Self-propelled particles that transport cargo through flowing blood and halt hemorrhage

Abstract

Delivering therapeutics deep into damaged tissue during bleeding is challenging because of the outward flow of blood. When coagulants cannot reach and clot blood at its source, uncontrolled bleeding can occur and increase surgical complications and fatalities. Self-propelling particles have been proposed as a strategy for transporting agents upstream through blood. Many nanoparticle and microparticle systems exhibiting autonomous or collective movement have been developed, but propulsion has not been used successfully in blood or used in vivo to transport therapeutics. We show that simple gas-generating microparticles consisting of carbonate and tranexamic acid traveled through aqueous solutions at velocities of up to 1.5 cm/s and delivered therapeutics millimeters into the vasculature of wounds. The particles transported themselves through a combination of lateral propulsion, buoyant rise, and convection. When loaded with active thrombin, these particles worked effectively as a hemostatic agent and halted severe hemorrhage in multiple animal models of intraoperative and traumatic bleeding. Many medical applications have been suggested for self-propelling particles, and the findings of this study show that the active self-fueled transport of particles can function in vivo to enhance drug delivery.

Keywords: Antifibrinolytic; Bubble propulsion; Calcium carbonate; Coagulation; Foaming; Hemostasis; Micromotors; Self-propelling particles; Surgery; drug delivery.

Fig. 1. CaCO₃ particles combined with an organic acid travel through aqueous solutions.

(A) Schematic showing CaCO₃ particles releasing CO₂ and propelling themselves and their cargo when placed in water. (B) Schematic showing how particle movement was measured in buffer and whole blood. (C and D) Images of particles appearing at the surface of a buffered solution (C) and whole blood (D). Scale bars, 2 mm. (E) Schematic and images of a steel hull propelled by CaCO₃ and TXA-NH₃⁺. Scale bar, 2 mm. (F) Images of immobilized CaCO₃ particles containing a fluorescently tagged cargo, FITC-dextran. Scale bars, 30 μm (green fluorescent particles) and 0.5 μm (scanning electron micrographs). (G) Schematic showing a mouse tail being amputated and treated with propelling CaCO₃ particles. Red rectangle denotes the field of view in (H). (H) Histological section of a treated tail showing particles located 6 mm inside the tail, blood vessels, and caudal vertebrae (CV). Fluorescence staining shows actin (red), nuclei (blue), and CaCO₃ particles (green). Scale bar, 200 μm.

Fig. 2. CaCO₃ particles travel upstream and at high velocities through stagnant and flowing solutions.

(A) Images of CaCO₃ particles transporting upward through a stagnant acidic solution. (B) Particle velocity increased as a function of the volume of attached bubbles. The red line denotes the velocities of bubbles predicted by a model equating buoyant and drag forces. The black solid line denotes a one-half power regression of the data. Dashed lines denote the 99% confidence band (black) and 90% prediction band (gray) of regression. (C) Images of particles carried upward by CO₂ bubbles. (D) Schematic showing how propulsion of particles with TXA-NH₃⁺ through flowing water was measured. (E) The particles traveled against flow velocities of up to 3 mm/s. (Inset) Fraction of particles that accumulated for each flow velocity at 20 s. (F) Schematic showing how propulsion of particles through a channel of flowing whole blood was measured at three angles. (G) Maximum distances that particles traveled upstream through flowing and stagnant blood at various angles. Scale bars, 2 mm.

Fig. 3. Propelled thrombin clots flowing blood plasma and halts severe hemorrhage.

(A and B) Schematic of clotting and occlusion of flowing blood plasma ex vivo in vertical (A) and horizontal (B) orientations. (C and D) Clotting of flowing human plasma ex vivo by thrombin-loaded particles at various flow rates in a vertical orientation (C) and at 0.13 mm/s in a horizontal orientation (D). n = 3. *P < 0.05. Error bars indicate SEM.

Fig. 4. Propelled thrombin is delivered deep into wounds and halts hemorrhage in vivo.

(A) Bleeding times in vivo after the tails of mice were amputated. (B) Schematic showing a mouse liver punctured and treated with propelled thrombin. (C) Volume of blood loss in a separate cohort of mice after their livers were punctured and treated. (D and E) Histological sections of livers treated with propelled thrombin (D) or nonpropelled thrombin (E). Fluorescence staining shows actin (red), nuclei (blue), and CaCO₃ particles (green). Scale bar, 200 μm. (F) Mass of CaCO₃ delivered to sites of liver puncture. (G) Schematic showing a pig’s punctured femoral artery being treated with gauze impregnated with propelled thrombin. (H) Survival of pigs after treatment. n = 5. *P < 0.05, **P < 0.01. Error bars indicate SEM.