Biomaterials Science Blog

Biodegradable stents (BDSs) have proven to be better compared to permanently implanted metallic stents for the treatment of endovascular diseases in children.  Currently, BDSs that are made out of polylactic acid (PLA) with degradation times of 2–3 years are not suitable for infants, where the ideal healing time for an artery is 3 to 6 months. Poly p-dioxanone (PPDO) is an ideal alternative owing to its availability, FDA approval in clinics and suitable degradation time of 6 months.  But, braided PPDO fiber stents still have lower stiffness than standard self-expanding metal ones.

Researchers from China have come up with a novel design strategy to reinforce the mechanical properties of PPDO fibers by using an elastomeric polycaprolactone (PCL) coating which can serve as a binder at to improve the compression performance. This self-expandable, fiber-based, composite braided biodegradable stent (CBBS) made of PPDO and PCL was then assessed for its physical properties, changes in mechanical properties during degradation, etc and compared with the control, cobalt–chromium-based alloy self-expanding stents (WALLSTENTs/WSs). CBBSs delivered in sheaths post dilation exhibited similar mechanical properties as WSs.

In vitro degradation studies showed that CBBSs post-dilation retained effective mechanical support and stent weight (almost 90%) for at least 16 weeks, which is adequate for arterial healing. These results corroborate with the hydrolysis mechanisms involved in degradation of PDDO, the main component and with in vivo histopathological evaluation.

Lastly, the stents were implanted in porcine models without resulting in any evidence of complications such as implant migration, thrombosis, dissection or aneurism. The mechanical performance of CBBS was also not worse than metallic stents in vivo. Angiographic analysis revealed vessel stenosis and an inflammatory response (intima proliferation) at 4 months due to hydrolysis induced degradation of the stent. But this inflammation was resolved at 12 months due to the complete degradation of CBBSs unlike the WSs. When different diameters of WSs were compared, the ones in oversized common iliac arteries exhibited higher luminal gain initially but there was stenosis and vascular injury compared to normal-sized abdominal aortas in the mid-term follow-up period

All the results combined demonstrate the advantages of these novel composite braided degradable stents over the standard metallic ones in terms of mechanical strength and appropriate degradation rate.

To find out more please read:

Oversized composite braided biodegradable stents with post-dilatation for pediatric applications: mid-term results of a porcine study

Jing Sun, Kun Sun,  Kai Bai, Sun Chen, Fan Zhao, Fujun Wang, Nanchao Honga and Hanbo Hu

Biomaterials Science, 2020, DOI: 10.1039/d0bm00567c

About the web writer:

Saswat Choudhury is a graduate student at the Indian Institute of Science Bangalore pursuing research on biomaterials and tissue engineering. He studies bioabsorbable polymers, design and characterization for biomedical applications. Besides research, he is also interested in science communication. You can find him on Twitter @saswatchoudhur1

Spinal cord injury (SCI) can be categorized as traumatic (90% of cases) or non-traumatic based on its origin. Traumatic SCI occurs when the primary injury is an external mechanical force (arising from traffic accidents, sports, violence etc.,), which damages the spinal cord and initiates a cascade of multiple secondary complications including neuronal/glial death with very slim chances of recovery. Current treatments for SCI are mainly palliative; however, studies involving surgical interventions for reconstructing injured sites via cell implantation have shown promise. Moreover, incorporating cells within engineered biomaterial substrates which act as extracellular matrix (ECM) substitutes not only lowers cell density requirements but also enables more accurate localised transplantation. Both natural and synthetic biomaterials are being investigated in this regard.

Researchers from the UK have recently developed Proliferate^®, a macroporous and biodegradable polymer based on cross-linked poly-ε-lysine (pεK) as a biomaterial candidate for SCI implantation. They demonstrate the biocompatibility of the material with CNS cells via in vitro and in vivo studies, both in the original form and on incorporating functional ECM peptides.

First, they synthesized the polymer in two formats: (i) as inserts suspended in 24-well plate culture wells for in vitro studies and (ii) in tubular form with parallel channels facilitating cell guidance for in vivo studies. The material exhibited a beaded, heterogeneous 3D topography with the porosity capable of being tuned by varying the degree of cross-linking.

Next, they cultured astrocytes on the Proliferate^® inserts in vitro and compared  cell morphology with controls grown on PLL-coated coverslips. Staining results showed that the astrocytes adopt a fibrous, ramified morphology typical of in vivo conditions when cultured on the inserts. In addition, the polymer supported differentiation, neuronal survival as well as neurite extension in myelinating cultures; however, myelination was slightly delayed in comparison with coverslip-based controls.

