This research was supported by funding from the University of Michigan - Israel Partnership for Research. The authors would like to thank Uri Merdler, Lior Debbi&#160;and Galia Ben David for their great assistance and support, Nadine Wang, Ph.D. and Pilar Herrera-Fierro, Ph.D. of the Lurie Nanofabrication Facility at the University of Michigan, as well as Luis Solorio, Ph.D. for enlightening discussions of photolithography techniques.

<ol>
	<li>Novosel, E. C., Kleinhans, C., Kluger, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vascularization+is+the+key+challenge+in+tissue+engineering.">Vascularization is the key challenge in tissue engineering.</a> Advanced Drug Delivery Reviews. 63 (4), 300-311 (2011).</li><li>Landau, S., Guo, S., Levenberg, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Localization+of+Engineered+Vasculature+within+3D+Tissue+Constructs.">Localization of Engineered Vasculature within 3D Tissue Constructs.</a> Frontiers in Bioengineering and Biotechnology. 6, 2 (2018).</li><li>Griffith, C. K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diffusion+Limits+of+an+in+Vitro+Thick+Prevascularized+Tissue.">Diffusion Limits of an in Vitro Thick Prevascularized Tissue.</a> Tissue Engineering. 11 (12), (2005).</li><li>Potente, M., Gerhardt, H., Carmeliet, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Basic+and+therapeutic+aspects+of+angiogenesis.">Basic and therapeutic aspects of angiogenesis.</a> Cell. 146 (6), 873-887 (2011).</li><li>Landau, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tropoelastin+coated+PLLA-PLGA+scaffolds+promote+vascular+network+formation.">Tropoelastin coated PLLA-PLGA scaffolds promote vascular network formation.</a> Biomaterials. 122, 72-82 (2017).</li><li>Lesman, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+vessel-like+networks+within+multicellular+fibrin-based+constructs.">Engineering vessel-like networks within multicellular fibrin-based constructs.</a> Biomaterials. 32 (31), 7856-7869 (2011).</li><li>Richards, D., Jia, J., Yost, M., Markwald, R., Mei, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Bioprinting+for+Vascularized+Tissue+Fabrication.">3D Bioprinting for Vascularized Tissue Fabrication.</a> Annals of Biomedical Engineering. 45 (1), 132-147 (2017).</li><li>Levenberg, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+vascularized+skeletal+muscle+tissue.">Engineering vascularized skeletal muscle tissue.</a> Nature Biotechnology. 23 (7), 879-884 (2005).</li><li>Rouwkema, J., Khademhosseini, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vascularization+and+Angiogenesis+in+Tissue+Engineering:+Beyond+Creating+Static+Networks.">Vascularization and Angiogenesis in Tissue Engineering: Beyond Creating Static Networks.</a> Trends in Biotechnology. 34 (9), 733-745 (2016).</li><li>Miller, J. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+casting+of+patterned+vascular+networks+for+perfusable+engineered+three-dimensional+tissues.">Rapid casting of patterned vascular networks for perfusable engineered three-dimensional tissues.</a> Nature Materials. 11, (2012).</li><li>Gariboldi, M. I., Butler, R., Best, S. M., Cameron, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+vasculature+Architectural+effects+on+microcapillary-like+structure+self-assembly.">Engineering vasculature Architectural effects on microcapillary-like structure self-assembly.</a> PLOS ONE. 14 (1), 1-13 (2019).</li><li>Blache, U., Guerrero, J., G&#252;ven, S., Klar, A. S., Scherberich, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microvascular+Networks+and+Models,+In+vitro+Formation.">Microvascular Networks and Models, In vitro Formation.</a> Vascularization for Tissue Engineering and Regenerative Medicine. , 1-40 (2018).</li><li>Wong, K. H. K., Chan, J. M., Kamm, R. D., Tien, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic+Models+of+Vascular+Functions.">Microfluidic Models of Vascular Functions.</a> Annual Review of Biomedical Engineering. 14 (1), 205-230 (2012).</li><li>Jansen, K. A., Bacabac, R. G., Piechocka, I. K., Koenderink, G. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cells+actively+stiffen+fibrin+networks+by+generating+contractile+stress.">Cells actively stiffen fibrin networks by generating contractile stress.</a> Biophysical Journal. 105 (10), 2240-2251 (2013).</li><li>Pollet, A. M. A. O., den Toonder, J. M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recapitulating+the+vasculature+using+Organ-on-Chip+technology.">Recapitulating the vasculature using Organ-on-Chip technology.</a> Bioengineering. 7 (1), (2020).</li><li>Hasan, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic+techniques+for+development+of+3D+vascularized+tissue.">Microfluidic techniques for development of 3D vascularized tissue.</a> Biomaterials. 35 (26), 7308-7325 (2014).</li><li>Jordahl, J. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Jet+Writing:+Functional+Microtissues+Based+on+Tessellated+Scaffold+Architectures.">3D Jet Writing: Functional Microtissues Based on Tessellated Scaffold Architectures.</a> Advanced Materials. 30 (14), 1707196 (2018).</li><li>Gauvin, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfabrication+of+complex+porous+tissue+engineering+scaffolds+using+3D+projection+stereolithography.">Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography.</a> Biomaterials. 33 (15), 3824-3834 (2012).</li><li>Coscoy, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microtopographies+control+the+development+of+basal+protrusions+in+epithelial+sheets.">Microtopographies control the development of basal protrusions in epithelial sheets.</a> Biointerphases. 13 (4), 041003 (2018).</li><li>Kaplan, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+prototyping+fabrication+of+soft+and+oriented+polyester+scaffolds+for+axonal+guidance.">Rapid prototyping fabrication of soft and oriented polyester scaffolds for axonal guidance.</a> Biomaterials. , (2020).</li><li>Steier, A., Mu&#241;iz, A., Neale, D., Lahann, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emerging+Trends+in+Information-Driven+Engineering+of+Complex+Biological+Systems.">Emerging Trends in Information-Driven Engineering of Complex Biological Systems.</a> Advanced Materials. 31 (26), 11806898 (2019).</li><li>Szklanny, A. 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The authors have nothing to disclose.

