REDONDO will develop reversibly cross-linked polyethylene (rPEX), rendering the up-to-date non-recyclable PEX a recyclable-by-design innovative polymer. Achievement of reversibility will be conducted via two different thermally reversible reactions for PE cross-linking: one based on C-S-S bonds and one exploiting the Diels Alder reaction. The produced rPEX will be characterized and its reversibility assessed. Key to this project is the adoption of the Safety-by-Design and Sustainability-by-Design frameworks, incorporating the outcomes of safety- and sustainability-related assessments already from the design stage in the polymer formulation process. Use of innovative green and bio-based additives (LPNs and MFC) will enable for further improvement of desired properties (thermal stability, mechanical properties and flammability) without introducing health risks or jeopardizing the environmental sustainability of the materials and processes. Nanolignin (LNPs) and nanocellulose (MFC) are being produced and studied as rPEX additives for the first time. An inventory of additives tool will enable further adoption of this approach. Two high potential impact end-use applications have been placed under focus: pipes and photovoltaic cables. The efficiency of the adopted approach and technologies will be demonstrated and validated according to the specification and requirements of end-users. Moreover, a number of analyses will be performed to evaluate the safety and sustainability of rPEX but also to establish the safety and sustainability-by-design criteria and the recyclability requirements. This will facilitate the extension of this concept to other end-use applications of rPEX or to the design and development of other cross-linked -not currently recyclable- polymers as reversible ones.