Official Achievement Announcement

We have successfully developed composite slotted semi‑rigid shafts based on high‑yield‑strength stainless steel (304V/316L) and super‑elastic nickel‑titanium alloy (NiTi), achieving breakthrough optimization in material mechanical properties. Through innovative material formulation and heat‑treatment processes, the product retains the super‑elasticity of NiTi alloy (8.5% recoverable strain) while raising the yield strength of stainless steel to 1250 MPa. Tests verify that the composite shaft delivers an elastic recovery rate of 99.8%, with performance degradation less than 3% after one million bending cycles, offering a revolutionary material solution for high‑frequency, high‑precision interventional surgeries.

R&D Background & Pain Points

Conventional single‑material slotted shafts suffer inherent limitations in material performance. Medical‑grade stainless steel (316L) features high yield strength (typically 690 MPa) yet limited elasticity, with a maximum recoverable strain of only 0.3–0.5%, prone to plastic deformation and fatigue cracks under repeated bending. NiTi alloy exhibits outstanding super‑elasticity (6–8% recoverable strain) but relatively low yield strength (400–800 MPa), which may cause excessive bending and kinking in complex anatomical pathways. Differences in thermal expansion coefficients between the two materials (17.3×10⁻⁶/°C for stainless steel vs. 10.4×10⁻⁶/°C for NiTi alloy) induce interfacial stress concentration in composite structures and shorten service life.Clinical studies show that surface oxide layers of pure NiTi shafts start peeling after more than 500 000 cycles, potentially releasing nickel ions to trigger allergic reactions; stainless steel shafts suffer permanent deformation and 25% reduction in bending stiffness after only 200 000 cycles. Material selection has become a critical bottleneck restricting shaft performance.

Core Technological Innovations

1. Gradient Composite Metallurgy TechnologyStainless‑steel‑NiTi alloy gradient composite tubes are fabricated via powder metallurgy and hot isostatic pressing to realize continuous material transition. From the inner to outer layer, NiTi content decreases gradiently from 100% to 0%, while stainless steel content increases from 0% to 100%. The transition layer thickness is precisely controlled at 30–80 μm. Molecular dynamics simulations optimize the interfacial structure, achieving an interfacial bonding strength of 500 MPa and gradient variation of thermal expansion coefficients to eliminate thermal stress concentration.
2. Precise Regulation of Nanocrystalline StructuresA combined process of high‑pressure torsion and low‑temperature annealing refines stainless steel grain sizes to below 30 nm. Strengthened by the Hall‑Petch effect, the nanocrystalline structure impedes dislocation motion, raising yield strength to 1250 MPa while maintaining 18% elongation. For NiTi alloy, two‑step aging treatment (350 °C × 1 h + 450 °C × 30 min) regulates precipitate size and distribution, confining phase‑transformation hysteresis within 3 °C and boosting super‑elasticity stability by 40%.
3. Multifunctional Composite Surface CoatingA multilayer gradient titanium‑nitrogen‑carbon coating is developed, forming a 2–3 μm functional layer on the surface via physical vapor deposition. The coating achieves a hardness of HV 2800 and a friction coefficient of 0.12, with excellent biocompatibility. Trace silver and copper ions (0.5–1.0 at% each) are doped into the coating for sustained‑release antibacterial functions, attaining >99.5% bacteriostatic rates against Staphylococcus aureus and Escherichia coli. Cytotoxicity tests comply with ISO 10993‑5 standards.

Working Mechanism

The advantages of composite shafts stem from multi‑scale synergistic effects. At the atomic scale, reversible martensitic transformation of NiTi alloy under stress provides super‑elasticity and shape‑memory effects; the nanocrystalline structure of stainless steel enhances strength and fatigue resistance via grain‑boundary strengthening and dislocation pinning. At the microscale, the gradient transition layer enables smooth variation of elastic modulus (40–60 GPa at the NiTi end, 190–210 GPa at the stainless steel end), matching biomechanical properties of different tissues and reducing stress‑shielding effects. At the macroscale, the composite structure delivers a mechanical response of balanced rigidity and flexibility: stainless steel provides axial pushing force and torsional rigidity to ensure 1:1 torque transmission; NiTi alloy offers radial compliance and shape‑recovery capability for immediate straightening after bending. The functional coating reduces protein and cell adhesion by lowering surface energy, while sustained release of silver‑copper ions forms an antibacterial microenvironment to lower infection risks.

Performance Validation

Material performance tests yield exceptional results. In super‑elasticity tests, the composite fully recovers under 8.5% strain, with a 35% smaller hysteresis loop area and reduced energy dissipation compared with pure NiTi. In fatigue tests under ±90° bending at 4 Hz, performance retention remains >97% after 1 million cycles. In corrosion tests immersed in simulated body fluid (PBS, pH 7.4, 37 °C) for 180 days, the nickel ion release rate is <0.05 μg/cm²·day, far below the ISO 10993‑12 limit of 1 μg/cm²·day.Animal experiments show mild inflammatory responses in surrounding tissues and a fibrous capsule thickness of only 40–60 μm (vs. 100–130 μm for the stainless steel control group) 12 months post‑implantation. In clinical trials of neurointerventional surgeries using composite shafts, the navigation success rate of microcatheters through tortuous blood vessels rises from 82% to 96%. In complex cardiac arrhythmia ablation surgeries, catheters maintain stable performance during 6 hours of continuous intracardiac operation, whereas conventional products suffer a 15% decline in bending stiffness after only 3 hours.

R&D Strategy & Philosophy

We adhere to the R&D philosophy: Performance defined by materials, functions realized by structures, and build a four‑dimensional MIPS innovation system (Material‑Interface‑Performance‑System). At the material level, we establish the world's first medical shaft material gene database containing 542 performance parameters of 213 alloys, predicting properties of new materials via machine learning. At the interface level, atomic‑scale bonding mechanisms are studied, with interfacial designs optimized through first‑principles calculations. At the performance level, multi‑scale simulation models are developed to predict mechanical behaviors from nanoscale to macroscale. At the system level, material properties are precisely matched with clinical requirements.Joint laboratories with the Institute of Metal Research (CAS) and Beihang University focus on fundamental research of shape‑memory alloys. Meanwhile, we implement material genome engineering to accelerate new‑material R&D via high‑throughput computation and experiments, shortening the development cycle from the traditional 6–10 years to 3–4 years.

Future Outlook

Medical materials will evolve toward intelligence, functionality and biomimicry. We are developing stimulus‑responsive smart materials whose mechanical properties adjust with body temperature, pH values or electric fields to enable real‑time intraoperative stiffness regulation. Self‑healing composite materials are being engineered to automatically release repair agents upon detecting microcracks for extended service life. Bioabsorbable magnesium alloys are explored for safe degradation within 9–12 months after completing device functions.By 2027, we will launch tissue‑adaptable smart shafts with surface‑modified extracellular matrix proteins (e.g., fibronectin, laminin) to promote endothelial cell adhesion and reduce thrombosis risks. In the longer term, 4D‑printed active materials will come into reality. These materials not only respond to external stimuli but also conduct biological signal communication with surrounding tissues to achieve true biological integration, pioneering new pathways for permanent implantable devices.