The need for a rich vasculature within embedded in engineered tissues is critical for construct survival and proper function1. Although engineering the vascular system has been the focus of a vast amount of research, much is left to investigate and understand24. In particular, when recreating a specific tissue, the microvasculature should behave and organize accordingly12. The most common approach for microvessels generation is co-seeding endothelial...

The field of tissue engineering is rapidly progressing towards the fabrication of engineered constructs to replace missing or damaged organs and tissues1. However, fully functional constructs have yet to be achieved, in part, since generating operational vascular networks for tissue nourishment remains an outstanding challenge. Without proper vascularization, engineered tissues are limited to a passive diffusion transport of oxygen and nutrients, constraining the maximum viable tissue thickness to the diffusion limit, approximately 200 &#181;m2. Such thicknesses are not suitable to repair large tissue defects or for full organ fabrication, which renders the presence of functional vascular network a mandatory characteristic for functional and implantable tissues3.
The vascular system is comprised of a wide variety of blood vessels, with different sizes, phenotypes, and organization, tightly related to the host tissue. Understanding the behavior, response and migration decisions made by the developing and sprouting vessels can instruct their integration in engineered tissues4. Currently, the most common approach for creating in vitro vascular networks is combining endothelial cells (ECs) with support cells (SCs, with the capability to differentiate into mural cells), seeded within a three-dimensional micro-environment. This environment provides chemical and physical cues to allow the cells to attach, proliferate and self-assemble into vessel networks2,5,6,7,8. When co-cultured, SCs secrete extracellular matrix (ECM) proteins while providing mechanical support to the ECs, which form the tubular structures. Furthermore, a cross-interaction between both cell types promote tubulogenesis, vessel sprouting and migration, in addition to the SCs maturation and differentiation into &#945;-smooth muscle actin-expressing (&#945;SMA) mural cells4. Vessel network development is most commonly studied in 3D environments created using hydrogels, porous polymeric scaffolds, or a combination thereof. The latter option equally provides a cell-friendly environment and the required mechanical support for both the cells and the ECM9.
A great amount of work has been carried out to study vascular development, including co-culturing the cells on hydrogels10, hydrogels-scaffold combinations11,12, 2D platforms, and microfluidic devices13. However, hydrogels can be easily deformed by the cell-exerted forces14, while 2D and microfluidics systems fail to recreate a closer-to-nature environment to obtain a more extrapolatable response15,16. Understanding how forming vessels react to their surrounding environment can provide critical insight that might allow for the fabrication of engineered environments with the capability of guiding the vessel development in a predictable manner. Understanding vascular formation phenomena is especially critical to keep pace with the rapid emergence of submicron-to-micron scale fabrication techniques, such as stereolithography, digital projection lithography, continuous liquid interface production, 3D melt-electro jetwriting, solution based 3D electro jet writing, and emerging bioprinting techniques17,18,19,20,21. Aligning the control of these micromanufacturing techniques with a deepened understanding of vascular biology is key to the creation of an appropriate engineered vasculature for a target tissue.
