The construction of three-dimensional micro-fluidic scaffolds of biodegradable polymers by solvent vapor based bonding of micro-molded layers

Preparation of fibre-reinforced PLA-collagen@PLA-PCL@PCL-gelatin three-layer vascular graft by EDC/NHS cross-linking and its performance study

In this study, a three-layer small diameter artificial vascular graft with a structure similar to that of natural blood vessels was first constructed by triple-step electrospinning technology, in which polylactic acid (PLA) and collagen (COL) were used for the inner layer, polylactic acid and polycaprolactone (PCL) was used for the middle layer and polycaprolactone and gelatin was used for the outer layer. The properties of the artificial vascular graft were adjusted by the EDC/NHS cross-linking agent through the reaction between the collagen or gelatine and EDC/NHS. The mechanical and hydrophilic properties of the cross-linked artificial vessels were substantially enhanced, with a maximum stress of 9.56 MPa in the axial direction and 9.31 MPa in the radial direction for the P/C (4:1) vascular graft, which exceeded that of many textile-based and natural vascular grafts. The increased hydrophilicity of the inner layer of the vessel before crosslinking was due to the addition of COL, and the inner layer of the artificial vessel after crosslinking had a substantial increase in hydrophilicity due to the production of a more hydrophilic urea derivative. The increased hydrophilicity led to easier cell adhesion to the inner layer of the artificial vessel, especially for the P/C (2:1) vascular graft, where the cell proliferation rate and adhesion were high due to COL incorporation and cross-linking. The three-layer vascular grafts studied did not lead to haemolysis. Therefore, the EDC/NHS cross-linked three-layer vascular graft had good mechanical properties, hydrophilicity, anticoagulation and could enhance cell adhesion and proliferation.

Hierarchical Structuring of Black Silicon Wafers by Ion-Flow-Stimulated Roughening Transition: Fundamentals and Applications for Photovoltaics

Ion-flow-stimulated roughening transition is a phenomenon that may prove useful in the hierarchical structuring of nanostructures. In this work, we have investigated theoretically and experimentally the surface texturing of single-crystal and multi-crystalline silicon wafers irradiated using ion-beam flows. In contrast to previous studies, ions had relatively low energies, whereas flow densities were high enough to induce a quasi-liquid state in the upper silicon layers. The resulting surface modifications reduced the wafer light reflectance to values characteristic of black silicon, widely used in solar energetics. Features of nanostructures on different faces of silicon single crystals were studied numerically based on the mesoscopic Monte Carlo model. We established that the formation of nano-pyramids, ridges, and twisting dune-like structures is due to the stimulated roughening transition effect. The aforementioned variety of modified surface morphologies arises due to the fact that the effects of stimulated surface diffusion of atoms and re-deposition of free atoms on the wafer surface from the near-surface region are manifested to different degrees on different Si faces. It is these two factors that determine the selection of the allowable "trajectories" (evolution paths) of the thermodynamic system along which its Helmholtz free energy, F, decreases, concomitant with an increase in the surface area of the wafer and the corresponding changes in its internal energy, U (dU>0), and entropy, S (dS>0), so that dF=dU - TdS<0, where T is the absolute temperature. The basic theoretical concepts developed were confirmed in experimental studies, the results of which showed that our method could produce, abundantly, black silicon wafers in an environmentally friendly manner compared to traditional chemical etching.

Computer-aided patterning of PCL microspheres to build modular scaffolds featuring improved strength and neovascularized tissue integration

