**4. Conclusions**

Over the last decade, there has been an impressive advancement on scaffolds-based formulations and strategies for bioengineer functional tissues and organs in vitro and in vivo. In this context, bottom-up approaches based on the rational assembly of modular units, in the form of cell-free/cell-laden μPs and/or layers, are, nowadays, the most promising and used approaches. Polymeric μPs offer the advantage in scaffolds' design of morphology and shape control, full pores interconnectivity, high mechanical properties, and biomolecules encapsulation and release. Furthermore, cell-laden μ-scaffolds demonstrated the capability to self-assemble in vitro to form μTPs made up of endogenous ECM and tunable in size and shape. These μTPs can be further assembled in large 3D patches. After μPs degradation, the resulting tissue can be used for in vitro study of complex tissue morphogenesis or for screening normal and dysfunctional tissues' response to specific biophysical and biochemical factors.

Soft-lithography and AM techniques enabled the CAD of cell-free scaffolds and cell-laden constructs down to nano-scale resolution, thereby overcoming limitations related to in vitro cell seeding and micro-architectural features' control. Although it was possible the fabrication of patient-specific devices suitable for clinical implantation, the regeneration of even complex biological tissues aided by these scaffolds is still far from being achieved and requires extensive research efforts on materials design and processing, automated systems integration, and processing times acceleration.

In conclusion, all the results highlighted in this work indicate that the next decades challenge will be to obtain a technology platform that enables users to fabricate ECM-mimicking architectures capable of controlling cell activities and directing their fate for clinical translation and successful engineering of tissues and organs.

**Author Contributions:** Conceptualization, A.S. and P.A.N.; writing—original draft preparation, A.S., G.C. and P.P.; writing—review and editing, A.S., G.C., P.P. and P.A.N.; visualization, A.S., G.C. and P.P.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