Here, we present a 3D system to study the response of new forming and sprouting vessels to the surrounding scaffold geometry, observing their sprout origin and subsequent migration22. By utilizing 3D scaffolds with tessellated compartment geometries, and a two-step seeding technique, we succeeded to create highly organized vascular networks in a clear and easy to analyze fashion. The tessellated geometries provide a high throughput system with individual units containing vessels that respond to their local environment. Using multicolored ECs, we tracked sprout formation origins and subsequent migration patterns, correlated to the compartment geometry and the SCs location22.
Although the proposed protocol has been prepared to analyze the effects of geometrical cues on vascularization behavior, this approach can be expanded and applied to a variety of new applications. The tessellated scaffold and the easily imageable networks allow for the straightforward analysis of different ECs and SCs interaction, the addition of specific organ cells and their interaction with the vascular networks, drug effect on vascular networks, and more. Our suggested system results very versatile and of simple fabrication and processing.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Angiotool freeware</td>
			<td>NIH-CCR</td>
			<td></td>
			<td>Free download at https://ccrod.cancer.gov/confluence/display/ROB2/Home</td>
		</tr>
		<tr>
			<td>Bovine albumin serum Probumin</td>
			<td>Millipore</td>
			<td>82-045-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Dental pulp stem cells</td>
			<td>Lonza</td>
			<td>PT-5025</td>
			<td></td>
		</tr>
		<tr>
			<td>ECM media + bullet kit</td>
			<td>Sciencell</td>
			<td>#1001</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol 96%</td>
			<td>Gadot-Group</td>
			<td>64-17-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Evicel fibrin sealant</td>
			<td>Johnson&#38;Johnson</td>
			<td>EVB05IL</td>
			<td>Provides both thrombin and fibrinogen (BAC2) solutions</td>
		</tr>
		<tr>
			<td>GlutaMAX</td>
			<td>Gibco</td>
			<td>35050061</td>
			<td></td>
		</tr>
		<tr>
			<td>Goat anti-mouse Cy3 antibody</td>
			<td>Jackson</td>
			<td>115-166-072</td>
			<td></td>
		</tr>
		<tr>
			<td>Goat anti-rabbit Alexa-Fluor 488</td>
			<td>Thermo- Fisher Scientific</td>
			<td>A11034</td>
			<td></td>
		</tr>
		<tr>
			<td>Human adipose microvascular cells</td>
			<td>Sciencell</td>
			<td>#7200</td>
			<td></td>
		</tr>
		<tr>
			<td>Human fibronectin</td>
			<td>Sigma</td>
			<td>F0895-5MG</td>
			<td>Stock concentration: 1 mg/mL</td>
		</tr>
		<tr>
			<td>ImageJ</td>
			<td>NIH</td>
			<td></td>
			<td>Free download at https://imagej.nih.gov/ij/download.html</td>
		</tr>
		<tr>
			<td>Isopropyl alcohol</td>
			<td>Gadot-Group</td>
			<td>67-63-0</td>
			<td></td>
		</tr>
		<tr>
			<td>Lift-off reagent</td>
			<td>Kayaku Advanced Materials, Inc</td>
			<td>G112850</td>
			<td>Commercial name Omnicoat</td>
		</tr>
		<tr>
			<td>Low-glucose DMEM</td>
			<td>Biological Industries</td>
			<td>01-050-1A</td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse anti-SMA antibody</td>
			<td>Dako</td>
			<td>M0851</td>
			<td></td>
		</tr>
		<tr>
			<td>NEAA</td>
			<td>Gibco</td>
			<td>11140068</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraformaldehyde solution 4% in PBS</td>
			<td>ChemCruz</td>
			<td>SC-281692</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin-Nystatin Solution</td>
			<td>Biological Industries</td>
			<td>03-032-1B</td>
			<td></td>
		</tr>
		<tr>
			<td>Phospate buffered saline (PBS)</td>
			<td>Sigma</td>
			<td>P5368-10PAK</td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit anti-vWF antibody</td>
			<td>Abcam</td>
			<td>ab9378</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>Silicon Valley Microelectronics (SVM)</td>
			<td></td>
			<td>Wafers 4&#34;, Type N-1-10, 500-550 microns thick</td>
		</tr>
		<tr>
			<td>SU-8 2050 photoresist</td>
			<td>Kayaku Advanced Materials, Inc</td>
			<td>Y11058</td>
			<td></td>
		</tr>
		<tr>
			<td>SU-8 developer</td>
			<td>Kayaku Advanced Materials, Inc</td>
			<td>Y020100</td>
			<td></td>
		</tr>
		<tr>
			<td>Tryton-X 100</td>
			<td>BioLab LTD</td>
			<td>57836</td>
		</tr>
	</tbody>
</table>

