Figure 1: Alginate hydrogels match the viscoelastic properties of mammalian tissues and conform to complex substrates.

(a) Schematic of the proposed device and its various components. The encapsulation layer is made from a stretchable hydrogel (blue) to which a viscoelastic electrically insulating polymer (pink) is covalently coupled. The conductive tracks (black) are fabricated from a macroporous hydrogel with carbon additives (inset), and interface with a flexible connector (gold). As all the components of the device are viscoelastic, the assembled array can be designed to match the modulus, and flow to conform to follow the tissue on which it is implanted.

(b) Rheological properties of fresh lamb cortical tissue and fresh rat cardiac tissue. Storage moduli (G’) (top), and loss moduli (G”) (bottom) shown as a function of strain (ε), at a frequency of 1 Hz, n=10 independent tissue sample sections. Mean and s.d. are plotted.

(c) Rheological properties of alginate hydrogels with varying levels of crosslinking agent indicated in the legend. Storage moduli (G’) (top), and loss moduli (G”) (bottom) shown as a function of strain (ε), at a frequency of 1 Hz, n=8 independent gels of each formulation. Mean and s.d. are plotted.

(d) Photographs of plastic (5 mm × 15 mm × 25 μm sheet of polyimide, bottom), elastomer (5 mm × 15 mm × 100 μm sheet of Ecoflex, centre), and viscoelastic (5 mm × 15 mm × 250 μm sheet of alginate, top) substrates, with the thickness adjusted so that the bending stiffnesses were comparable. Substrates were coated with blue dye prior to application, and images (left) demonstrate the shapes taken by each material immediately following placement onto the agarose brain model and images (right) after removal of the substrates, and the dye transferred from each substrate to the tissue demonstrated regions of close contact.

(e) Quantification of the area on model brains to which dye was transferred for each material (plastic, elastic, viscoelastic), as a metric of direct contact between the substrates and the porcine brain model. Values are normalized to that of the viscoelastic alginate substrate (n=3 substrates of each material).

(f) Photographs of viscoelastic (alginate sheets, 5 mm × 5 mm × 200 μm) and elastic (Ecoflex sheets, right: 5 mm × 5 mm × 100 μm) substrates, both when present on the porcine brain model and immediately after removal. The two substrates had matched bending stiffness and were placed on the brain models for two weeks prior to removal and immediate imaging. Scale bar represents 5 mm.