4D polycarbonates via stereolithography as scaffolds for soft tissue repair

The study addresses the unmet clinical need for minimally invasive, resorbable, and biocompatible scaffolds that can reliably fill soft tissue voids (e.g., after breast cancer surgery) while enabling tissue ingrowth and maintaining mechanical support during healing. Existing porous implants and foams often have uneven pore distributions and require post-processing, which limits nutrient diffusion and reproducibility. While 3D printing can create interconnected, well-defined porosity, suitable photopolymerizable materials with long-term in vivo biocompatibility are scarce. Commonly used acrylate/epoxide systems have low toxicity thresholds, and widely used degradable biomaterials like PLLA are difficult to process by photopolymerization and degrade to acidic products. The authors hypothesize that aliphatic polycarbonate-based, photopolymerizable resins with shape memory (4D behavior) can be engineered to produce soft, elastomeric, surface-eroding scaffolds that are cytocompatible/biocompatible, conform to irregular voids with low expansion forces, and degrade to non-acidic byproducts while supporting soft tissue regeneration.

Prior porous implants (e.g., siloxane maxillofacial devices, polyurethane occlusive devices, collagen-derived adipose scaffolds) provide mechanical support but are limited by variable pore morphologies and post-processing requirements. Additive manufacturing, particularly stereolithography/DLP, offers precise, reproducible porosity with micron-scale control but lacks material systems that combine printability, tunable mechanics, degradability, and biocompatibility. Traditional photopolymers (acrylates/epoxides) present toxicity concerns; PLLA is popular but challenging for photoprocessing and yields acidic degradation byproducts. 4D printing introduces time-dependent behaviors (shape memory, swelling, controlled degradation) beneficial for minimally invasive deployment; however, existing minimally invasive foams often suffer from poor pore interconnectivity, limited nutrient diffusion, and variable shape-change reproducibility. There are few examples of clinically relevant 4D-printed, minimally invasive scaffolds that combine non-inflammatory degradation, controlled mechanics, and reliable shape response.

Materials design and synthesis: Allyl- and norbornene-functional aliphatic cyclic carbonate monomers were synthesized: TMPAC and NTC. Organocatalytic ring-opening polymerization (ROP) using DBU/H2O produced low-molar-mass (Mn ~2 kDa, Ð ~1.1) oligocarbonates: polyTMPAC, polyNTC, and copolymers. Structures were confirmed by 1H NMR, 13C NMR, FT-IR, and SEC. Resin formulation: Oligomers were combined with multifunctional thiol crosslinker PETMP to enable radical thiol-ene photopolymerization. Urethane-containing reactive diluents (isophorone- and hexamethylene-based) were used to modulate viscosity (<10 Pa·s) and network properties. A 405 nm photoinitiator and a paprika-extract photoinhibitor ensured controlled layer curing during DLP. Chain-extended poly(carbonate urethane)s (via diisocyanates) were also explored but had higher viscosities and were not the primary focus. Photopolymerization and printing: Resins were photocured by 405 nm exposure. Photorheology tracked gelation (loss factor, storage modulus, complex viscosity). DLP stereolithography fabricated porous scaffolds with designed pore sizes (200–1500 µm) and surface areas (1–3 cm²). µCT verified pore fidelity (within ~5% of design). Additional foamed scaffolds were made by gas-blowing for comparison. Cytocompatibility: 2D spin-coated films (0.4 wt% in CHCl3 on coverslips) and 3D scaffolds were evaluated with multiple cell lines: murine fibroblasts (NOR-10), human fibroblasts (Hs 792), murine macrophages (IC21), murine adipocytes (D16), and murine pre-osteoblasts (MC3T3). Assays followed ISO 10993 protocols, including direct/indirect contact, PrestoBlue proliferation, live/dead staining, and confocal imaging of adhesion/spreading. A pyramidal structure with smooth vs stair-step sides assessed surface-morphology effects on proliferation. Cell infiltration in printed vs foamed scaffolds was compared. Thermomechanical characterization: DMA (tension and compression) determined glass transition (dry and PBS-plasticized at 37 °C), tan δ, storage modulus, and relaxation behavior. Tensile tests on dogbones (10 mm/min) measured elastic modulus, strength, and strain to failure. Compressive cyclic tests (up to 100 cycles) in PBS at 37 °C characterized scaffold elasticity, energy dissipation, and stability. Shape memory and expansion behavior: Shape fixation and recovery were quantified by DMA and optical tracking of compressed (up to ~80%) scaffolds at ambient and 37 °C in PBS. Expansion forces were measured during recovery. Alginate hydrogels (elastic modulus ~60 kPa) served as adipose-like tissue models; void-filling efficiency and gel deformation were assessed in eye-shaped incisions and in 3D-printed rigid molds. Simplified computational models compared with experimental gel deformation. Degradation studies: In vitro hydrolytic degradation assessed by static gravimetry and dynamic mechanical immersion (films deformed at 1 Hz in 5 M NaOH at 37 °C) to monitor phase ratio and storage modulus until failure. Printed scaffold strut erosion was tracked microscopically. In vivo subcutaneous implantation in rats compared PLLA controls, nonporous PTMPCTX disks, and porous PTMPCTX and PNTCTX disks (500 µm pores) over 1–4 months. Post-explant swelling ratios, gel fraction (crosslink density), FT-IR and 1H NMR analyses estimated mass loss and degradation mode; ex vivo strut diameters were measured. Histology and scoring: H&E and Masson’s Trichrome stains evaluated tissue response, adipocyte infiltration, lobule formation, capsule thickness, and vascularization. Inflammatory cell presence was scored per ISO 10993-6 Annex E. Ethics approvals and surgical procedures followed EU Directive 2010/63/EU and UK regulations.