In the past decade, modular scaffolds prepared by assembling biocompatible and biodegradable building blocks (e.g. microspheres) have found promising applications in tissue engineering (TE) towards the repair/regeneration of damaged and impaired tissues. Nevertheless, to date this approach has failed to be transferred to the clinic due to technological limitations regarding microspheres patterning, a crucial issue for the control of scaffold strength, vascularization and integration in vivo. In this work, we propose a robust and reliable approach to address this issue through the fabrication of polycaprolactone (PCL) microsphere-based scaffolds with in-silico designed microarchitectures and high compression moduli. The scaffold fabrication technique consists of four main steps, starting with the manufacture of uniform PCL microspheres by fluidic emulsion technique. In the second step, patterned polydimethylsiloxane (PDMS) moulds were prepared by soft lithography. Then, layers of 500 µm PCL microspheres with geometrically inspired patterns were obtained by casting the microspheres onto PDMS moulds followed by their thermal sintering. Finally, three-dimensional porous scaffolds were built by the alignment, stacking and sintering of multiple (up to six) layers. The so prepared scaffolds showed excellent morphological and microstructural fidelity with respect to the in-silico models, and mechanical compression properties suitable for load bearing TE applications. Designed porosity and pore size features enabled in vitro human endothelial cells adhesion and growth as well as tissue integration and blood vessels invasion in vivo. Our results highlighted the strong impact of spatial patterning of microspheres on modular scaffolds response, and pay the way about the possibility to fabricate in silico-designed structures featuring biomimetic composition and architectures for specific TE purposes.

Graphene nanosheets as reinforcement and cell-instructive material in soft tissue scaffolds

Mechanical strength of polymeric scaffolds deteriorates quickly in the physiological mileu. This can be minimized by reinforcing the polymeric matrix with graphene, a planar two-dimensional material with unique physicochemical and biological properties. Association between the sheet and polymer chains offers a range of porosity commensurate with tissue requirements. Besides, studies suggest that corrugated structure of graphene offers desirable bio-mechanical cues for tissue regeneration. This review covers three important aspects of graphene-polymer composites, (a) the opportunity on reinforcing the polymer matrix with graphene, (b) challenges associated with limited aqueous processability of graphene, and (c) physiological signaling in the presence of graphene. Among numerous graphene materials, our discussion is limited to graphene oxide (GO) and reduced graphene oxide (rGO) nanosheets. Challenges associated with limited dispersity of hydrophobic sheets within the polymeric matrix have been discussed at molecular level.

Vibration sensing the mammalian way: an artificial Pacinian corpuscle

A vibration sensor is presented mimicking the structure of the Pacinian corpuscle. A multi-step casting process is used to create a 5 mm diameter sensor with a liquid metal core, elastomer dielectric, and graphite counter electrode creating a spherical capacitive sensing element with sensitivities on the order of 10 Δ pF/mm^-1. A model for the capacitance change of the spherical capacitor as it is formed is developed and its findings support the sensitivities observed. Various elastomer dielectric compositions with integrated barium titanate nanoparticles are tested to increase the dielectric constant. The biological acoustic filter within the corpuscle is mimicked using alternating cast layers of oligomers and elastomers around the spherical sensor element. Vibration sensing is characterized over the low frequency range of 10-300 Hz and the minimum detectable sensitivity is found to be 1 µm with a low power requirement of 7 mW. The artificial Pacinian corpuscle has potential applications in tactile sensing and seismic monitoring devices.

The effect of different surface treatment methods on the physical, chemical and biological performances of a PGA scaffold

In order to improve the adhesion between a PGA scaffold and islet cells, it is necessary to find a suitable method to modify the scaffold. In this study, the PGA scaffold surface was modified by plasma, polylysine coating and plasma combined with polylysine coating (P–P-PGA). The surface adhesion of the modified PGA scaffold was examined, and the stretchability and infiltration of the PGA scaffold were also tested. Then, the PGA scaffold treated under the optimal treatment conditions was selected to co-culture with rat islet cells, and the survival activity of the rat islet cells on the untreated PGA scaffold and the P–P-PGA scaffold was examined via the MTT method. Rhodamine staining and DAPI staining were used to detect the number of islet cells adhered to four groups of scaffolds at different culture time points. The PGA-islet graft in the leg muscle of rats was stained with HE to perform the PGA-islet graft pathological examination. The experimental results showed that when the plasma treatment power was 240 W, the processing time was 4 min; the concentration of the polylysine coating solution was 2 mg ml⁻¹, the tensile strength of the PGA scaffold was 320.45 MPa and the amount of infiltration of the PGA scaffold by the serum medium presented the maximum value: 3.17 g g⁻¹. The MTT survival activity test results showed that after 3 d of culture, the survival activity of the islet cells of the treated PGA scaffold culture group (2.02 ± 0.13) was significantly different from the survival activity of the islet cells of the untreated PGA scaffold culture group (1.93 ± 0.10). The survival activities of the islet cells in the experimental groups (1.60 ± 0.13, 1.40 ± 0.12) were still higher than those of the control groups (0.96 ± 013, 0.69 ± 0.09) at 15 and 21 d. The results of the rhodamine and DAPI staining showed that with the increase in culture time, the number of the adherent cells in each group increased, and the number of the adherent islet cells in the experimental group was higher than that in the untreated group. The HE staining results showed that the islet cells on the P–P-PGA scaffold were more than those on the untreated PGA scaffold. After modification of the PGA scaffold, the adhesion of the islet cells improved, which was conducive to the growth of islet cells. These results confirmed that the plasma combined with polylysine coating treatment could enhance the adhesion of the PGA scaffold surface, so that the scaffold and the islet cells exhibited better adhesion and biocompatibility, and the modified PGA scaffold (P–P-PGA) could be used as a promising islet cell scaffold.