stepwise cell seeding on tessellated scaffolds to study sprouting blood vessels

The cardiovascular system is a key player in human physiology, providing nourishment to most tissues in the body; vessels are present in different sizes, structures, phenotypes, and performance depending on each specific perfused tissue. The field of tissue engineering, which aims to repair or replace damaged or missing body tissues, relies on controlled angiogenesis to create a proper vascularization within the engineered tissues. Without a vascular system, thick engineered constructs cannot be sufficiently nourished, which may result in cell death, poor engraftment, and ultimately failure. Thus, understanding and controlling the behavior of engineered blood vessels is an outstanding challenge in the field. This work presents a high-throughput system that allows for the creation of organized and repeatable vessel networks for studying vessel behavior in a 3D scaffold environment. This two-step seeding protocol shows that vessels within the system react to the scaffold topography, presenting distinctive sprouting behaviors depending on the compartment geometry in which the vessels reside. The obtained results and understanding from this high throughput system can be applied in order to inform better 3D bioprinted scaffold construct designs, wherein fabrication of various 3D geometries cannot be rapidly assessed when using 3D printing as the basis for cellularized biological environments. Furthermore, the understanding from this high throughput system may be utilized for the improvement of rapid drug screening, the rapid development of co-cultures models, and the investigation of mechanical stimuli on blood vessel formation to deepen the knowledge of the vascular system.

The presented protocol, using stereolithography techniques, allows for the fabrication of tessellated scaffolds made of SU-8 photoresist. Scaffolds with distinct compartment geometries (squares, hexagons, and circles), and highly accurate and repeatable features were obtained (Figure 1).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61995/61995fig01.jpg" /> 
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Stepwise Cell Seeding on Tessellated Scaffolds to Study Sprouting Blood Vessels

Engineered tissues heavily rely on proper vascular networks to provide vital nutrients and gases and remove metabolic waste. In this work, a stepwise seeding protocol of endothelial cells and support cells creates highly organized vascular networks in a high-throughput platform for studying developing vessel behavior in a controlled 3D environment.

The cardiovascular system is a key player in human physiology, providing nourishment to most tissues in the body; vessels are present in different sizes, ...

stepwise-cell-seeding-on-tessellated-scaffolds-to-study-sprouting

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JoVE Journal

Bioengineering

3.4K Views.  Technion. Engineered tissues heavily rely on proper vascular networks to provide vital nutrients and gases and remove metabolic waste. In this work, a stepwise seeding protocol of endothelial cells and support cells creates highly organized vascular networks in a high-throughput platform for studying developing vessel behavior in a controlled 3D environment.

Video: Stepwise Cell Seeding on Tessellated Scaffolds to Study Sprouting Blood Vessels

Autologous Endothelial Progenitor Cell-Seeding Technology and Biocompatibility Testing For Cardiovascular Devices in Large Animal Model