- Resin inks and rapid curing: Visible-light (405 nm) thiol-ene resins gelled rapidly; photorheology showed storage modulus increase from 179.6 ± 17.5 Pa to 1.5 ± 0.4 MPa and complex viscosity from 3.1 ± 0.1 Pa·s to 23.1 ± 8.3 MPa·s within seconds. 1H NMR confirmed near-complete allyl/thiol consumption within 30 s. - Print fidelity: DLP produced porous scaffolds with designed pore sizes 200–1500 µm; µCT measured pore sizes within ~5% of CAD values. - Cytocompatibility: Across murine/human fibroblasts, macrophages, adipocytes, and pre-osteoblasts, no significant differences in viability or proliferation over 7 days; cells adhered and spread well. 3D printed scaffolds supported cell proliferation throughout thickness for pores 250–1500 µm; gas-blown foams exhibited surface-only proliferation with limited infiltration, indicating superior interconnectivity in printed scaffolds. Surface morphology (stair-step vs smooth) did not affect proliferation, implicating chemistry as key for cytocompatibility. - Tunable thermomechanics: Tg (dry) tuned from ~0.3 °C (PTMPCTX) to ~88.2 °C (PNTCTX); plasticized Tg from ~−20.6 °C to ~87.1 °C. Tensile properties spanned elastic modulus ~15.2 MPa (PTMPCTX) to ~776 MPa (P(TMPCTX25-NTCTX75)); UTS up to ~22 MPa; strain at break from ~40% (PNTCTX) to ~144% (PTMPCTX). Scaffolds tolerated compressive strains up to ~85–90% with full shape recovery. - Cyclic durability: PTMPCTX in PBS at 37 °C over 100 compression cycles showed minimal changes (elastic modulus ~1.1 MPa; UTS 8.4→8.0 MPa from cycle 1 to 100). PNTCTX (stiffer) showed partial recovery reduction under ambient cyclic loading that stabilized after ~15 cycles. - Shape memory and void filling: All compositions exhibited 100% strain fixation below Tg and 100% recovery above Tg; at 37 °C in PBS, compositions showed shape memory with kinetics dependent on NTC content/Tg. PTMPCTX recovered 100% strain within ~45 s but induced ~15% alginate deformation; void filling ~90% depending on shape. PNTCTX recovered more slowly (passive ~50 min at 37 °C; active ~10 min with 50 °C water), achieving 100% void filling and ~90% strain recovery at the scaffold center with low expansion forces. - Expansion forces: PTMPCTX peak force 0.52 ± 0.24 N at 25 °C; initial relaxation rate ~1.3 mN s−1 (first 10 min). PNTCTX peak force 0.71 ± 0.19 N at ~3 min; relaxation rate ~0.3 mN s−1 (first 30 min), with tan δ/storage modulus changes consistent with controlled creep—favorable for self-fitting with minimal tissue compression. - Degradation: Materials underwent surface erosion. In vitro, strut cross-sections decreased progressively in 5 M NaOH at 37 °C; dynamic loading accelerated erosion. In vivo (rat, subcutaneous), swelling remained minimal and stable over 4 months; gel fraction remained >99% initially, with all SMPs showing ~80% mass remaining at 4 months (extrapolated total mass loss ~20 months). PLLA controls showed negligible mass loss in same period. - Host response and integration: Adipocyte infiltration evident by 1 month; distinct adipocyte lobules within scaffold pores by 2 months, with ~40% of infiltrated tissue composed of lobules. Capsule thickness around implants <200 µm; porous polycarbonate scaffolds exhibited ~50 µm capsules, approximately half of solid polycarbonates and smaller than PLLA (~120 µm). Macrophage profiles indicated healing rather than severe inflammation. Neovascularization observed by 2 months with small mature vessels at 4 months. No calcification or necrosis observed.

