The Advanced Polymer Research Lab focuses on fundamental issues in polymer thermomechanics, interface chemistry, radiation processing, semiconductor processing and wave scattering in metamaterials to push forward applied research in biomedical device design, neural interfaces, flexible electronics and defense. Following are additional information regarding our areas of interest.
Interfacial Phenomena for Flexible Electronics
Fundamental Science: A novel processing method is described using photolithography to pattern thin-film flexible electronics on shape memory polymer substrates with mechanical properties tailored to improve biocompatability and enhance adhesion between the polymer substrate and metal layers. We adapt standard semiconductor wafer processing techniques to decrease the maximum processing temperatures to 40 °C and enable robust device design onto a variety of softening substrates with tunable moduli. First, device components are deposited and patterned onto a sacrifical substrate. Then a polymer network is polymerized from monomers directly onto the fabricated devices by copolymerization of, for example, methyl acrylate, isobornyl acrylate and acrylic acid with poly(ethylene glycol) diacrylate, such that the interfacial adhesion is enhanced over traditional flexible electronic processing techniques. As neural tissue deforms around stiff implants, tissue moves relative to the implant causing continuous injury and eliciting a chronic immune response. A substrate system with a tunable swelling profile is synthesized by varying the amount of acrylic acid (0-20 wt %), while adjusting other monomers, to keep Tg constant. This enables precise control of modulus of the substrate in physiological conditions and is used to minimize modulus mismatch between probe and tissue. The resulting devices are stiff enough ( shear modulus of ~700 MPa) to assist with device implantation and then soften in vivo (~300 kPa) approaching the modulus of brain tissue (~10 kPa) within 24 hrs. Acute in vivo studies demonstrate that these materials are capable of recording neural activity. Softening multi-electrode arrays offer a highly customizable interface which can be optimized to improve biocompatibility, enabling the development of robust, reliable neural electrodes for neural engineering and neuroscience.
Takeaway point #1 for applications: Demonstrated acute recordings from softerning neural interfaces that can be implanted at a modulus above 1 GPA (~Plexiglass) and soften in vivo to a modulus below 1 MPA (~silicone rubber).
Takeaway point #2 for applications: Developed a transfer process to greatly enhance adhesion of flexible electronics.
Improved Biological Response of Flexible Electronics
Fundamental Science: Neural interfaces provide a communication platform for direct interaction with the nervous system. Communication with the central nervous system has enabled treatment of numerous conditions such as epilepsy and depression, control of prosthetic devices and the advancement of the field of neuroscience. However, devices designed to record extracellular neural activity generally fail within one year of implantation. This failure has been widely attributed to gliosis, the chronic reactive biological response to the invasive foreign probe, which leads to death of neurons and encapsulation of the implant resulting in a loss in the signal-to-noise-ratio over time. A number of factors contribute to the timeframe and extent of the observed gliosis: size, stiffness, surface chemistry, insertion procedure and mechanical constraints provided by electrical contacts have been shown to have a direct effect on glial scarring.
Takeaway point for biomedical applications: We present a shape memory polymer (SMP) thiol-ene click chemistry platform to build biocompatible, functionalizable, self-softening substrates for 3D flexible neural electronics made by robust photolithographic techniques. We are already in testing in vivo cortical probes, sciatic-tibial-sural ‘Y’electrodes and longitudinal intrafascicular electrodes and are developing dorsal root ganglion self-coiling nerve cuff electrodes that can be inserted at 1 GPA and soften toward 300kPa.
Triple Shape Memory Polymers
Fundamental Science: Triple shape memory polymers (TSMPs) are a growing subset of a class of smart materials known as shape memory polymers, which are capable of changing shape and stiffness in response to a stimulus. A TSMP can change shapes twice and can fix two metastable shapes in addition to its permanent shape. In this work, a novel TSMP system comprised of both permanent covalent crosslinks and supramolecular hydrogen bonding crosslinks has been synthesized via a one-pot method. Triple shape properties arise from the combination of the glass transition of (meth)acrylate copolymers and the dissociation of self-complementary hydrogen bonding moieties, enabling broad and independent control of both glass transition temperature (Tg) and crosslink density. Specifically, ureidopyrimidone methacrylate and a novel monomer, ureidopyrimidone acrylate, were copolymerized with various alkyl acrylates and bisphenol A ethoxylate diacrylate. Control of Tg from 0 to 60 °C is demonstrated; concentration of hydrogen bonding moieties is varied from 0 to 40 wt%; concentration of the diacrylate is varied from 0 to 30 wt%. Toughness ranges from 0.06 to 0.14 MPa and is found to peak near 20 wt% of the supramolecular crosslinker. A widely tunable class of amorphous triple-shape memory polymers has been developed and characterized through dynamic and quasi-static thermomechanical testing to gain insights into the dynamics of supramolecular networks.