Implantable cardiovascular devices are manufactured from artificial materials (e.g. titanium (Ti), expanded polytetrafluoroethylene), which pose the risk of thromboemboli formation1,2,3. We have developed a method to line the inside surface of Ti tubes with autologous blood-derived human or porcine endothelial progenitor cells (EPCs)4. By implanting Ti tubes containing a confluent layer of porcine EPCs in the inferior vena cava (IVC) of pigs, we tested the improved biocompatibility of the cell-seeded surface in the prothrombotic environment of a large animal model and compared it to unmodified bare metal surfaces5,6,7 (Figure 1). This method can be used to endothelialize devices within minutes of implantation and test their antithrombotic function in vivo. 
Peripheral blood was obtained from 50 kg Yorkshire swine and its mononuclear cell fraction cultured to isolate EPCs4,8. Ti tubes (9.4 mm ID) were pre-cut into three 4.5 cm longitudinal sections and reassembled with heat-shrink tubing. A seeding device was built, which allows for slow rotation of the Ti tubes. 
We performed a laparotomy on the pigs and externalized the intestine and urinary bladder. Sharp and blunt dissection was used to skeletonize the IVC from its bifurcation distal to the right renal artery proximal. The Ti tubes were then filled with fluorescently-labeled autologous EPC suspension and rotated at 10 RPH x 30 min to achieve uniform cell-coating9. After administration of 100 USP/ kg heparin, both ends of the IVC and a lumbar vein were clamped. A 4 cm veinotomy was performed and the device inserted and filled with phosphate-buffered saline. As the veinotomy was closed with a 4-0 Prolene running suture, one clamp was removed to de-air the IVC. At the end of the procedure, the fascia was approximated with 0-PDS (polydioxanone suture), the subcutaneous space closed with 2-0 Vicryl and the skin stapled closed. 
After 3 - 21 days, pigs were euthanized, the device explanted en-block and fixed. The Ti tubes were disassembled and the inner surfaces imaged with a fluorescent microscope.
We found that the bare metal Ti tubes fully occluded whereas the EPC-seeded tubes remained patent. Further, we were able to demonstrate a confluent layer of EPCs on the inside blood-contacting surface.
Concluding, our technology can be used to endothelialize Ti tubes within minutes of implantation with autologous EPCs to prevent thrombosis of the device. Our surgical method allows for testing the improved biocompatibility of such modified devices with minimal blood loss and EPC-seeded surface disruption.

A method for seeding titanium blood-contacting biomaterials with autologous cells and testing biocompatibility is described. This method uses endothelial progenitor cells and titanium tubes, seeded within minutes of surgical implantation into porcine venae cavae. This technique is adaptable to many other implantable biomedical devices.

Implantable cardiovascular devices are manufactured from artificial materials (e.g. titanium (Ti), expanded polytetrafluoroethylene), which pose the risk ...

Design of a Cyclic Pressure Bioreactor for the Ex Vivo Study of Aortic Heart Valves

The aortic valve, located between the left ventricle and the aorta, allows for unidirectional blood flow, preventing backflow into the ventricle. Aortic valve leaflets are composed of interstitial cells suspended within an extracellular matrix (ECM) and are lined with an endothelial cell monolayer. The valve withstands a harsh, dynamic environment and is constantly exposed to shear, flexion, tension, and compression. Research has shown calcific lesions in diseased valves occur in areas of high mechanical stress as a result of endothelial disruption or interstitial matrix damage1-3. Hence, it is not surprising that epidemiological studies have shown high blood pressure to be a leading risk factor in the onset of aortic valve disease4. 
The only treatment option currently available for valve disease is surgical replacement of the diseased valve with a bioprosthetic or mechanical valve5. Improved understanding of valve biology in response to physical stresses would help elucidate the mechanisms of valve pathogenesis. In turn, this could help in the development of non-invasive therapies such as pharmaceutical intervention or prevention. Several bioreactors have been previously developed to study the mechanobiology of native or engineered heart valves6-9. Pulsatile bioreactors have also been developed to study a range of tissues including cartilage10, bone11 and bladder12. The aim of this work was to develop a cyclic pressure system that could be used to elucidate the biological response of aortic valve leaflets to increased pressure loads. 
The system consisted of an acrylic chamber in which to place samples and produce cyclic pressure, viton diaphragm solenoid valves to control the timing of the pressure cycle, and a computer to control electrical devices. The pressure was monitored using a pressure transducer, and the signal was conditioned using a load cell conditioner. A LabVIEW program regulated the pressure using an analog device to pump compressed air into the system at the appropriate rate. The system mimicked the dynamic transvalvular pressure levels associated with the aortic valve; a saw tooth wave produced a gradual increase in pressure, typical of the transvalvular pressure gradient that is present across the valve during diastole, followed by a sharp pressure drop depicting valve opening in systole. The LabVIEW program allowed users to control the magnitude and frequency of cyclic pressure. The system was able to subject tissue samples to physiological and pathological pressure conditions. This device can be used to increase our understanding of how heart valves respond to changes in the local mechanical environment.

Design of a Cyclic Pressure Bioreactor for the Ex Vivo Study of Aortic Heart Valves

A cyclic pressure bioreactor capable of subjecting heart valve tissue to physiological and pathological pressure conditions has been designed. A LabVIEW program allows users to control pressure magnitude, amplitude and frequency. This device can be used to study the mechanobiology of heart valve tissue or isolated cells.

The aortic valve, located between the left ventricle and the aorta, allows for unidirectional blood flow, preventing backflow into the ventricle. Aortic ...

