Obstruction of blood vessels (thrombosis) potentially leads to severe consequences like stroke, heart attack, or pulmonary embolism and is a leading cause of death globally.^1,2 Thrombolytic therapy aims to remove blood clots either mechanically using catheters (thrombectomy) or by using thrombolytic agents to dissolve the clots.^1,2 Tissue plasminogen activator (tPA) is a commonly used thrombolytic drug, but it may increase the risk of brain bleeding.^1,3 To address this, researchers from The Chinese University of Hong Kong (CUHK) developed retrievable, magnetic tPA-anchored nanorobots (tPA-nbots).¹ These tPA-nbots may enable more precise delivery of the drug closer to the blood clot, thereby reducing the required dose and minimizing side effects.¹ This approach can enhance thrombolysis efficiency and precision, while also shortening the overall treatment duration.¹

tPA is a serine protease normally found on endothelial cells that dissolves clots by catalyzing the conversion of plasminogen to plasmin.^1,2 However, it can disrupt the blood-brain barrier, induce intracerebral hemorrhage and increase mortality.^1,3 Nanorobots that can transport tPA more efficiently to clot sites may reduce the required dose and side effects.¹ However, tPA and nanorobots retained in the blood vessels may cause long-term side effects, hence their removal after treatment is needed.¹ In this study, retrievable, magnetic tPA-nbots were developed and their ability to achieve enhanced thrombolysis under a balloon catheter-assisted system was demonstrated in in vitro, ex vivo and in vivo models.¹

To achieve imaging-guided delivery and retrieval of the tPA-nbots, a system comprising a catheter, fluoroscopy and mechanical arm-equipped magnetic control unit was first developed.¹ This allows the tPA-nbots to be loaded into the catheter-accessible M1/M2 designated blood vessels before being released and guided by an external magnetic field under fluoroscopic imaging towards blood clots in the smaller M3/M4 segments.¹ The tPA-nbots can then be steered back to the tip of the catheter and removed from the blood vessels, reducing the side effects considerably.¹

The tPA-nbots (~300nm) were fabricated in 4 steps.¹ A magnetic core was first fabricated, followed by coating with a silica shell, and then soft and biocompatible polyethylene glycol tails were added to provide a protein-coupling site.¹ The protein tPA was then chemically bound to the surface of the Fe₃O₄@mSiO₂ spheres, resulting in the tPA-nbots.¹ The anchored tPA retained enzymatic activity comparable to pure tPA.¹ A rotating magnetic field was then used to trigger the swarming motion to direct the tPA-nbots to the clot.¹ The optimal parameters required to efficiently recapture the tPA-nbots were also studied, including the magnetic field strength and injection speed of the nanorobots.¹

In vitro, testing showed the tPA-nbots were capable of removing blood clots of varying stiffness in 1.5mm vessels, comparable to the M3 segment.¹ While the thrombolysis rate decreased slightly as the clot became stiffer, the tPA-nbots could still effectively dissolve the clots.¹ They were also able to remove blood clots with large aspect ratios or in complex three-dimensional vessels.¹ Using an in vitro vascular model with established blood flow, the tPA-nbot microswarm could completely remove an induced blood clot.¹ Furthermore, the nanorobots were successfully recaptured with a high retrieval rate of 91.91%.¹

The authors next tested the ex vivo thrombolysis and retrieval of the tPA-nbots using a human placenta model, which has a comparable vessel distribution to the human brain and is used for neurosurgery training.¹ In this established model, the tPA-nbots could successfully remove blood clots and restore blood flow across a range of vessel diameters from 1.5mm to 4.0mm.¹ While larger blood vessel diameter benefited the retrieval of the tPA-nbots, high blood flow velocity could lead to retrieval failure.¹ To address this, the researchers used a catheter balloon to temporarily reduce the blood flow, which effectively aided in the successful retrieval of the tPA-nbots.¹

The thrombolysis efficacy of the tPA-nbots was further validated in vivo using animal models.¹ In rats, a blood clot was induced in the femoral vein and in rabbits, a clot was induced in the carotid artery.¹ In both cases, the tPA-nbots were able to effectively degrade the clots, restoring the blood vessels to their original state.¹ Additionally, the in vivo safety profile of the tPA-nbots was evaluated by intravenously injecting them into mice.¹ Compared to control mice injected with saline, the blood cell counts and biochemical marker levels of the mice injected with the tPA-nbots remained within normal ranges, even after an extended observation period of 15 days.¹ No tissue damage or obvious accumulation of the tPA-nbots was observed in the major organs, demonstrating the non-toxic and biocompatible nature of the nanorobots.¹

In conclusion, the researchers developed a retrievable magnetic colloidal microswarm of tPA-nbots that could achieve rapid and efficient thrombolysis, even in hard-to-reach clot locations beyond the reach of conventional catheter methods.¹ This innovative robotic system offers high spatial precision for enhanced thrombolysis with low side effects, representing a promising advancement in the field.¹