Written by Christopher Johnson, PhD, Emory University, 2020
Edited by ­Savannah Fletcher, MD, PGY-1

Definitions

Shear thinning biomaterial (STB): This paper expands upon previous investigations of a biocompatible biomaterial made of porcine gelatin and silicon nanoparticles that spontaneously form a hydrogel upon mixing, due to electrostatic interactions between the two gel components2. This material is shear-thinning, meaning that when a shear force is applied, such as injection through a catheter, the material is “thinned” causing it to flow. Upon the removal of the shear force, when the material exits the catheter tip, it spontaneously forms a hydrogel that conforms to the vessel anatomy leading to complete occlusion and subsequent embolization.
In vitro: Experiments performed outside of a “normal” biological context.
In vivo: Experiments performed in living organisms.
Hydrogel: A material that is a polymer network composed of hydrophilic elements where water is the dispersion medium.
Anticoagulation: Inhibition of the body’s endogenous clotting mechanisms by reducing available clotting factors (e.g. warfarin), or directly inhibiting factors within the clotting cascade (direct oral anticoagulants – apixaban, dabigatran, rivaroxaban) thereby prolonging the time it takes for thrombosis to occur.

Introduction

Vascular injury leading to uncontrolled bleeding is traditionally treated with either open surgical repair or endovascular embolization. Endovascular embolization is achieved through the placement of materials such as metallic coils, liquid agents (glue, onyx), gel foam, embospheres that trigger thrombosis leading to vessel occlusion. Re-bleeding rates can be as high as 47%, especially in patients that are pharmacologically anticoagulated or with disseminated intravascular coagulation (DIC). Metallic coils can interfere with post procedural imaging and are at risk of migrating from the original deployment site. Non-embolic strategies such as N-butil-2-cyanocrylate (NCBA) glue have also been developed and used clinically, but require time to cure and polymerize.

The goal of this study was to engineer an injectable biomaterial capable of embolization that is independent of the coagulation cascade and does not introduce artifacts to subsequent Magnetic Resonance (MR), Computed Tomography (CT) , and ultrasound images1. This basic science study characterizes a novel embolic biomaterial and provides proof of concept evidence in vivo using both murine and porcine models.

Results

Material characterization was performed to analyze the properties of this novel STB. The STB remained stable over time, which meant that the material could undergo multiple cycles of shear force application without changes in performance, measured using intermittent high strain rheometry. The mean amounts of force required to deploy the material through 18G and 23G needles as well as 5F and 4F catheters were 27 +/- 0.6 N and 24 +/-3.1 N respectively. Successful deployment was confirmed using an in vitro model system that showed increased flow pressure (31.7 +/- 1.9 kPa) of anticoagulated blood within the system, which corresponded to occlusion, or embolization. Coil deployment within the same system did not result in blood flow pressure changes compared to control (both roughly 16 kPa). The biocompatibility of the material was tested using coagulation and hemolysis assays. The material was placed in whole blood and time to coagulation and red blood cell hemolysis were measured, providing metrics to predict how the material may interact with blood upon intravascular deployment. The testing conditions lacked blood flow, which is one important limitation in its relatability to in vivo conditions. Qualitatively, the STB performed similarly to several coil types when evaluating clot formation after 3-5 min in a static, in vitro environment. Importantly, the addition of contrast dye and sterilization procedures did not affect the materials hemostatic properties.

In order to successfully translate this embolization strategy to patients, in vivo studies need to demonstrate embolization efficacy. First, the femoral artery of a mouse was occluded by injecting the STB through a 30G needle. Vaso-occlusion was confirmed with laser doppler micro-perfusion imaging, which indicated reduced blood flow to the mouse hindlimb after material deployment. Microcomputed tomography mCT imaging of the mouse was also performed, which showed the material remained at the intended injection site. These mouse studies were then followed up with embolization of the lumbar artery of a pig by delivering the STB through a 5F catheter. Post-injection fluoroscopic images showed complete arterial occlusion. Several veins were also embolized. Histological analysis of vessels at 24 days revealed that the vessels remained occluded. To test if the STB produced durable occlusion and did not embolize to sites distant from the deployment site, the material was injected in pig forearm veins where it could embolize to the lungs. Pulmonary CT scans were performed, which showed no evidence of pulmonary embolism. Vascular remodeling after STB deployment was investigated. Histological sections were obtained from vessels that had been occluded at different times and sections were taken from different locations within a vessel. Histologic analysis revealed that the STB was completely replaced by remodeled tissue over the course of 24 days. If a vessel was embolized in multiple places, the most proximal sites were remodeled more than distal sites, with the distal sites still containing the STB.

Conclusions

The STB described in this paper is a promising alternative to coil embolization, especially in patients undergoing anticoagulation therapy or in coagulopathic states. Coil embolization can be problematic in this patient population due to an inability of the clotting cascade to activate upon coil placement, resulting in poor embolization. The STB can be successfully delivered through a standard catheter and is able to embolize both arteries and veins while remaining localized to the injection site. Importantly, the STB is able to conform to complex vessel anatomy and deployment of the material does not interfere with the use of other imaging modalities. The shear-thinning nature of the material poses a risk of material mobilization at supraphysiologic vascular pressures. The occlusion of a vessel causes an increase in pressure at the occlusion site, which could dislodge the embolic material leading to embolization of unwanted vessels. Further studies will be required to compare the safety and efficacy of the STB to coil embolization, as well as other embolic materials, such as NBCA glue, in animals before translation to human studies. Successful studies in humans would allow for embolization that is independent of the patient’s coagulation status and would not interfere with subsequent imaging studies. Translation of this material into current clinical practice would provide additional options to stop emergent bleeding in critically ill patients in DIC and patients on anticoagulation therapy that may not be surgical candidates.

References

  1. Avery R, Albadawi H, Akbari M, Zhang Y, Duggan M, Sahani D, Olsen B, Khademhosseini A, Oklu R. An injectable shear-thinning biomaterial for endovascular embolization. Sci Transl Med. 2016;8:365ra156. DOI: 10.1126/scitranslmed.aah5533
  2. Gaharwar A, Avery R, Assmann A, Paul A, McKinley G, Khademhosseini A, Olsen B. Shear-thinning nanocomposite hydrogels for the treatment of hemorrhage. 2014;8:9833-9842. DOI: 10.1021/nn503719n