Beyond their development, organoids have integrated mechanisms to self-maintain their function through their lifespan. The hair-bearing organoid homeostasis mechanisms are described to illustrate this unique property of selfmicrofabrication production units. In vivo, hair follicles contain bulge stem cells that differentiate and eventually replenish the hair matrix cells, which will form the solid hair shaft 261 (Fig. ). This stem cell homeostasis strategy shares common characteristics with other adult tissue homeostasis 262 . A small pool of cells remains quiescent, maintaining their stemness via microenvironmental signals from their niche, including bone morphogenetic protein (BMP), transforming growth factor beta (TGFβ) and Wnt inhibition . The anagen phase activates bulge stem cells 264 , which migrate downward through various CellµEnvs guiding their differentiation 265 . They become transit amplifying cells for rapid proliferation, and then progenitors before reaching the DP and giving rise to matrix cells 266 . At the dermal side of the hair follicle bulb, DP cells regulate signaling and nutrient supply, influencing the surrounding CellµEnv 267 (Fig. ). DP cells interact with matrix cells using diffusing signals through the basement membrane and regulate their differentiation . Matrix cells proliferate rapidly and undergo a keratinization process while migrating away from the DP signals . They increase the expression of type I and II keratin proteins, which assemble in heterodimers, then tetramers and finally align headto-tail in intermediate filaments . Keratohyalin granules begin to form, increasing keratin's concentration and storing it within the cells in preparation for their solidification. Inside the granules, filaggrin and trichohyalin proteins help bundle filaments into microfibrils while transglutaminase stabilizes them by cross-linking cysteines into disulfide bonds 271 , which contributes to the mechanical stiffening and hardening of the hair 272 . During cell migration, keratinocytes' nuclei and organelles slowly degrade, halting metabolic activity. The cells are progressively transformed and compacted together in a multilayer solid material composed of microstructured keratin fibers . The hair-bearing organoid is currently used as a model in development, wound repair, and hair loss research and creates opportunities beyond medical applications . A keratinization organoid model offers a promising foundation for studying other keratin-based biological materials found in vertebrates (e.g. nails, horns, scales, shells, beaks, feathers, and hooves 273 ) and invertebrates (e.g. marine organisms 274 ). The hair organoid produces a hair shaft with all seven concentric keratinized cell layers found in vivo 2 (Fig. ). None of the material processing steps, including the keratinization or crosslinking, have been directly engineered nor regulated as it is in artificial systems. The complex organogenesis and maintenance of this materialproducing organoid require minimal inputs and leverages existing self-organization programs 260 . To the best of our knowledge, no synthetic system can autonomously regulate the formation of such hierarchical material at this scale, nor its production unit, highlighting the innovative potential of selfmicrofabrication. However, developing organoids in new species raises fundamental challenges 275 , developed in the next section.