1900367 (1 of 10) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mme-journal.de COMMUNICATION Cryogel-Based Electronic–Tissue Interfaces with Soft, Highly Compressible, and Tunable Mechanics Rosa Ghatee, Anita Tolouei, Jennifer Fijalkowski, Abdulrahman Alsasa, Justin Hayes, Walter Besio, and Stephen Kennedy* Dr. R. Ghatee, Dr A. Tolouei, J. Fijalkowski, J. Hayes, Prof. S. Kennedy Department of Chemical Engineering University of Rhode Island Kingston, RI 02881, USA E-mail: smkennedy@uri.edu A. Alsasa, Prof. W. Besio, Prof. S. Kennedy Department of Electrical, Computer and Biomedical Engineering University of Rhode Island Kingston, RI 02881, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201900367. DOI: 10.1002/mame.201900367 the symptoms exhibited by patients suf- fering from neurological diseases often require the use of electrodes to stimulate and/or record signals from neural tis- sues. [2] For instance, deep brain stimula- tion (DBS) uses implanted electrodes to electrically stimulate targets within the brain, [3,4] which can reduce symptoms exhibited by patients with Parkinson’s dis- ease, essential tremor, dystonia, chronic pain, major depression, and obsessive compulsive disorder. [5] Such strategies require an electrode–tissue interface whose electrical conductivity remains sufficiently high to transmit neural sig- nals over time. [6] Metallic electrodes are typically used as the electrode material in these neuroprosthetic applications due to their high electrical conductivity. However, the stiffness of metallic electrodes can be problematic, especially in applications that require prolonged electrode–tissue interface and/or applications that demand electrode implantation within tissues. Spe- cifically, stiff electrodes can have difficulty remaining in contact with soft, curvilinear surfaces (e.g., against the skin in electro- encephalography [EEG] and electrocardiography [ECG] appli- cations). Conductive pastes (such as Ten20 Conductive EEG Paste) are typically used as interfaces to help facilitate a stable electrical pathway between electrodes and soft surface tissues. However, they involve messy and time-consuming electrode preparation before use, dry up during use, and are therefore not well-suited for applications requiring recording/stimulating over prolonged periods of time (e.g., for wearable sensors and electronics). For applications that demand in vivo electrode implantation (e.g., DBS), maintenance of an electrically con- ductive path across the electrode–tissue interface is further complicated by the body’s ability to build fibrous tissue around the implanted electrode, which can greatly reduce interfacial conductivity. [2,7–9] Fibrous tissue development at the electrode– tissue interface is thought to be the result of several factors. First, surgical introduction of a stiff metal electrode to sensi- tive tissues can be traumatic and can incite a strong inflamma- tory response. As time progresses, the mechanical mismatch between the stiff metallic electrode and soft tissue can lead to micromotion, shear stresses, continued inflammation, and a stronger foreign body reaction. [2,8] These responses can lead Electrically conductive materials with soft, tough, and tunable mechanics have utility in a wide range of applications including neuroprosthetics. Such materials can serve as interfaces between electrical components and tissues, providing mechanical matches with and better conformations to soft, irregularly shaped surfaces. Hydrogels can potentially provide these attributes while remaining hydrated for long periods of time—providing a long-term and stable electronic–tissue interface. Additionally, in applications that demand implantation, hydrogels can be formulated to locally deliver enhancing therapeutics. Here, hydrogels are developed by entrapping a conducting polymer within a crosslinked poly(acrylic acid) (pAAc) network. Critically, these hydrogels are cast under freezing conditions which produces cryogels that exhibit macroporous, soft, and highly tunable mechanics (0.2–20 kPa, by varying pAAc and crosslinker concentrations). Additionally, these cryogels are tough enough to survive over 90% compression, which enables survival after being passed through 16-gauge needles. Cryogels also exhibit electrical conductivities that are sufficient to record alpha waves from the scalp of human subjects. Growth of fibroblasts cultures in the presence of these cryogels produce statistically similar viabilities compared to controls and do not disrupt fibroblast cell cycles. Finally, cryogels are capable of being loaded with and delivering proteins that can potentially combat inflammation. Electrically conductive materials that exhibit soft, deformable, but tough mechanical properties are highly desirable in a wide range of applications that require electrical recording from or stimulation of tissues. For example, such materials could enhance neurological disorder treatment strategies—a family of disorders that affect up to a billion people worldwide and are on the rise due to gradual increases in life expectancies. [1] Neuroprosthetic devices that can restore functionality or reduce Macromol. Mater. Eng. 2019, 1900367