The results demonstrate that aliphatic polycarbonate-based, thiol-ene photopolymer resins can be additively manufactured into highly porous scaffolds that are elastomeric, shape-memory capable, and degrade by surface erosion to non-acidic products. The tunable Tg and mechanical properties enable tailoring of working times and expansion forces to minimize tissue compression while achieving self-fitting in irregular voids, addressing a key limitation of traditional foams and nonresponsive implants. Cytocompatibility across relevant cell types, deep cellular infiltration in printed architectures, and favorable in vivo tissue responses (reduced capsule thickness, adipocyte lobule formation, and vascularization) support their suitability for adipose tissue repair and broader soft tissue applications. The surface-erosion degradation with minimal swelling preserves structural support over months while allowing progressive tissue replacement, thereby meeting the design goal of consistent mechanical support during healing without acidic byproducts. Collectively, the approach addresses the identified gaps in materials for DLP-based tissue scaffolds and provides a platform for minimally invasive, patient-specific soft tissue reconstruction.

This work introduces a 4D-printable aliphatic polycarbonate platform that combines rapid visible-light photopolymerization, precise porous architectures, tunable thermomechanics and Tg, robust shape memory with controllable expansion forces, excellent cytocompatibility/biocompatibility, and surface-erosion degradation to non-acidic byproducts. The scaffolds self-fit into soft tissue voids with minimal deformation of surrounding tissue and support adipose tissue ingrowth and neovascularization in vivo while reducing fibrous capsule formation compared to controls. Future research should optimize formulation-specific working windows for targeted clinical indications, expand in vivo studies to larger animal models and anatomically relevant defect sites, evaluate long-term functional outcomes and complete resorption timelines, and explore applications in other minimally invasive devices (e.g., laparoscopic tools, catheters, compressible stents/patches) through further tuning of composition, porosity, and surface chemistry.

- The in vivo evaluation was limited to a subcutaneous rat model over 4 months; mass-loss timelines beyond this period are extrapolated (~20 months) rather than directly measured. Translation to human anatomical sites and load/strain environments remains to be validated. - Shape memory performance is temperature-dependent; at very low or high temperatures, practical utilization may be limited. Formulation selection requires application-specific optimization of Tg and recovery kinetics. - Very small pore sizes (<250 µm) were difficult to fabricate reproducibly due to resin viscosity and overcuring, limiting exploration of finer pore architectures. - Expansion force measurements and void-filling tests were conducted in alginate gels and rigid printed molds; while informative, these are surrogates for complex soft tissues and surgical environments. - Cytocompatibility was assessed over 7–14 days and with select cell lines; longer-term cell–material interactions and immunomodulation were not comprehensively characterized. - Stiffer compositions (e.g., PNTCTX) showed slower recovery and partial recovery reduction under rapid cyclic loading at ambient conditions, which may necessitate controlled activation strategies in practice.