Finally, they implanted the tubular form of the biomaterial into adult rat contusion SCI for in vivo assessment at two timepoints i.e. 7 weeks and 6 months post-implantation. The Proliferate^®  implants induced extensive vascularisation and cellular infiltration with no significant difference being observed in microglial response surrounding non-implanted injury cavities and construct-implanted injuries. Although, construct-tissue borders were permissive to astrocyte growth and migration, most cell guidance channels were observed to disintegrate with time and organized axonal growth seen only in intact channels.

Taken together, the results indicate the potential of this novel material, both as a solo implant as well as a substrate for delivery of essential biomolecules to the injury site for facilitation of axonal regeneration following SCI.

To find out more please read:

A novel poly-ε-lysine based implant, Proliferate^®, for promotion of CNS repair following spinal cord injury

Sara Hosseinzadeh, Susan L. Lindsay, Andrew G. Gallagher, Donald A. Wellings, Mathis O. Riehle, John S. Riddell and Susan C. Barnett

Biomater. Sci., 2020, 8, 3611-3627, DOI: 10.1039/D0BM00097C

Interfacing living cells with inorganic nanowire (NW) array substrates is one of the latest areas of exploration in life sciences with potential applications in electrical stimulation, biosensors, cell injection, axonal guidance and so on. A growing body of evidence has identified the role of substrate nanotopography in regulating various cellular phenotypes including cell morphology, adhesion, proliferation, differentiation and intracellular signaling. However, cellular interactions with high surface area vertical nanowires are relatively unexplored and further studies are necessary to fully reveal the correlations between NW array geometry and stem cell behavior.

Researchers from Germany have recently interfaced human induced pluripotent stem cell (hiPSC)-derived neurons with tailor-made silicon nitride NW array substrates, achieving highly efficient neuronal differentiation and generating electrophysiologically mature neurons within 4 weeks of culture.

First, they fabricated NW arrays using a top-down dry reactive ion etching (RIE) approach in 3 different arrangements – random, hexagonal and rectangular. The NW lengths were fixed to 1.2 μm with pitches of 1.8 μm and 4 μm, resulting in low density (LD) and high density (HD) arrangements respectively. The cells were transferred onto the NW substrates after 14 days in vitro (DIV) and cultured for another 14-16 DIV before performing functional characterization.

Next, they assessed viability of cells cultured on NW substrates and found that both material used as well as substrate topology had no negative impact on cell viability compared with controls cultured on glass coverslips. Furthermore, on studying cellular outgrowth and morphology, they observed that cells rested on NW tips in the case of HD arrays whereas they encapsulated the NWs in LD arrays, thus indicating the effect of NW density on regulating the settling regime of the cells.

Finally, they tested the electrophysiological integrity of the hiPSC-derived neurons via patch clamping and observed that the neurons cultured on the NW substrates fired characteristic action potentials and demonstrated no significant differences in electrophysiological parameters compared with controls.

Taken together, the results indicate the potential of this platform in stem cell research and regenerative medicine for interfacing human stem cell-derived neurons with tailor-made nanostructured substrates to achieve desired cell behaviors.

To find out more please read:

Interfacing human induced pluripotent stem cell-derived neurons with designed nanowire arrays as a future platform for medical applications

Jann Harberts, Undine Haferkamp, Stefanie Haugg, Cornelius Fendler, Dennis Lam, Robert Zierold, Ole Pless and Robert H. Blick

Biomater. Sci., 2020, 8, 2434-2446, DOI: 10.1039/D0BM00182A

Managing cancer requires visualization of tumors using a plethora of imaging modalities such as positron-emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), photoacoustic tomography and optical imaging. Upconversion nanoparticles (UCNPs), a new generation of optical nanomaterials which convert near-infrared (NIR) radiation to visible light by a process called “upconversion luminescence” (UCL), are garnering a lot of attention in cancer diagnostics due to their ability to selectively label cancer cells.

Researchers from Suzhou, China have recently coated tumor targeting UCNPs with red blood cell (RBC) membranes to render them stealthy, effectively preventing them from immune attack and clearance by the host system. Subsequently, they assessed the utility of these RBC-UCNPs for targeted multimodality imaging of 4T1 breast cancer, a triple-negative breast cancer.

