Gold Nanorod-Based Engineered Cardiac Patch for Suture-Free Engraftment by Near IR.

Although cardiac patches hold a promise for repairing the infarcted heart, their integration with the myocardium by sutures may cause further damage to the diseased organ. To address this issue, we developed facile and safe, suture-free technology for the attachment of engineered tissues to organs. Here, nanocomposite scaffolds comprised of albumin electrospun fibers and gold nanorods (AuNRs) were developed. Cardiac cells were seeded within the scaffolds and assembled into a functioning patch. The engineered tissue was then positioned on the myocardium and irradiated with a near IR laser (808 nm). The AuNRs were able to absorb the light and convert it to thermal energy, which locally changed the molecular structure of the fibrous scaffold, and strongly, but safely, attached it to the wall of the heart. Such hybrid biomaterials can be used in the future to integrate any engineered tissue with any defected organs, while minimizing the risk of additional injury for the patient, caused by the conventional stitching methods.

Engineered cardiac patches are considered a promising approach for regenerating the infarcted heart.[1,2] In this concept, cardiac cells are seeded within 3-dimensional (3D) biomaterial scaffolds, which provide a physical, structural, and biochemical supporting microenvironment. The scaffolds encourage cell–cell and cell–matrix interactions, which lead to the formation of a functioning tissue.[3] Once the cardiac patches are engineered, they are usually attached to the scar tissue of the heart by a surgical operation, involving synthetic sutures or staples.[4,5] Although these surgical procedures can strongly attach a patch to a desired site, several limitations still remain. These include the blockage of blood supply to the patch, bleeding, injury to a healthy tissue, and risk of infection. Such trauma may cause an additional deterioration of the left ventricle’s function, expanding the damage.

In recent years, in order to attach tissues, several alternatives to surgical procedures were used.[6−9] For example, biological glues, such as medical grade cyanoacrylates were used to adhere tissues in the stomach. Although, the materials strongly adhere to tissues, they are associated with toxicity.[10] In addition, the strong adhesion leads to an area with stiff mechanical properties, which do not match the elastic properties of the myocardium,[11] impeding proper contractile function and provoking inflammation. To address this limitation, several biological glues were developed.[12−17] For example, an elegant elastomeric biodegradable and biocompatible, gecko-inspired tissue adhesive tape was developed.[18] The glue relied on the synergy between chemistry and topography, showing an effective sealing of surgical colon and stomach defects. In another study, a nanoparticle solution was used to glue two hydrogels by connecting to their polymer chains and reorganizing their structure.[19] Other approaches include neat glues that do not contain any solvents, and their mechanical properties can be tuned.[20] However, although all these materials exhibit excellent adhesive properties, several properties limit their application for engineering thick tissues. For example, they are not porous, do not supply a supporting fibrous biomimetic microenvironment for tissue assembly, and in some cases their water-free environment does not allow cell penetration into their core.

Gold nanostructures can also be used for tissue welding and wound sealing. For example, gold nanoshells or particles can strongly absorb light, heat up and locally melt tissues, and fuse them.[21,22] However, such technologies were designed to fuse tissues by heating without a special consideration for cell viability. In addition, the particles are in a solution and a 3D assembly of tissues within is limited.

Recently, we have shown that gold nanostructures such as spheres or particles with a higher aspect ratio can be incorporated into 3D scaffolds to increase the transfer of the electrical signal between electrogenic cells.[23−26] Here, we sought to develop a hybrid scaffold composed of albumin electrospun fibers and gold nanorods (AuNRs). After a functional cardiac patch was engineered within the fibrous scaffold, we exploited the ability of AuNRs to absorb near IR radiation and as a consequence locally heat up. Such irradiation has been previously shown to be highly localized, triggering a temperature raise to 57 °C in the particle, decaying exponentially within a few nanometers.[27] This allowed efficient soldering of the engineered tissue with the host (Figure ) without the need for conventional sutures.

Figure 1

Overview of the concept. (A) Gold nanorods adsorption to albumin electrospun fiber scaffolds. The rods are able to absorb the light and convert it to thermal energy. (B) Cardiac cells are seeded within the nanocomposite scaffolds to form the (C) cardiac patch. (D) After maturation of the tissue, the patch is placed on the heart and irradiated with an 808 nm laser for suture-free integration.

AuNRs were synthesized to allow a rapid and efficient conversion of an 808 nm laser radiation into thermal energy. The AuNRs were characterized by TEM and UV–vis-NIR spectroscopy. The particles were monodispersed with a mean size of ∼60 nm × 20 nm (Figure a). The UV–vis-NIR spectrum of the AuNRs showed a sharp and strong absorption centered at ∼810 nm (Figure b). Next, we sought to integrate the AuNRs to 3D albumin electrospun fiber scaffolds, which were previously shown to promote functional cardiac tissue assembly.[23] Albumin was dissolved in trifluoroethanol and distilled water, and β-mercaptoethanol was subsequently added after 24 h. The solution was electrospun at room temperature and delivered at a flow rate of 2 mL/h under an electrical field of 12.5 kV, as described before.[28] As shown, the scaffold was composed of ribbon-like fibers with a mean width of 2.4 μm and a thickness of 0.5 μm (Figure c,d). Next, we sought to integrate the AuNRs with the protein-based scaffolds. The scaffolds were placed within a AuNRs solution for 60 min to allow for quick adsorption. As shown, the rods spontaneously attached to the scaffolds leaving a transparent solution and resulting in a brown scaffold (Figure e). Surface analysis by scanning electron microscope images and EDX revealed a homogeneous distribution of the AuNRs on the surface of the scaffold fibers (Figure f–h).

