title: Clinical microneedle injection of methyl nicotinate: stratum corneum penetration.
abstract: OBJECTIVE
  In recent years, microneedles were proposed as a method to painlessly deliver drugs past the stratum corneum. Microneedles have been fabricated in several designs, but limited studies have tested microneedle injections in humans. In this work, we compare microneedle injections with topical application (TA) to investigate if microneedles enhance in vivo drug delivery past the stratum corneum.
  
  
  METHODS
  In vitro tests were used to measure microneedle pressures and injection volumes. In vivo microneedle injections were performed on the volar forearm of 11 healthy volunteers. Two sets of microneedles, pointed and symmetric, were used to develop microneedle/syringe apparatuses that were used to inject approximately 1 microL of 0.1 M methyl nicotinate, and were compared against TA. A Laser Doppler Perfusion Monitor was used to record maximum blood flow and the time to maximum blood flow at the treatment sites.
  
  
  RESULTS
  Pointed and symmetric microneedle-injected sites showed a significantly faster time to maximum blood flow than TA. The pointed microneedle injections also resulted in a higher maximum blood flux. Volunteers reported feeling pressure but no pain from the microneedles during the injections.
  
  
  CONCLUSIONS
  The microneedles aid in bypassing the stratum corneum and enhance drug delivery through it. The design of the microneedle influences its delivery capabilities, because the pointed microneedles seem to be less susceptible to clogging during the injection.
authors.affiliation: Department of Bioengineering, University of California, Berkeley, CA, USA.
authors.forename: Raja K
authors.initials: RK
authors.lastname: Sivamani
authors.forename: Boris
authors.initials: B
authors.lastname: Stoeber
authors.forename: Gabriel C
authors.initials: GC
authors.lastname: Wu
authors.forename: Hongbo
authors.initials: H
authors.lastname: Zhai
authors.forename: Dorian
authors.initials: D
authors.lastname: Liepmann
authors.forename: Howard
authors.initials: H
authors.lastname: Maibach
mesh_terms: D000287:Administration, Topical; D000293:Adolescent; D000328:Adult; D001783:Blood Flow Velocity / Q000187:drug effects*; D004817:Epidermis / Q000187:drug effects* / Q000502:physiology*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D008297:Male; D008845:Microinjections / Q000295:instrumentation* / Q000379:methods*; D008904:Miniaturization; D009339:Needles; D009539:Nicotinic Acids / Q000008:administration & dosage*; D010812:Physical Stimulation / Q000379:methods; D012867:Skin / Q000098:blood supply; D012869:Skin Absorption / Q000187:drug effects / Q000502:physiology; D013314:Stress, Mechanical
chemical_list: D009539:Nicotinic Acids; C008473:methyl nicotinate