First, they isolated cell membranes from the RBCs, reconstructing them into vesicles which were used to encapsulate UCNPs via extrusion. Folic acid (FA) molecules were inserted into the surface of these RBC-UCNPs to assess the tumor-targeting ability of nanoparticles. Upconversion fluorescence imaging revealed that RBC-FA-UCNPs intravenously injected into mice bearing 4T1 subcutaneously transplanted tumors exhibited quick accumulation, long-term retention and reduced uptake by the immune system.

Next, they investigated the feasibility of using these biomimetic nanoparticles in MRI and PET imaging for the detection of tumors in vivo. They found that the MR signal was significantly enhanced by the FA-RBC-UCNPs, indicating the increased circulation time of particles at the tumor site. Furthermore, a combination of pre-targeting strategy and in vivo click chemistry was utilized to mediate PET imaging, which indicated that the biomimetic nanoparticles displayed a higher tumor uptake of the tracer compared with controls, on application of a short half-life radionuclide.

Finally, they conducted in vivo toxicity studies in mice over a span of 30 days, to assess cytotoxicity of the nanoparticles. Blood chemistry, hematology, and histological analyses indicated non-significant induction of toxicity and organ damage, in turn demonstrating the biocompatibility of the biomimetic nanoparticles and their suitability for clinical utilization.

Taken together, the results indicate the potential of this platform for further applications in realizing early diagnosis, bioimaging and treatment of tumors, especially for deep-seated lesions.

To find out more please read:

Red blood cell membrane-coated upconversion nanoparticles for pretargeted multimodality imaging of triple-negative breast cancer

Mengting Li, Hanyi Fang, Qingyao Liu, Yongkang Gai, Lujie Yuan, Sheng Wang, Huiling Li, Yi Hou, Mingyuan Gao and Xiaoli Lan

Biomater. Sci., 2020,8, 1802-1814, DOI: 10.1039/d0bm00029a

Obtaining real-time visual information and feedback is vital for surgeons while removing tumors located in the brain, where a lack of precision can lead to catastrophic surgical complications and reduced life expectancy. During surgery, the human eye can identify only anatomical structures and is unable to detect features at the molecular level, which makes it challenging for surgeons to differentiate tumor tissue from surrounding normal brain tissue. Fluorescence-guided surgery attempts to overcome this limitation and relies on the administration of a fluorescent dye which accumulates within the tumor and produces light which in turn is captured and visualized using a camera.

Researchers from Guangzhou, China have recently engineered microglial cells into optical imaging agent vehicles, achieving more accurate brain tumor imaging for fluorescence-guided surgery compared with the commercially used tracer 5-aminolevulinic acid (5-ALA).

First, they activated BV2 microglial cells with citric-acid coated iron oxide nanoparticles (CIONPs) and loaded them with near-infrared fluorescent dye DiD (DiDBV2-Fe). Priming the cells with iron oxide nanoparticles downregulated M2 markers associated with the immunoresponse, and upregulated expression levels of genes that promote transportation of cells across the blood–brain barrier (BBB), thus achieving a two-fold favorable effect.

Next, they assessed the administration of DiDBV2-Fe in glioblastoma-bearing mice models via two routes:. intravenous and intracarotid artery injection. The latter route resulted in more efficient accumulation of activated cells in the brain tumor, 2.2 times higher than that of 5-ALA, 8 hours after application. Maximum fluorescence intensity images of brain tissues acquired at various timepoints from 2 to 24 hours using near-infrared imaging revealed clear tumor border demarcation. Confocal microscopy of harvested brain tumor sections showed noticeable co-localization of DiDBV2-Fe with the Ki67 positive tumor cells along with a significantly higher tumor-to-brain fluorescence ratio compared with 5-ALA (4.54 vs. 1.81).

Finally, they evaluated the in vivo preliminary safety of DiDBV2-Fe in comparison to 5-ALA. Administering DiDBV2-Fe did not induce acute liver injury, phototoxic or hypersensitivity reactions until a certain threshold was reached. In addition, the engineered microglia did not induce gene expression changes of the detected immunoregulatory proteins, unlike 5-ALA which induced both phototoxic and photoallergic reactions.

Taken together, the results indicate that these engineered microglial cells can serve as biological homing vehicles – in seeking out tumors and delivering optical imaging agents, which in turn can help surgeons navigate and identify tumor tissue via fluorescence during surgery.

To find out more please read:

Engineering microglia as intraoperative optical imaging agent vehicles potentially for fluorescence-guided surgery in gliomas

Ling Guo, Xiaochen Zhang, Runxiu Wei, Gaojie Li, Bingzhi Sun, Hongbo Zhang, Dan Liu, Cuifeng Wang and Min Feng

Biomater. Sci., 2020, 8, 1117-1126.