Preparation of 3D Fibrin Scaffolds for Stem Cell Culture Applications

Stem cells are found in naturally occurring 3D microenvironments in vivo, which are often referred to as the stem cell niche 1. Culturing stem cells inside of 3D biomaterial scaffolds provides a way to accurately mimic these microenvironments, providing an advantage over traditional 2D culture methods using polystyrene as well as a method for engineering replacement tissues 2. While 2D tissue culture polystrene has been used for the majority of cell culture experiments, 3D biomaterial scaffolds can more closely replicate the microenvironments found in vivo by enabling more accurate establishment of cell polarity in the environment and possessing biochemical and mechanical properties similar to soft tissue.3 A variety of naturally derived and synthetic biomaterial scaffolds have been investigated as 3D environments for supporting stem cell growth. While synthetic scaffolds can be synthesized to have a greater range of mechanical and chemical properties and often have greater reproducibility, natural biomaterials are often composed of proteins and polysaccharides found in the extracelluar matrix and as a result contain binding sites for cell adhesion and readily support cell culture. Fibrin scaffolds, produced by polymerizing the protein fibrinogen obtained from plasma, have been widely investigated for a variety of tissue engineering applications both in vitro and in vivo 4. Such scaffolds can be modified using a variety of methods to incorporate controlled release systems for delivering therapeutic factors 5. Previous work has shown that such scaffolds can be used to successfully culture embryonic stem cells and this scaffold-based culture system can be used to screen the effects of various growth factors on the differentiation of the stem cells seeded inside 6,7.
 This protocol details the process of polymerizing fibrin scaffolds from fibrinogen solutions using the enzymatic activity of thrombin. The process takes 2 days to complete, including an overnight dialysis step for the fibrinogen solution to remove citrates that inhibit polymerization. These detailed methods rely on fibrinogen concentrations determined to be optimal for embryonic and induced pluripotent stem cell culture. Other groups have further investigated fibrin scaffolds for a wide range of cell types and applications - demonstrating the versatility of this approach 8-12.

This work details the preparation of 3D fibrin scaffolds for culturing and differentiating plutipotent stem cells. Such scaffolds can be used to screen the effects of various biological compounds on stem cell behavior as well as modified to contain drug delivery systems.

Stem cells are found in naturally occurring 3D microenvironments in vivo, which are often referred to as the stem cell niche 1. Culturing stem cells ...

Self-reporting Scaffolds for 3-Dimensional Cell Culture

Culturing cells in 3D on appropriate scaffolds is thought to better mimic the in vivo microenvironment and increase cell-cell interactions. The resulting 3D cellular construct can often be more relevant to studying the molecular events and cell-cell interactions than similar experiments studied in 2D. To create effective 3D cultures with high cell viability throughout the scaffold the culture conditions such as oxygen and pH need to be carefully controlled as gradients in analyte concentration can exist throughout the 3D construct. Here we describe the methods of preparing biocompatible pH responsive sol-gel nanosensors and their incorporation into poly(lactic-co-glycolic acid) (PLGA) electrospun scaffolds along with their subsequent preparation for the culture of mammalian cells. The pH responsive scaffolds can be used as tools to determine microenvironmental pH within a 3D cellular construct. Furthermore, we detail the delivery of pH responsive nanosensors to the intracellular environment of mammalian cells whose growth was supported by electrospun PLGA scaffolds. The cytoplasmic location of the pH responsive nanosensors can be utilized to monitor intracellular pH (pHi) during ongoing experimentation.

Biocompatible pH responsive sol-gel nanosensors can be incorporated into poly(lactic-co-glycolic acid) (PLGA) electrospun scaffolds. The produced self-reporting scaffolds can be used for in&#160;situ monitoring of microenvironmental conditions whilst culturing cells upon the scaffold. This is beneficial as the 3D cellular construct can be monitored in real-time without disturbing the experiment.

Culturing cells in 3D on appropriate scaffolds is thought to better mimic the in vivo microenvironment and increase cell-cell interactions. The resulting ...

Improved Method for the Preparation of a Human Cell-based, Contact Model of the Blood-Brain Barrier

The blood-brain barrier (BBB) comprises impermeable but adaptable brain capillaries which tightly control the brain environment. Failure of the BBB has been implied in the etiology of many brain pathologies, creating a need for development of human in vitro BBB models to assist in clinically-relevant research. Among the numerous BBB models thus far described, a static (without flow), contact BBB model, where astrocytes and brain endothelial cells (BECs) are cocultured on the opposite sides of a porous membrane, emerged as a simplified yet authentic system to simulate the BBB with high throughput screening capacity. Nevertheless the generation of such model presents few technical challenges. Here, we describe a protocol for preparation of a contact human BBB model utilizing a novel combination of primary human BECs and immortalized human astrocytes. Specifically, we detail an innovative method for cell-seeding on inverted inserts as well as specify insert staining techniques and exemplify how we use our model for BBB-related research.

