The invention relates to a multi-electrode array comprising a plurality of electrodes, the electrodes being spaced apart from each other by spacer means, the electrodes being secured to the spacer means, the spacer means being encapsulated within a housing. The invention also relates to an implantable device comprising a multi-electrode array of the invention, a method of manufacturing electrodes for use in an array of the invention and a method for manufacturing a multi-electrode array of the invention. The invention also relates to a method for monitoring the effect of a test substance on a biological tissue using a multi-electrode array of the invention.