  Bioinspired artificial surfaces with tailored adhesive properties have
attracted significant interest. While fibrillar adhesive pads mimicking gecko
feet are optimized for strong reversible adhesion, monolithic microsphere
arrays mimicking the slippery zone of the pitchers of carnivorous plants of the
genus Nepenthes show anti-adhesive properties even against tacky counterpart
surfaces. In contrast to the influence of topography, the influence of relative
humidity (RH) on adhesion has been widely neglected. Some previous works deal
with the influence of RH on the adhesive performance of fibrillar adhesive
pads. Commonly, humidity-induced softening of the fibrils enhances adhesion.
However, little is known on the influence of RH on solid anti-adhesive
surfaces. We prepared polymeric nanoporous monolithic microsphere arrays
(NMMAs) with microsphere diameters of a few 10 {\mu}m to test their
anti-adhesive properties at RHs of 2 % and 90 %. Despite the presence of
continuous nanopore systems through which the inner nanopore walls were
accessible to humid air, the topography-induced anti-adhesive properties of
NMMAs on tacky counterpart surfaces were retained even at RH = 90 %. This
RH-independent robustness of the anti-adhesive properties of NMMAs
significantly contrasts the adhesion enhancement by humidity-induced softening
on nanoporous fibrillar adhesive pads made of the same material.