Establishment of human models of the blood-brain barrier (BBB) can benefit research into brain conditions associated with BBB failure. We describe here an improved technique for preparation of a contact BBB model, which permits coculturing of human astrocytes and brain endothelial cells on the opposite sides of a porous membrane.

The blood-brain barrier (BBB) comprises impermeable but adaptable brain capillaries which tightly control the brain environment. Failure of the BBB has ...

Helical Organization of Blood Coagulation Factor VIII on Lipid Nanotubes

Cryo-electron microscopy (Cryo-EM)1 is a powerful approach to investigate the functional structure of proteins and complexes in a hydrated state and membrane environment2.
Coagulation Factor VIII (FVIII)3 is a multi-domain blood plasma glycoprotein. Defect or deficiency of FVIII is the cause for Hemophilia type A&#160;-&#160;a severe bleeding disorder. Upon proteolytic activation, FVIII binds to the serine protease Factor IXa on the negatively charged platelet membrane, which is critical for normal blood clotting4. Despite the pivotal role FVIII plays in coagulation, structural information for its membrane-bound state is incomplete5. Recombinant FVIII concentrate is the most effective drug against Hemophilia type A and commercially available FVIII can be expressed as human or porcine, both forming functional complexes with human Factor IXa6,7.
In this study we present a combination of Cryo-electron microscopy (Cryo-EM), lipid nanotechnology and structure analysis applied to resolve the membrane-bound structure of two highly homologous FVIII forms: human and porcine. The methodology developed in our laboratory to helically organize the two functional recombinant FVIII forms on negatively charged lipid nanotubes (LNT) is described.&#160;The representative results demonstrate that our approach is sufficiently sensitive to define the differences in the helical organization between the two highly homologous in sequence (86% sequence identity) proteins. Detailed protocols for the helical organization, Cryo-EM and electron tomography (ET) data acquisition are given. The two-dimensional (2D) and three-dimensional (3D) structure analysis applied to obtain the 3D reconstructions of human and porcine FVIII-LNT is discussed. The presented human and porcine FVIII-LNT structures show the potential of the proposed methodology to calculate the functional, membrane-bound organization of blood coagulation Factor VIII at high resolution.

We present a combination of Cryo-electron microscopy, lipid nanotechnology, and structure analysis applied to resolve the membrane-bound structure of two highly homologous FVIII forms: human and porcine. The methodology developed in our laboratory&#160;to helically organize the two functional recombinant FVIII forms on negatively charged lipid nanotubes (LNT) is described.

Cryo-electron microscopy (Cryo-EM)1 is a powerful approach to investigate the functional structure of proteins and complexes in a hydrated state and ...

Concentric Gel System to Study the Biophysical Role of Matrix Microenvironment on 3D Cell Migration

The ability of cells to migrate is crucial in a wide variety of cell functions throughout life&#160;from embryonic development and wound healing&#160;to tumor and cancer metastasis. Despite intense research efforts, the basic biochemical and biophysical principles of cell migration are still not fully understood, especially in the physiologically relevant three-dimensional (3D) microenvironments. Here, we describe an in vitro assay designed to allow quantitative examination of 3D cell migration behaviors. The method exploits the cell&#8217;s mechanosensing ability and propensity to migrate into previously unoccupied extracellular matrix (ECM). We use the invasion of highly invasive breast cancer cells, MDA-MB-231, in collagen gels as a model system. The spread of cell population and the migration dynamics of individual cells over weeks of culture can be monitored using live-cell imaging and analyzed to extract spatiotemporally-resolved data. Furthermore, the method is easily adaptable for diverse extracellular matrices, thus offering a simple yet powerful way to investigate the role of biophysical factors in the microenvironment on cell migration.

The mechanical properties and microstructure of the extracellular matrix strongly affect 3D migration of cells. An in vitro method to study the spatiotemporal cell migration behavior in biophysically variable environments, at both population and individual cell levels, is described.

The ability of cells to migrate is crucial in a wide variety of cell functions throughout life&#160;from embryonic development and wound healing&#160;to ...

