  Stereolithography (SLA) and digital light processing (DLP) are powerful
additive manufacturing techniques that address a wide range of applications
including regenerative medicine, prototyping, and manufacturing. Unfortunately,
these printing processes introduce micrometer-scale anisotropic inhomogeneities
due to the resin absorptivity, diffusivity, reaction kinetics, and swelling
during the requisite photoexposure. Previously, it has not been possible to
characterize high-resolution mechanical heterogeneity as it develops during the
printing process. By combining DLP 3D printing with atomic force microscopy in
a hybrid instrument, heterogeneity of a single, in situ printed voxel is
characterized. Here, we describe the instrument and demonstrate three
modalities for characterizing voxels during and after printing. Sensing
Modality I maps the mechanical properties of just-printed, resin-immersed
voxels, providing the framework to study the relationships between voxel sizes,
print exposure parameters, and voxel-voxel interactions. Modality II captures
the nanometric, in situ working curve and is the first demonstration of in situ
cure depth measurement. Modality III dynamically senses local rheological
changes in the resin by monitoring the viscoelastic damping coefficient of the
resin during patterning. Overall, this instrument equips researchers with a
tool to develop rich insight into resin development, process optimization, and
fundamental printing limits.
