A method of producing an array of sharp tips including a biocompatible coating, wherein the biocompatible coating is thicker at a sharp end of the sharp tips than at a broader section of the sharp tips, the method including providing an array of sharp tips, and coating the sharp end of the sharp tips differentially from coating the broader section of the sharp tips. A method of treating skin including producing a hollow in the skin by heating and mechanically compressing epidermis while retaining a covering of stratum corneum. A method of treating tissue including heating a tip to a temperature suitable for producing a crater in the tissue, advancing the tip toward the tissue, detecting when the tip comes into contact with the tissue by detecting a change in mechanical resistance to the advancing, and measuring the mechanical resistance to the advancing. Related apparatus and methods are also described.