Official Achievement Announcement

We have successfully developed composite slotted semi‑rigid shafts fabricated from high‑yield‑strength stainless steel (304V/316L) and super‑elastic nickel‑titanium (NiTi) alloy, achieving groundbreaking optimization of 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, providing a revolutionary material solution for high‑frequency, high‑precision interventional surgeries.

R&D Background & Pain Points

Conventional single‑material slotted shafts suffer from inherent limitations in material performance. Medical‑grade 316L stainless steel 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, 10.4×10⁻⁶/°C for NiTi alloy) induce interfacial stress concentration in composite structures and shorten service life.

Clinical studies show that the surface oxide layer of pure NiTi shafts begins to peel after more than 500 000 cycles, potentially releasing nickel ions and triggering allergic reactions. Stainless steel shafts develop permanent deformation and a 25% decline in bending stiffness after only 200 000 cycles. Material selection has become a critical bottleneck restricting shaft performance.

Core Technological Innovations

• Gradient Composite Metallurgy TechnologyStainless‑steel‑NiTi alloy gradient composite tubes are manufactured 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, gradient variation of thermal expansion coefficients, and elimination of thermal stress concentration.
• 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 the size and distribution of precipitated phases, confining phase‑transformation hysteresis within 3 °C and improving super‑elasticity stability by 40%.
• 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 (PVD). 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 to deliver sustained‑release antibacterial performance, attaining bacteriostatic rates over 99.5% against Staphylococcus aureus and Escherichia coli. Cytotoxicity tests comply with the ISO 10993‑5 standard.

Working Mechanism

The advantages of composite shafts stem from multi‑scale synergistic effects. At the atomic scale, reversible martensitic transformation of NiTi alloy occurs under stress, providing 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 integrating 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, instantly rebounding to a straight profile 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 mitigate infection risks.

Performance Validation

Material performance tests yield remarkable 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 exceeds 97% after one million cycles. In corrosion tests, after 180‑day immersion in simulated body fluid (PBS, pH 7.4, 37 °C), the nickel ion release rate is less than 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 (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 uphold the R&D philosophy: Performance is defined by materials, functions are realized by structures, and establish the four‑dimensional MIPS innovation system (Material‑Interface‑Performance‑System). At the material level, we build the world's first medical shaft material gene database containing 542 performance parameters of 213 alloys, and predict properties of new materials via machine learning. At the interface level, we study atomic‑scale bonding mechanisms and optimize interfacial design through first‑principles calculations. At the performance level, we develop multi‑scale simulation models to predict mechanical behaviors from nanoscale to macroscale. At the system level, we precisely match material properties with clinical requirements.

We have built joint laboratories with the Institute of Metal Research, Chinese Academy of Sciences, and Beihang University, focusing on fundamental research of shape‑memory alloys. Meanwhile, we implement material genome engineering to accelerate new‑material development through high‑throughput computation and experiments, shortening the R&D 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 value or electric fields, enabling real‑time intraoperative stiffness regulation. Self‑healing composite materials are being developed 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‑adaptive smart shafts with surface‑modified extracellular matrix proteins (e.g., fibronectin, laminin) to promote endothelial cell adhesion and reduce thrombosis risks. In the long run, 4D‑printed active materials will become reality. Such 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.