In order to improve the adhesion between a PGA scaffold and islet cells, it is necessary to find a suitable method to modify the scaffold.

Fabrication of poly (L-lactic acid)/gelatin composite tubular scaffolds for vascular tissue engineering

The in vitro fabrication of fully functional 3D vascular tissue construct represents one of the most fundamental challenges in vascular tissue engineering. Polymer blending is an effective method for developing, desirable bio-composites for tissue engineering. This study employs the blending of desired characteristics of a synthetic polymer, poly (L-lactic acid) (PLLA) and a biopolymer, gelatin for enhancing cell adhesion sites. Aligned and random PLLA/gelatin nanofibers were fabricated using electrospinning technique. Morphological and chemical characterization of the nanofibrous scaffolds was carried out and the size of fibers ranged from 100 to 500 nm. The SEM, fluorescent staining and viability assays revealed an increase in viability and proliferation of Human Umbilical Vein Endothelial Cells (HUVECs) and Smooth Muscle Cells (SMCs) proportional to gelatin content. The aligned fiber morphology helps cells to orient and elongate along their long axis. Thus the results were suggestive of the fact that topographically aligned nanofibrous scaffolds control cellular organization and possibly provide a good support for achieving the vital organization and physical properties of blood vessel.

Tri-layered elastomeric scaffolds for engineering heart valve leaflets

Tissue engineered heart valves (TEHVs) that can grow and remodel have the potential to serve as permanent replacements of the current non-viable prosthetic valves particularly for pediatric patients. A major challenge in designing functional TEHVs is to mimic both structural and anisotropic mechanical characteristics of the native valve leaflets. To establish a more biomimetic model of TEHV, we fabricated tri-layered scaffolds by combining electrospinning and microfabrication techniques. These constructs were fabricated by assembling microfabricated poly(glycerol sebacate) (PGS) and fibrous PGS/poly(caprolactone) (PCL) electrospun sheets to develop elastic scaffolds with tunable anisotropic mechanical properties similar to the mechanical characteristics of the native heart valves. The engineered scaffolds supported the growth of valvular interstitial cells (VICs) and mesenchymal stem cells (MSCs) within the 3D structure and promoted the deposition of heart valve extracellular matrix (ECM). MSCs were also organized and aligned along the anisotropic axes of the engineered tri-layered scaffolds. In addition, the fabricated constructs opened and closed properly in an ex vivo model of porcine heart valve leaflet tissue replacement. The engineered tri-layered scaffolds have the potential for successful translation towards TEHV replacements.

ADVANCED TISSUE ENGINEERING STRATEGIES FOR VASCULARIZED PARENCHYMAL CONSTRUCTS

The fabrication of vascularized parenchymal organs to alleviate donor shortage in organ transplantation is the holy grail of tissue engineering. However, conventional tissue-engineering strategies have encountered huge challenges in recapitulating complex structural organization of native organs (e.g., orderly arrangement of multiple cell types and vascular network), which plays an important role in engineering functional vascularized parenchymal constructs in vitro. Recent developments of various advanced tissue-engineering strategies have exhibited great promise in replicating organ-specific architectures into artificial constructs. Here, we review the recent advances in top-down and bottom-up strategies for the fabrication of vascularized parenchymal constructs. We highlight the fabrication of microfluidic scaffolds potential for nutrient transport or vascularization as well as the controlled multicellular arrangement. The advantages as well as the limitations associated with these strategies will be discussed. It is envisioned that the combination of microfluidic concept in top-down strategies and multicellular arrangement concept in bottom-up strategies could potentially generate new insights for the fabrication of vascularized parenchymal organs.

