Miniaturized Human Tendons: A Bioengineering Perspective on on-Chip Models

Tendon tissues exhibit complex physiology and poor regenerative capacity, making the treatment of tendinopathies and injuries a significant challenge in orthopedics and sports medicine. It is increasingly recognized that tendons harbor a heterogeneous cell population composed of distinct subtypes with specialized functions, within a dynamic extracellular matrix (ECM) with context-dependent composition and architecture. However, the biological mechanisms driving the onset and progression of tendinopathy - often leading to fibrosis and reduced biomechanical competence - remain poorly understood. A major limitation in this field is the lack of reductionist yet representative human in vitro models that can provide mechanistic insights into tendon homeostasis, development, and disease, which are often difficult to study in vivo using animal models. To address this gap, we have applied a range of bioengineering approaches to develop human-based microphysiological systems that replicate the multicellular organization and three-dimensional architecture of tendon tissues. This talk will focus on the design principles and fabrication strategies we have explored to develop tendon-on-chip models using two distinct platforms: conventional microfluidic devices and automated 3D writing within support matrices. While differing in technical implementation, both approaches share the goal of recreating the anisotropic architecture, cell organization, and phenotype of the intrinsic (core) tendon compartment. Given their importance in both tendon development and degeneration, these models incorporate vascular-like structures that emulate the interface between intrinsic and extrinsic tendon compartments, allowing the study of bidirectional crosstalk between these tendon cell populations. Additionally, we demonstrate how these platforms can be leveraged to investigate adaptive immune cell responses at the onset of tendon inflammation, focusing on interactions between tendon core tenocytes and circulating T cells. We will discuss the advantages and limitations of each system, as well as key findings regarding the impact of multicellular crosstalk on progenitor cell tenogenesis, tenocyte phenotype, T-cell recruitment, and inflammatory secretome composition. Overall, our 3D tendon-on-chip models provide powerful new tools for investigating the biological mechanisms and signaling pathways underlying tendon physiology and pathology, as well as for evaluating potential therapeutic strategies. This research was supported by the EU Horizon 2020 ERC under grants No. 772817 and 101171765, as well as the FCT projects 10.54499/2020.03410.CEECIND/CP1600/CT0013 and 10.54499/2022.05526.PTDC. The TENET COST Action CA22170 is also acknowledged for fostering collaboration and networking opportunities.