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We present the first hollow out-of-wafer-plane silicon microneedles having openings in the shaft rather than having an orifice at the tip. These structures are well suited for transdermal microfluidic applications, e.g., drug or vaccine delivery. The developed deep-reactive ion etching (DRIE) process allows fabrication of two dimensional, mechanically highly resistant needle arrays offering low resistance to liquid flows and a large exposure area between the fluid and the tissue. The presented process does not require much wafer handling and only two photolithography steps are required. Using a 3/spl times/3 mm/sup 2/ chip in a typical application, e.g., vaccine delivery, a 100 /spl mu/l volume of aqueous fluid injected in 2 s would cause a pressure drop of less than 2 kPa. The presented needles are approximately 210 /spl mu/m long.