Update on vascularization in tissue engineering

Vascularization of engineered tissues is critical for success. Adequate and physiologically regulated blood supply is important for viability of the implanted tissue but even more important for the proper function of parenchymal cells, which is the desired clinical outcome for most applications in regenerative medicine. Several methods are being developed to stimulate revascularization of engineered tissue. Prevascularized scaffolds with a hierarchical vascular pattern, allowing surgical hook-up of the inflow and outflow tracts, that are already preseeded and cultured with primary vascular cells or precursors will be required for larger tissues or tissues with an immediate high metabolism, such as myocardium. The preimplantation presence of a mature vasculature will improve differentiation and maturation of the parenchyma, thus meeting the functional demands of the host. This may also be true for smaller or metabolically less-active tissues, yet for viability and immediate function they may rely on facilitated postimplantation ingrowth of the host vasculature.

3D structural patterns in scalable, elastomeric scaffolds guide engineered tissue architecture

Microfabricated elastomeric scaffolds with 3D structural patterns are created by semiautomated layer-by-layer assembly of planar polymer sheets with through-pores. The mesoscale interconnected pore architectures governed by the relative alignment of layers are shown to direct cell and muscle-like fiber orientation in both skeletal and cardiac muscle, enabling scale up of tissue constructs towards clinically relevant dimensions.

Fabricated micro-nano devices for in vivo and in vitro biomedical applications

In recent years, the innovative use of microelectromechanical systems (MEMSs) and nanoelectromechanical systems (NEMSs) in biomedical applications has opened wide opportunities for precise and accurate human diagnostics and therapeutics. The introduction of nanotechnology in biomedical applications has facilitated the exact control and regulation of biological environments. This ability is derived from the small size of the devices and their multifunctional capabilities to operate at specific sites for selected durations of time. Researchers have developed wide varieties of unique and multifunctional MEMS/NEMS devices with micro and nano features for biomedical applications (BioMEMS/NEMS) using the state of the art microfabrication techniques and biocompatible materials. However, the integration of devices with the biological milieu is still a fundamental issue to be addressed. Devices often fail to operate due to loss of functionality, or generate adverse toxic effects inside the body. The in vitro and in vivo performance of implantable BioMEMS such as biosensors, smart stents, drug delivery systems, and actuation systems are researched extensively to understand the interaction of the BioMEMS devices with physiological environments. BioMEMS developed for drug delivery applications include microneedles, microreservoirs, and micropumps to achieve targeted drug delivery. The biocompatibility of BioMEMS is further enhanced through the application of tissue and smart surface engineering. This involves the application of nanotechnology, which includes the modification of surfaces with polymers or the self-assembly of monolayers of molecules. Thereby, the adverse effects of biofouling can be reduced and the performance of devices can be improved in in vivo and in vitro conditions.

Layer-by-layer micromolding of natural biopolymer scaffolds with intrinsic microfluidic networks

A three-dimensional (3D) microfluidic network plays an important role in engineering thick organs. However, most of the existing methods are limited to mechanically robust synthetic biomaterials and only planar or simple microfluidic networks have been incorporated into soft natural biopolymers. Here we presented an automatic layer-by-layer micromolding strategy to reproducibly fabricate 3D microfluidic porous scaffolds directly from the aqueous solution of soft natural biopolymers. Process parameters such as the liquid volume for each layer and contact displacement were investigated to produce a structurally stable 3D microfluidic scaffold. Microscopic characterization demonstrated that the microfluidic channels were interconnected in 3D and successfully functioned as a convective pathway to transport a polymer solution. Endothelial cells grew relatively well in the porous microfluidic channels. It is envisioned that this method could provide an alternative way to reproducibly build complex 3D microfluidic networks into extracellular matrix-like scaffolds for the fabrication of soft vascularized organs.

