A microstructure array comprising: a planar substrate having a top surface and a bottom surface; a reservoir in fluid communication with the top surface of the planar substrate; a plurality of microstructures projecting from the bottom surface of the planar substrate, each of the plurality of microstructures comprising: a respective solid body portion tapering from a respective base to a respective distal tip positioned at a respective height from the bottom surface of the planar substrate, the respective solid body portion from the respective base to the respective distal tip defining a respective microstructure surface; a respective first delivery channel or slit extending from the top surface of the planar substrate to a respective first channel or slit opening within the respective microstructure surface, thereby fluidly connecting the reservoir to the respective first channel or slit opening; and a respective second delivery channel or slit extending from the top surface of the planar substrate to a respective second channel or slit opening within the microstructure surface, thereby fluidly connecting the reservoir to the respective second channel or slit opening; characterized in that the microstructure array further comprises: a first electrode in electrical contact with the reservoir and a second electrode configured to electrically contact a tissue positioned against the bottom surface of the planar substrate, wherein the solid body portion of each of the plurality of microstructures is formed from silicon-using photolithography and etching, wherein the first delivery channel or slit and second delivery channel or slit have an inner diameter of 1 to 999 nm, wherein the respective first channel or slit opening is positioned within a first plane parallel to the planar substrate, wherein the respective second channel or slit opening is positioned within a second plane parallel to the planar substrate, and wherein the first plane is distally spaced apart from the second plane by a distance of from 20% to 60% of the height from the bottom surface of the planar substrate; wherein the respective first channel or slit opening is positioned at the respective distal tip. a planar substrate having a top surface and a bottom surface; a reservoir in fluid communication with the top surface of the planar substrate; a plurality of microstructures projecting from the bottom surface of the planar substrate, each of the plurality of microstructures comprising: a respective solid body portion tapering from a respective base to a respective distal tip positioned at a respective height from the bottom surface of the planar substrate, the respective solid body portion from the respective base to the respective distal tip defining a respective microstructure surface; a respective first delivery channel or slit extending from the top surface of the planar substrate to a respective first channel or slit opening within the respective microstructure surface, thereby fluidly connecting the reservoir to the respective first channel or slit opening; and a respective second delivery channel or slit extending from the top surface of the planar substrate to a respective second channel or slit opening within the microstructure surface, thereby fluidly connecting the reservoir to the respective second channel or slit opening; characterized in that the microstructure array further comprises: a respective solid body portion tapering from a respective base to a respective distal tip positioned at a respective height from the bottom surface of the planar substrate, the respective solid body portion from the respective base to the respective distal tip defining a respective microstructure surface; a respective first delivery channel or slit extending from the top surface of the planar substrate to a respective first channel or slit opening within the respective microstructure surface, thereby fluidly connecting the reservoir to the respective first channel or slit opening; and a respective second delivery channel or slit extending from the top surface of the planar substrate to a respective second channel or slit opening within the microstructure surface, thereby fluidly connecting the reservoir to the respective second channel or slit opening; characterized in that the microstructure array further comprises: a first electrode in electrical contact with the reservoir and a second electrode configured to electrically contact a tissue positioned against the bottom surface of the planar substrate, wherein the solid body portion of each of the plurality of microstructures is formed from silicon-using photolithography and etching, wherein the first delivery channel or slit and second delivery channel or slit have an inner diameter of 1 to 999 nm, wherein the respective first channel or slit opening is positioned within a first plane parallel to the planar substrate, wherein the respective second channel or slit opening is positioned within a second plane parallel to the planar substrate, and wherein the first plane is distally spaced apart from the second plane by a distance of from 20% to 60% of the height from the bottom surface of the planar substrate; wherein the respective first channel or slit opening is positioned at the respective distal tip.