Biomedical electronic devices can be interfaced with the human body to measure physiological signals [1,2] or to provide electrical stimulation for treatment. [3,4] Electronic materials that can be seamlessly interfaced with human tissues or cells are, therefore, essential to study or stimulate the nervous system, or to serve as neural tissue scaffolds. But, materials challenges remain to be addressed to bridge the gap between "soft" biological tissues and "hard" electronics. The mechanical mismatch between biological tissues and electronics can lead to scar formation between the electronics and the target tissue, which significantly hampers the performance and efficacy of bioelectronic stimulation and recording. [3,5,6] Conductive hydrogels are a promising strategy for interfacing electronic materials with biological tissues. [6,7] Due to their high water content (70-99 wt%), hydrogels can be as soft and flexible as biological tissues, including skin, muscle, heart, spinal cord, and brain (E < 100 kPa). [6] To impart electrical conductivity, conductive nanoparticles, such as metal nanowires (NWs) [8,9] carbon nanotubes (CNTs), [7] and conductive polymers [5,7,10] can be incorporated inside the hydrogel. NWs and CNTs, while effective at achieving electronically conductive hydrogels, were shown to lead to heterogeneities in the network and hydrogels with a high elastic modulus; overall leading to a poor interface with biological entities. [8] Compared to these nanocomposite hydrogels, conductive hydrogels made from conducting polymers (CPs), including poly(3,4-ethylenedioxythiophene) (PEDOT), [11,12] polypyrrole (PPy), [13][14][15] or polyaniline (PANI) [15,16] offer a higher compatibility with biological systems, by virtue of their flexibility and potential for ionic-as well as electronic-conductivity.Several methods have been envisioned for the preparation of conductive hydrogels from CPs. [17,18] The conducting polyelectrolyte complex of PEDOT with poly(styrene sulfonate) (PEDOT:PSS) can be gelled directly from its aqueous solution by increasing the ionic strength with bivalent ions Mg 2þ , Ca 2þ , or multivalent ions Fe 3þ , Ce 4þ . [19,20] These conductive hydrogels, however, are only weakly crosslinked leading to poor strength and contain residual ions, which could lead to potential inflammation and cell toxicity. [21] Thus, the purification process requires a large amount of distilled water for at least a week to wash out excess ions. [20] Another method is the use of secondary dopants, including sulfuric acid and dimethyl sulfoxide, as both
Articular cartilage derives its load-bearing strength from the mechanical and physiochemical coupling between the collagen network and negatively charged proteoglycans, respectively. Current disease modeling approaches and treatment strategies primarily focus on cartilage stiffness, partly because indentation tests are readily accessible. However, stiffness measurements via indentation alone cannot discriminate between proteoglycan degradation versus collagen degradation, and there is a lack of methods to monitor physiochemical contributors in full-stack tissue. To decouple these contributions, here, we developed a platform that measures tissue swelling in full-depth equine cartilage explants using piezoresistive graphene strain sensors. These piezoresistive strain sensors are embedded within an elastomer bulk and have sufficient sensitivity to resolve minute, real-time changes in swelling. By relying on simple DC resistance measurements over optical techniques, our platform can analyze multiple samples in parallel. Using these devices, we found that cartilage explants under enzymatic digestion showed distinctive swelling responses to a hypotonic challenge and established average equilibrium swelling strains in healthy cartilage (4.6%), cartilage with proteoglycan loss (0.5%), and in cartilage with both collagen and proteoglycan loss (−2.6%). Combined with histology, we decoupled the pathologic swelling responses as originating either from reduced fixed charge density or from loss of intrinsic stiffness of the collagen matrix in the superficial zone. By providing scalable and in situ monitoring of cartilage swelling, our platform could facilitate regenerative medicine approaches aimed at restoring osmotic function in osteoarthritic cartilage or could be used to validate physiologically relevant swelling behavior in synthetic hydrogels.
Recent advances recognize that the viscoelastic properties of epithelial structures play important roles in biology and disease modeling. However, accessing the viscoelastic properties of multicellular structures in mechanistic or drug-screening...
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