An intra-pulposus exposed electrode surface (280) is configured to be implanted in a nucleus pulposus (42) of an intervertebral disc (30), and a plurality of extra-pulposus exposed electrode surfaces (44) is configured to be implanted outside the nucleus pulposus (42), in electrical communication with the intervertebral disc (30). Control circuitry (50) separately controls at least two of the plurality of extra-pulposus exposed electrode surfaces (44), and to repeatedly alternatively assume: (a) a pressure-increasing mode of operation, in which fluid is electroosmotically driven into the nucleus pulposus (42) to increase pressure in the intervertebral disc (30), by applying a first mean voltage of less than 1.23 V between the exposed electrode surfaces (280, 44), and (b) an oxygen-generating mode of operation, in which oxygen is generated within the nucleus pulposus (42) by electrolysis, by applying a second mean voltage of at least 1.23 V between the exposed electrode surfaces (280, 44). Other embodiments are also described.