Takeaway point for applications: We demonstrate precision control of an intermediate modulus plateau between rubbery and glassy states to build temperature insensitivity regions into shape memory polymers for defense and other applications.
High-Strain Polymer Networks
Fundamental Science: Shape-memory polymers (SMPs) are self-adjusting, smart materials in which shape changes can be accurately controlled at specific, tailored temperatures. In this study, the glass transition temperature (Tg) is adjusted between 28 ˚C and 55 ˚C through synthesis of copolymers of methyl acrylate (MA), methyl methacrylate (MMA) and isobornyl acrylate (IBoA). Acrylate compositions with both crosslinker densities and photoinitiator concentrations optimized at fractions of a mole percent, demonstrate fully recoverable strains at 807% for a Tg of 28 ˚C, at 663% for a Tg of 37 ˚C and at 553% for a Tg of 55 ˚C. A new compound, 4,4’-di(acryloyloxy)benzil (referred to hereafter as Xini) in which both polymerizable and initiating functionalities are incorporated in the same molecule, was synthesized and polymerized into acrylate shape-memory polymers and thermomechanically characterized yielding fully recoverable strains above 500%. The materials synthesized in this work were compared to an industry standard thermoplastic SMP, Mitsubishi’s MM5510, which showed failure strains of similar magnitude but without full shape recovery: residual strain after a single shape-memory cycle caused large-scale disfiguration. The materials in this study are intended to enable future applications where both recoverable high strain capacity and the ability to accurately and independently position Tg are required.
Takeaway point for applications: Demonstrated highest fully recoverable SMP in the literature recovering strains above 800%.
Mnemosynation - Radiation Crosslinked SMPs
Fundamental Science: Shape-memory polymers (SMPs) are active smart materials with tunable stiffness changes at specific, tailored temperatures. The use of thermoset SMPs has been limited in commodity applications because a variety of common low-cost plastics processing techniques are not possible with network polymers. In this study of thermoset SMPs, beyond adjusting the glass transition temperature (Tg) between 25 and 75 °C and tuning the recoverable force between 0.5 and 13 MPa, a novel manufacturing process, Mnemosynation, is described. The customizable mechanical properties of traditional SMPs are coupled with traditional plastic processing techniques to enable a new generation of mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable Tg. The results of this study are intended to enable future advanced applications where mass manufacturing, the ability to accurately and independently position Tg and the ability to tune recoverable force in SMPs are required.
Takeaway point for applications: Thermoset SMPs can be mass-manufactured at low costs, using traditional plastics processing techniques such as injection molding.
Etymology of Mnemosynation: Mnemosynation was named for Mnemosyne, the Greek goddess of memory, much like Vulcanization was named after the Roman god of fire. Both processes enable high throughput manufacturing of thermoplastic precursors that are post-crosslinked to yield robust materials with enhanced thermomechancial properties. Early references to Mnemosyne, appeared as early as 700 B.C. in Hesiod's Theogeny. In legend, Mnemosyne was the mother of the nine muses by Zeus and a titaness whose parents were Gaia and Uranus.
Fundamental Science: Shape-memory polymers (SMPs) are smart materials that can be designed to retain a metastable state and upon activation, recover a preprogrammed shape. In this study, poly(methyl acrylate) (PMA) is blended with poly(ethylene glycol) diacrylate (PEGDA) of several molecular weights in various concentrations and subsequently exposed to ionizing radiation. PEGDA sensitizes the radiation crosslinking of PMA, lowering the minimum absorbed dose for gelation and increasing the rubbery modulus, after crosslinking. Minimum dose for gelation, as determined by the Charlesby-Pinner equation, decreases from 25.57 kGy for unblended PMA to 2.06 kGy for PMA blended with 10.00 mole% PEGDA. Moreover, increasing the blend concentration of PEGDA increases the crosslinking density of the resulting networks. Sensitizer length, namely the Mn of PEGDA, also affects crosslinking and final mechanical properties. Increasing the length of the PEGDA molecule at a constant molar ratio increases the efficacy of the molecule as a radiation sensitizer as determined by the increase in gel fraction and rubbery modulus across doses. However, at a constant weight ratio of PEGDA to PMA, shorter PEGDA chains sensitize more crosslinking because they have more reactive ends per weight fraction. Sensitized samples of PMA with PEGDA were tested for shape-memory properties and showed shape fixity of greater than 99%. Samples had a glass transition temperature near 28 °C and recovered between 97% and 99% of the induced strain when strained to 50%.
Takeaway points for applications: Sensitizer dose plays a critical role in determining stiffness and recoverable force in the rubbery regime of SMPs manufactured through Mnemosynation.
Metamaterials and Scattering Theory
Fundamental Science: Stay tuned...
Takeaway points for applications: Metamaterials can create resonant bandgaps at specific wave frequencies to fundamentally interact with incident waves and alter their properties. This has use in acoustic and electromagnetic applications.