  The microfluidic probe (MFP) is a non-contact, scanning microfluidic
technology for local (bio)chemical processing of surfaces based on
hydrodynamically confining nanoliter volumes of liquids over tens of
micrometers. We present here a compact MFP (cMFP) that can be used on a
standard inverted microscope and assist in the local processing of tissue
sections and biological specimens. The cMFP has a footprint of 175 X 100 X 140
mm^3 and can scan an area of 45 X 45 mm^2 on a surface with an accuracy of +-15
um. The cMFP is compatible with standard surfaces used in life science
laboratories such as microscope slides and Petri dishes. For ease of use, we
developed self-aligned mounted MFP heads with standardized chip-to-world and
chip-to-platform interfaces. Switching the processing liquid in the flow
confinement is performed within 90 seconds using a selector valve with a
dead-volume of approximately 5 uL. We further implemented height-compensation
that allows a cMFP head to follow non-planar surfaces common in tissue and
cellular ensembles. This was shown by patterning different macroscopic
copper-coated topographies with height differences up to 750 um. To illustrate
the applicability to tissue processing, 5 um thick M000921 BRAF V600E+ melanoma
cell blocks were stained with hematoxylin to create contours, lines, spots,
gradients of the chemicals, and multiple spots over larger areas. The local
staining was performed in an interactive manner using a joystick and a
scripting module. The compactness, user-friendliness and functionality of the
cMFP will enable it to be adapted as a standard tool in research, development
and diagnostic laboratories, particularly for the interaction with tissues and
cells.
