In an alternative configuration (not illustrated) each of the first and second electrode may be formed from a hydrogel that is configured to absorb mechanical loads and dissipate energy from mechanical loads. In this alternative configuration the hydrogel comprises a polymer matrix and the polymer matrix comprises at least two crosslink structures. The crosslink structures are configured to dissipate energy from any mechanical loads that are acting on the first electrode and/or second electrode. Mechanical loads may be any axial loads (e.g. tension, compression) or bending loads or torsional loads or shearing loads. The crosslink structures comprise a first covalent crosslink structure that includes covalent bonds or hydrogen bonds and a second ionic crosslink structure that includes ionic bonds. The first crosslink structure is defined by a first polymer comprising an acrylamide and the second crosslink structure is defined by an alginate.

In this alternative configuration the first polymer comprises a polyacrylamide (PAAm) polymer that is covalently cross linked thereby defining the covalent crosslink structure. The second polymer comprises an aluminium alginate (Al-alginate) polymer that is ionically cross linked thereby defining the ionic crosslink structure. In this alternative configuration the hydrogel may also include an aqueous solution comprising a crosslinking agent e.g. a salt or an acid or Ca ions or Mg ions or Zn ions or Fe ions or Al ions.