Optimizing Design Parameters of P4HB 3D-printed Scaffolds for Long-lasting Reconstructed Nipples

PURPOSE: Surgically creating nipples is a crucial step in breast reconstruction. However, autologous tissue techniques are limited due to scar contracture and significant loss of neo-nipple projection. The use of biodegradable polymers Poly-4-Hydroxybutyrate (P4HB), with a long track record of clinical use as a surgical mesh (Phasix), has been previously proved to promote fast tissue ingrowth as 3D-printed nipple scaffolds with cartilage filled as interior support, in parallel with a stable projection maintenance of the reconstructed nipple. Herein we aimed to determine and optimize the design parameters of the 3D-printed P4HB cylindrical scaffold to facilitate neotissue formation in order to better mimic the biomechanical properties of the native nipple while maintaining long-term projection. METHODS: P4HB nipple scaffolds were 3D-printed (diameter: 1.0cm, height: 1.0cm) with different internal 3D conformations of P4HB filaments (infill density, filament diameter, external shell present/no shell) and implanted subcutaneously in rats using a CV flap technique. Cook Biodesign® Nipple Cylinder (G1), previously-studied P4HB scaffolds with the internal latticework (G2), P4HB scaffolds (only external shell present/empty, G3), manually-rolled Phasix mesh scaffolds (G4) and mechanically-thermoformed Phasix mesh scaffolds (G9) were implanted for comparison. The infill density of G5/G6, G7 and G8 is 20, 25 and 30%. The filament diameter of G6 is 0.15mm, and 0.2mm for the other 3D groups. RESULTS: Nipple reconstructions with internal 3D-printed P4HB latticework and Phasix mesh were well maintained in diameter over 6 months (~100%). Similarly, the projection reservation was significant improved over the first 3 months among those groups (>90%) and only slightly decreased in G4 and G6 after 6 months (80% and 86% respectively). In contrast, the Cook Biodesign® nipple (G1) lost 25%, 54% and 60% projection at 1, 3 and 6 months (p<0.05), due to the insufficient internal support to resist skin contraction because of rapid absorption of the SIS (small intestine submucosa) substrate. After 6 months, nearly 100% of the interior space of 3D-printed scaffolds was filled by tissue ingrowth and the inflammatory tissue seen at 1 month was replaced by mostly healthy fibrovascular tissue with adipocytes generated on the outermost layer between filaments. The starting stiffnesses were different between 3D-printed groups (ranging from 1.21 to 9.54MPa), all declined over time which were approaching that of native nipples (0.27MPa) by 6 months (ranging from 0.73 to 2.94MPa), as the P4HB polymers gradually degraded and were replaced by tissue ingrowth. CONCLUSIONS: Engineered neo-nipples with P4HB 3D-printed scaffolds demonstrated notable diameter and projection maintenance over 6 months, with no significant differences observed between different design parameters. As the P4HB filaments degrade over time and the scaffold loses structural integrity, the adipose fibrovascular tissue formed within provides the structure which allows the engineered nipple to maintain proper shape, volume and biomechanical qualities that approaches that of native nipples. Long-term observation out to 1 year is ongoing to verify the translatability of this novel 3D-P4HB nipple scaffold.