Despite the tremendous technical advancements in 3D bioprinting, the concept of fabricating 3D structures and functional tissues directly in live animals remains a visionary challenge. We show that 3D cell-laden hydrogels can be efficiently bioprinted across tissues and within tissues of living animals.We developed photo-sensitive polymers that allow in vitro and in vivo fabrication of hydrogels into pre-existing structures, by bio-orthogonal two-photon cycloaddition and crosslinking at wavelengths longer than 850 nm, without byproducts. By this technique, that we name intravital 3D bioprinting, after injection of these polymers in vivo it is possible to fabricate complex 3D structures inside tissues of living mice, including the dermis across epidermis, the skeletal muscle across epimysium or the brain across meninges. The use of commonly available multi-photon microscopes allows accurate (XYZ) positioning and orientation of bioprinted structures into specific anatomical sites. Finally, we show that intravital 3D bioprinting of donor muscle-derived stem cells allows de novo formation of myofibers in host animals. We envision that this strategy will offer an alternative in vivo approach to conventional bioprinting technology, holding great promises to substantially change the paradigm of 3D bioprinting for pre-clinical and clinical use.

Introduction In people with haemophilia (PWH) with severe arthropathy, total joint replacement (TJR) can be undertaken if conservative management fails. Post‐operative rehabilitation treatment is an important part of the comprehensive management of patients undergoing TJR. Aim To compare post‐operative standard rehabilitation (SR) and SR plus water rehabilitation (WR) in PWH undergoing TJR. Methods PWH who were admitted to our centre between June 2003 and December 2016 for rehabilitation after TJR were included in the study. Rehabilitation included SR (ie, manual and mechanical mobilization, scar tissue massage, light muscle strengthening exercises and walking training with and without crutches) with or without WR. WR exercises with floats of different size and volume were performed when possible. Range of motion (ROM), muscle strength, pain level, perceived health status and length of hospital stay were analysed retrospectively. Results A total of 184 patients (233 rehabilitation programmes were enrolled in the study, corresponding to 160 after total knee replacement [TKR], 37 after total ankle replacement [TAR] and 36 after total hip replacement [THR]). Fifty‐eight (25%) patients were treated with WR in addition to SR (32 for TKR, 19 for TAR and 7 for THR) with an average of 5.7 hours of WR. Muscle strength, pain and perceived health status improved significantly after rehabilitation. Conclusion This non‐randomized study seems to indicate that WR plus SR improves muscle strength, pain and perceived health status more than SR alone in PWH undergoing TJR. It would be necessary, however, to carry out randomized comparative studies to confirm these provisional conclusions.