Figure 2

The nanocomposite scaffolds. (a) HRTEM micrographs of AuNRs. Scale bar: 50 nm. (b) UV–Visible-NIR spectra wave scan of AuNRs. (c) SEM micrographs of electrospun albumin fiber scaffolds. Scale bar: 100 μm. (d) SEM micrographs of electrospun albumin fiber scaffolds. Scale bar: 10 μm. (e) AuNRs adsorption to electrospun albumin scaffolds at t = 0 (upper panel) and t = 60 min (lower panel). (f and g) ESEM micrographs of AuNRs adsorbed to an electrospun albumin fiber scaffold. Scale bars: f = 2 μm, g = 500 nm. (h) EDX confirming the existence of Au on the albumin fibers.

We next investigated the ability of near IR irradiation to attach the composite scaffolds to pieces of pig myocardium. Several parameters may affect the temperature of the particles in the scaffolds, including the AuNR concentration, laser flux, and time of irradiation.[29] Therefore, we first sought to examine the laser power flux that was needed to attach the AuNR-albumin electrospun fiber scaffold to the tissue. Fluxes of 1, 1.2, and 1.5 W/cm2 were applied on thin scaffolds, and a mechanical tester, which pulled the tissue and the scaffold to opposite sides (Supplementary Figure 1), was used to evaluate the adhesion strength between the layers. As shown, a flux of 1.5 W/cm2 enabled to attach the scaffolds significantly stronger than the 1 and 1.2 W/cm2 fluxes (p = 0.0009 and p = 0.0008, respectively; Figure a). We next examined the effect of irradiation time (60, 90, and 120 s, at 1.5 W/cm2) on the attachment of the scaffolds to the tissue. As shown, a time-dependent effect was observed (Figure b) with a significantly stronger attachment after 120 s.

Figure 3

Characterization of the engineered tissue integrated with a porcine cardiac tissue. (a) Stress versus laser power flux. (b) Stress versus time of NIR irradiation. (c) Schematics of the geometry of the albumin fiber scaffold used for tissue welding. (d) Stress versus thickness of albumin scaffold. (e) Cardiac cell viability before and after irradiation. (f) Immunofluorescence image of cardiac α-sarcomeric actinin (pink), connexin-43 (green), and cell nuclei (blue). Scale bar: 20 μm. n ≥ 10 in all experiments.

Thick albumin scaffolds do not allow proper penetration of the light through them. Therefore, we designed thick scaffolds with thin edges (Figure c). We speculated that the strength of thin layers of albumin scaffolds[30] would allow to attach the entire patch in place. To evaluate the ability of thin-layer scaffolds to attach the tissue, we have fabricated scaffolds with a thickness of 60–80 μm or 100–120 μm. The scaffolds were placed on the tissues and irradiated at 1.5 W/cm2, 120 s. As shown, the 60–80 μm scaffolds attached significantly stronger to the tissue, as compared to the 100–120 μm scaffolds (Figure d; p < 0.0001), probably due to better penetration of the light.

Near IR irradiation is considered safe for cells and tissues.[29,31,32] However, we wanted to ensure that the thermal energy generated by the AuNRs after irradiated with near IR for 120 s at 1.5 W/cm2 is not affecting the cells. Cardiac cells were isolated from the left ventricles of neonatal rat hearts and seeded within the hybrid scaffolds. After 7 days of incubation, a proper period for cell organization and tissue assembly, the cardiac patches were irradiated. As shown, exposure to near IR did not affect cell viability (Figure e). Following tissue irradiation, the patches were fixed and immunostained for cardiac sarcomeric actinin and connexin 43 (Cx43), proteins associated with cell contraction and electrical coupling, respectively. As shown by confocal microscope images (Figure f and Supplementary Figure 2), the cardiac cells exhibited massive actinin striation, indicating the ability of the assembled tissue to contract. Furthermore, pronounced Cx43 expression was observed between the cardiomyocytes, indicating the ability of the cells to function synchronously. Such hallmarks further indicated that the exposure to near IR and the local heating by the AuNRs did not harm the engineered tissue.

We next evaluated the ability of the hybrid scaffolds to attach to the heart in wet and dynamic conditions. Here, the chest of a Sprague–Dawley rat was opened; the heart was exposed, and the hybrid patch was placed on the left ventricle and irradiated with near IR for 120 s (Figure a,b and supplementary movie 1). As shown, the thin edges of the patch adhered to the heart (Figure b). Scanning electron microscope images of the sliced heart revealed a tight interaction between the heart and the patch (Figure c). Furthermore, as judged by SEM images and H&E staining of thin sections, the thin edges were able to strongly secure the thick tissue layer to the heart, ensuring a strong fixation (Figure c–e). As shown, no changes in the scaffolds’ structure could be observed. This further indicates the safety of the procedure. In the future, following implantation, the cells in the nonirradiated locations in the patch will secrete ECM proteins that will further strengthen the integration with the host tissue.

Figure 4

Cardiac patch integration. (a) Schematic representation of the integration process. (b) The cardiac patch after integration with rat heart. (c) SEM micrographs of the cross-section of the heart, revealing the interaction between the patch and the heart. Scale bar: 200 μm. (d) Higher magnification of the integration point. Scale bar: 50 μm. (e) H&E staining of the cross-sectioned heart. Scale bar: 500 μm.

In conclusion, a new hybrid material comprised of fibrous electrospun scaffolds and AuNRs was developed. Upon irradiation with near IR, the particles were locally heated, allowing efficient soldering, perhaps by melting the polymer, or by denaturation of albumin and collagen upon heating and their interlock,[33,34] providing an elegant, suture-free engraftment of a cardiac patch to the heart. Such technology may assist in the future to integrate various engineered tissues or pure biomaterials with any defected organ, minimizing the risk of additional injury for the patient.