Controlled release of bupivacaine HCl through microchannels of biodegradable drug delivery device

Local and prolonged delivery of local analgesics is much desired for post-operative pain management. For delivery of local analgesics at a constant rate over couple of days, a microfluidic device comprised of a drug reservoir and microchannels for drug release was developed using a biodegradable polymer, 85/15 poly(lactic-co-glycolic acid). Unlike conventional methods relying on material property, this device enables convenient modulation of the release speed of drugs by a simple change of the channel geometry such as the length and cross-sectional area. Bupivacaine was selected as our model local analgesic drug and its diffusional transport through microchannels was studied using the microfluidic devices. However, since the salt form of bupivacaine, bupivacaine hydrochloride, has pH-dependent solubility, its precipitation in microchannels had an adverse impact on the release performance of the microfluidic drug delivery devices. Thus, in this investigation, the diffusional transport and precipitation of bupivacaine hydrochloride in microfluidic channels were studied using in vitro release experiments and optical analysis. Furthermore, a concept of co-delivery of bupivacaine hydrochloride together with acidic additives was demonstrated to achieve a zero-order delivery of bupivacaine hydrochloride without the clogging of microchannels by its precipitation.

Longitudinal in vivo imaging to assess blood flow and oxygenation in implantable engineered tissues

The functionality of vascular networks within implanted prevascularized tissues is difficult to assess using traditional analysis techniques, such as histology. This is largely due to the inability to visualize hemodynamics in vivo longitudinally. Therefore, we have developed dynamic imaging methods to measure blood flow and hemoglobin oxygen saturation in implanted prevascularized tissues noninvasively and longitudinally. Using laser speckle imaging, multispectral imaging, and intravital microscopy, we demonstrate that fibrin-based tissue implants anastomose with the host (severe combined immunodeficient mice) in as short as 20 h. Anastomosis results in initial perfusion with highly oxygenated blood, and an increase in average hemoglobin oxygenation of 53%. However, shear rates in the preformed vessels were low (20.8±12.8 s(-1)), and flow did not persist in the vast majority of preformed vessels due to thrombus formation. These findings suggest that designing an appropriate vascular network structure in prevascularized tissues to maintain shear rates above the threshold for thrombosis may be necessary to maintain flow following implantation. We conclude that wide-field and microscopic functional imaging can dynamically assess blood flow and oxygenation in vivo in prevascularized tissues, and can be used to rapidly evaluate and improve prevascularization strategies.

Rapid vascularization of tissue-engineered vascular grafts in vivo by endothelial cells in co-culture with smooth muscle cells

A major challenge facing the development of tissue-engineered vascular grafts (TEVGs), promising living replacements for diseased vascular structures, is enhancing angiogenesis. To promote rapid vascularization, endothelial cells (ECs) were co-cultured with smooth muscle cells (SMCs) in decellularized small intestinal submucosa scaffolds to regenerate angiogenic-TEVGs (A-TEVGs). Observation of the A-TEVGs at 1 month post-implantation revealed that a rich network of neocapillaries lining the blood vessel wall had developed; that the ECs of the neovasculatures had been derived from previously seeded ECs and later invading ECs of the host's vascular bed; that tissue vascularization had not significantly impaired mechanical properties; and that the maximal tensile strength of the A-TEVGs was of the same order of magnitude as that of native porcine femoral arteries. These results indicate that of the co-culturing of ECs with SMCs could enhance vascularization of TEVGs in vivo, possibly increasing graft perfusion and host integration.

Fabrication of a hybrid microfluidic system incorporating both lithographically patterned microchannels and a 3D fiber-formed microfluidic network

A device containing a 3D microchannel network (fabricated using sacrificial melt-spun microfibers) sandwiched between lithographically patterned microfluidic channels offers improved delivery of soluble compounds to a large volume compared to a simple stack of two microfluidic channel layers. With this improved delivery ability comes an increased fluidic resistance due to the tortuous network of small-diameter channels.