Imaging Cell Viability on Non-transparent Scaffolds &#8212; Using the Example of a Novel Knitted Titanium Implant

Intervertebral disc degeneration and disc herniation is one of the major causes of lower back pain. Depletion of extracellular matrix, culminating in nucleus pulposus (NP) extrusion leads to intervertebral disc destruction. Currently available surgical treatments reduce the pain but do not restore the mechanical functionality of the spine. In order to preserve mechanical features of the spine, total disc or nucleus replacement thus became a wide interest. However, this arthroplasty era is still in an immature state, since none of the existing products have been clinically evaluated.
This study intends to test the biocompatibility of a novel nucleus implant made of knitted titanium wires. Despite all mechanical advantages, the material has its limits for conventional optical analysis as the resulting implant is non-transparent. Here we present a strategy that describes in vitro visualization, tracking and viability testing of osteochondro-progenitor cells on the scaffold. This protocol can be used to visualize the efficiency of the cleaning protocol as well as to investigate the biocompatibility of these and other non-transparent scaffolds. Furthermore, this protocol can be used to show adherence pattern of cells as well as cell viability and proliferation rates on/in the scaffold. This in vitro biocompatibility testing assay provides a propitious tool to analyze cell-material interaction in non-transparent and opaque scaffolds.

Here we present a fluorophore based imaging technique to detect cell viability on a non-transparent titanium scaffold as well as to detect glimpses of the scaffold impurities. This protocol troubleshoots the drawback of imaging cell-cell or cell-metal interactions on non-transparent scaffolds.

Intervertebral disc degeneration and disc herniation is one of the major causes of lower back pain. Depletion of extracellular matrix, culminating in ...

Synthesis of Biocompatible Liquid Crystal Elastomer Foams as Cell Scaffolds for 3D Spatial Cell Cultures

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block co-polymers featuring cholesterol units as side-chain pendant groups, resulting in smectic-A (SmA) liquid crystal elastomers (LCEs). Foam-like scaffolds, prepared using metal templates, feature interconnected microchannels, making them suitable as 3D cell culture scaffolds. The combined properties of the regular structure of the metal foam and of the elastomer result in a 3D cell scaffold that promotes not only higher cell proliferation compared to conventional porous templated films, but also better management of mass transport (i.e., nutrients, gases, waste, etc.). The nature of the metal template allows for the easy manipulation of foam shapes (i.e., rolls or films) and for the preparation of scaffolds of different pore sizes for different cell studies while preserving the interconnected porous nature of the template. The etching process does not affect the chemistry of the elastomers, preserving their biocompatible and biodegradable nature. We show that these smectic LCEs, when grown for extensive time periods, enable the study of clinically relevant and complex tissue constructs while promoting the growth and proliferation of cells.

This study presents a methodology to prepare 3D, biodegradable, foam-like cell scaffolds based on biocompatible side-chain liquid crystal elastomers (LCEs). Confocal microscopy experiments show that foam-like LCEs allow for cell attachment, proliferation, and the spontaneous alignment of C2C12s myoblasts.

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block ...

Melt Electrospinning Writing of Three-dimensional Poly(&#949;-caprolactone) Scaffolds with Controllable Morphologies for Tissue Engineering Applications

This tutorial reflects on the fundamental principles and guidelines for electrospinning writing with polymer melts, an additive manufacturing technology with great potential for biomedical applications. The technique facilitates the direct deposition of biocompatible polymer fibers to fabricate well-ordered scaffolds in the sub-micron to micro scale range. The establishment of a stable, viscoelastic, polymer jet between a spinneret and a collector is achieved using an applied voltage and can be direct-written. A significant benefit of a typical porous scaffold is a high surface-to-volume ratio which provides increased effective adhesion sites for cell attachment and growth. Controlling the printing process by fine-tuning the system parameters enables high reproducibility in the quality of the printed scaffolds. It also provides a flexible manufacturing platform for users to tailor the morphological structures of the scaffolds to their specific requirements. For this purpose, we present a protocol to obtain different fiber diameters using melt electrospinning writing (MEW) with a guided amendment of the parameters, including flow rate, voltage and collection speed. Furthermore, we demonstrate how to optimize the jet, discuss often experienced technical challenges, explain troubleshooting techniques and showcase a wide range of printable scaffold architectures.

This protocol serves as a comprehensive guideline to fabricate scaffolds via electrospinning with polymer melts in a direct writing mode. We systematically outline the process and define the appropriate parameter settings for achieving targeted scaffold architectures.

This tutorial reflects on the fundamental principles and guidelines for electrospinning writing with polymer melts, an additive manufacturing technology ...