is tethered to non-fouling substrates. This surface modification directs cell-mediated co-assembly of robust fibrillar FN and type I collagen (COL) matrices reminiscent of ECM, and increases in cell proliferation rates. Furthermore, the effect of this peptide is surface-directed, as addition of the soluble peptide has no effect on matrix assembly. We have also identified a critical surface density of the immobilized peptide to elicit the full activity. These results contribute to the development and design of biomimetic surface modifications that direct cell function for biomedical and biotechnology applications.Ph.D.Committee Chair: Andres J. Garcia; Committee Chair: David M. Collard; Committee Member: Elliot Chaikof; Committee Member: Loren Williams; Committee Member: Marcus Wec

Biomimetic mechanically adaptive nanocomposites

The development of a new class of mechanically adaptive nanocomposites has been inspired by biological creatures such as sea cucumbers, which have the ability to reversibly change the stiffness of their dermis. Several recent studies have related this dynamic mechanical behaviour to the distinctive nanocomposite architecture of the collagenous tissue, in which interactions among rigid collagen fibrils, embedded in a viscoelastic matrix of fibrillin microfibrils, are regulated by neurosecretory proteins. Here we review the development of a new family of artificial polymer nanocomposites that mimic the architecture and the mechanic adaptability of the sea cucumber dermis. The new materials are based on low-modulus matrix polymers that are reinforced with a percolating cellulose nanofiber network. Owing to the abundance of surface hydroxyl groups, the cellulose nanofibers display strong interactions between themselves, causing the evenly dispersed percolating nanocomposites to display a high stiffness. The nanofiber–nanofiber interactions can be largely switched off by the introduction of a chemical regulator that allows for competitive hydrogen bonding, resulting in a significant decrease in the stiffness of the material