  Acoustical tweezers open major prospects in microbiology for cells and
microorganisms contactless manipulation, organization and mechanical properties
testing since they are biocompatible, label-free and can exert forces several
orders of magnitude larger than their optical counterpart at equivalent wave
power. Yet, these tremendous perspectives have so far been hindered by the
absence of selectivity of existing acoustical tweezers -- i.e., the ability to
select and move objects individually -- and/or their limited resolution
restricting their use to large particle manipulation only. Here, we report
precise selective contactless manipulation and positioning of human cells in a
standard microscopy environment, without altering their viability. Trapping
forces of up to $\sim$ 200 pN are reported with less than 2 mW of driving
power. The unprecedented selectivity, miniaturization and trapping force are
achieved by combining holography with active materials and fabrication
techniques derived from the semi-conductor industry to synthesize specific
wavefields (called focused acoustical vortices) designed to produce stiff
localized traps. We anticipate this work to be a starting point toward
widespread applications of acoustical tweezers in fields as diverse as tissue
engineering, cell mechano-transduction analysis, neural network study or mobile
microorganisms imaging, for which precise manipulation and/or controlled
application of stresses is mandatory.