The fabrication and cell culture of three-dimensional rolled scaffolds with complex micro-architectures

Cell cultures for tissue engineering are traditionally prepared on two-dimensional or three-dimensional scaffolds with simple pores; however, this limits mass transportation, which is necessary for cell viability and function. In this paper, an innovative method is proposed for fabricating porous scaffolds with designed complex micro-architectures. Channels devised by computer-aided design were used to simulate features of blood vessels in native rat liver. Rapid prototyping and microreplication were used to produce a negative polydimethylsiloxane mold, and then a planar porous scaffold with predefined microchannel parameters was obtained by freeze-drying a silk fibroin/gelatin solution of an optimized concentration. After seeding with rat primary hepatocytes, the planar scaffold was rolled up to build spatial channels. By reconstructing the three-dimensional channel model in the scaffold in the form of micro-computed topography data and observing the cross-sections of the scroll, we confirmed that the bent channels were still interconnected, with restricted deviations. A comparison of the primary hepatocyte culture in the scaffolds with and without the devised channels proved that our design influenced cell organization and improved cell survival and proliferation. This method can be used for the construction of complex tissues for implantation and for culturing cells in vitro for biological tests and observations.

PLGA/hydrogel biopapers as a stackable substrate for printing HUVEC networks via BioLP

Two major challenges in tissue engineering are mimicking the native cell-cell arrangements of tissues and maintaining viability of three-dimension (3D) tissues thicker than 300 µm. Cell printing and prevascularization of engineered tissues are promising approaches to meet these challenges. However, the printing technologies used in biofabrication must balance the competing parameters of resolution, speed, and volume, which limit the resolution of thicker 3D structures. We suggest that high-resolution conformal printing techniques can be used to print 2D patterns of vascular cells onto biopaper substrates which can then be stacked to form a thicker tissue construct. Towards this end we created 1 cm × 1 cm × 300 µm biopapers to be used as the transferable, stackable substrate for cell printing. 3.6% w/v poly-lactide-co-glycolide was dissolved in chloroform and poured into molds filled with NaCl crystals. The salt was removed with DI water and the scaffolds were dried and loaded with a Collagen Type I or Matrigel. SEM of the biopapers showed extensive porosity and gel loading throughout. Biological laser printing (BioLP) was used to deposit human umbilical vein endothelial cells (HUVEC) in a simple intersecting pattern to the surface of the biopapers. The cells differentiated and stretched to form networks preserving the printed pattern. In a separate experiment to demonstrate "stackability," individual biopapers were randomly seeded with HUVECs and cultured for 1 day. The mechanically stable and viable biopapers were then stacked and cultured for 4 days. Three-dimensional confocal microscopy showed cell infiltration and survival in the compound multilayer constructs. These results demonstrate the feasibility of stackable "biopapers" as a scaffold to build 3D vascularized tissues with a 2D cell-printing technique.

Three-dimensional biodegradable microscaffolding: scaffold characterization and cell population at single cell resolution

Engineering artificial tissue scaffolds with a similar organization to that of the natural tissue is a key element to the successful recapitulation of function. However, three-dimensional (3-D) fabrication of tissue scaffolds containing complex microarchitectures still remains a challenge. In addition, little attention has been paid to the issue of how to incorporate cells within 3-D tissue scaffolds that contain precisely engineered architectures. Here we report a 3-D biodegradable microscaffolding (3D-BMS) technology and its process characterization as well as a microscale cellular loading technology as an efficient way to massively populate biodegradable polymers with cells at single cell resolution. In this study a particular emphasis was given to characterization of the material properties of the biodegradable polymers undergoing the 3D-BMS processes. Optimal process conditions were identified in order to avoid any unwanted change in material properties, such as crystallinity and scaffold strength, that have a direct impact on the degradation speed and physical integrity of the constructed scaffolds. For precise control of the cell distribution within the microstructured scaffolds a high precision microsieve structure was designed to localize rat hepatocytes and human articular chondrocytes in the biodegradable polymers. Cell suspensions were passed at a predetermined flow rate through biodegradable polymer layers that contained tapered microholes in a massively parallel process. This high resolution cell seeding method allows accurate manipulation of cell placement in thin layers of biodegradable polymers.

