  Light-based additive manufacturing holds great potential in the field of
bioprinting due to its exceptional spatial resolution, enabling the
reconstruction of intricate tissue structures. However, printing through
biological tissues is severely limited due to the strong optical scattering
within the tissues. The propagation of light is scrambled to form random
speckle patterns, making it impossible to print features at the
diffraction-limited size with conventional printing approaches. The poor tissue
penetration depth of ultra-violet or blue light, which is commonly used to
trigger photopolymerization, further limits the fabrication of high
cell-density tissue constructs. Recently, several strategies based on wavefront
shaping have been developed to manipulate the light and refocus it inside
scattering media to a diffraction-limited spot. In this study, we present a
high-resolution additive manufacturing technique using upconversion
nanoparticles and a wavefront shaping method that does not require measurement
from an invasive detector, i.e., it is a non-invasive technique. Upconversion
nanoparticles convert near-infrared light to ultraviolet and visible light. The
ultraviolet light serves as a light source for photopolymerization and the
visible light as a guide star for digital light shaping. The incident light
pattern is manipulated using the feedback information of the guide star to
focus light through the tissue. In this way, we experimentally demonstrate that
near-infrared light can be non-invasively focused through a strongly scattering
medium. By exploiting the optical memory effect, we further demonstrate
micro-meter resolution additive manufacturing through highly scattering media
such as a 300-{\mu}m-thick chicken breast. This study provides a proof of
concept of high-resolution additive manufacturing through turbid media with
potential application in tissue engineering.
