  Additive manufacturing (AM) techniques have gained interest in the tissue
engineering field thanks to their versatility and unique possibilities of
producing constructs with complex macroscopic geometries and defined patterns.
Recently, composite materials - namely heterogeneous biomaterials identified as
continuous phase (matrix) and reinforcement (filler) - have been proposed as
inks that can be processed by AM to obtain scaffolds with improved biomimetic
and bioactive properties. Significant efforts have been dedicated to
hydroxyapatite (HA)-reinforced composites, especially targeting bone tissue
engineering, thanks to the chemical similarities of HA with respect to mineral
components of native mineralized tissues. Here we review applications of AM
techniques to process HA-reinforced composites and biocomposites for the
production of scaffolds with biological matrices, including cellular tissues.
The primary outcomes of recent investigations in terms of morphological,
structural, and in vitro and in vivo biological properties of the materials are
discussed. We classify the approaches based on the nature of the matrices
employed to embed the HA reinforcements and produce the tissue substitutes and
report a critical discussion on the presented state of the art as well as the
future perspectives, to offer a comprehensive picture of the strategies
investigated as well as challenges in this emerging field.