Microfluidic cell culture models for tissue engineering

Microfluidic systems have emerged as revolutionary new platform technologies for a range of applications, from consumer products such as inkjet printer cartridges to lab-on-a-chip diagnostic systems. Recent developments have opened the door to a new set of opportunities for microfluidic systems, in the field of tissue and organ engineering. Advances in the design of physiologically relevant structures and networks, fabrication processes for biomaterials suitable for in vivo use, and techniques for scaling towards large, three-dimensional constructs, are converging towards therapeutic applications of microfluidic technologies in engineering complex tissues and organs. These advances herald a new generation of microfluidics-based approaches designed for specific tissue and organ applications, incorporating microvascular networks, structures for transport and filtration, and a three-dimensional microenvironment suitable for supporting phenotypic cell behavior, tissue function, and implantation and host integration.

Elastomeric polypeptide-based biomaterials

Elastomeric proteins are characterized by their large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Motivated by their unique mechanical properties, there has been tremendous research in understanding and manipulating elastomeric polypeptides, with most work conducted on the elastins but more recent work on an expanded set of polypeptide elastomers. Facilitated by biosynthetic strategies, it has been possible to manipulate the physical properties, conformation, and mechanical properties of these materials. Detailed understanding of the roles and organization of the natural structural proteins has permitted the design of elastomeric materials with engineered properties, and has thus expanded the scope of applications from elucidation of the mechanisms of elasticity to the development of advanced drug delivery systems and tissue engineering substrates.

Vascularization--the conduit to viable engineered tissues

Long-term viability of thick three-dimensional engineered tissue constructs is a major challenge. Addressing it requires development of vessel-like network that will allow the survival of the construct in vitro and its integration in vivo owing to improved vascularization after implantation. Resulting from work of various research groups, several approaches were developed aiming engineered tissue vascularization: (1) embodiment of angiogenesis growth factors in the polymeric scaffolds for prolonged release, (2) coculture of endothelial cells with target tissue cells and angiogenesis signaling cells, (3) use of microfabrication methods for creating designed channels for allowing nutrients to flow and/or for directing endothelial cells attachment, and (4) decellularization of organs and blood vessels for creating extracellular matrix. A synergistic effect is expected by combining several of these approaches as already demonstrated in some of the latest studies. Current paper reviews the progress in each approach and recent achievements toward vascularization of engineered tissues.

A three-layered electrospun matrix to mimic native arterial architecture using polycaprolactone, elastin, and collagen: a preliminary study

Throughout native artery, collagen, and elastin play an important role, providing a mechanical backbone, preventing vessel rupture, and promoting recovery under pulsatile deformations. The goal of this study was to mimic the structure of native artery by fabricating a multi-layered electrospun conduit composed of poly(caprolactone) (PCL) with the addition of elastin and collagen with blends of 45-45-10, 55-35-10, and 65-25-10 PCL-ELAS-COL to demonstrate mechanical properties indicative of native arterial tissue, while remaining conducive to tissue regeneration. Whole grafts and individual layers were analyzed using uniaxial tensile testing, dynamic compliance, suture retention, and burst strength. Compliance results revealed that changes to the middle/medial layer changed overall graft behavior with whole graft compliance values ranging from 0.8 to 2.8%/100 mm Hg, while uniaxial results demonstrated an average modulus range of 2.0-11.8 MPa. Both modulus and compliance data displayed values within the range of native artery. Mathematical modeling was implemented to show how changes in layer stiffness affect the overall circumferential wall stress, and as a design aid to achieve the best mechanical combination of materials. Overall, the results indicated that a graft can be designed to mimic a tri-layered structure by altering layer properties.

Functionally graded beta-TCP/PCL nanocomposite scaffolds: in vitro evaluation with human fetal osteoblast cells for bone tissue engineering

The engineering of biomimetic tissue relies on the ability to develop biodegradable scaffolds with functionally graded physical and chemical properties. In this study, a twin-screw-extrusion/spiral winding (TSESW) process was developed to enable the radial grading of porous scaffolds (discrete and continuous gradations) that were composed of polycaprolactone (PCL), beta-tricalciumphosphate (beta-TCP) nanoparticles, and salt porogens. Scaffolds with interconnected porosity, exhibiting myriad radial porosity, pore-size distributions, and beta-TCP nanoparticle concentration could be obtained. The results of the characterization of their compressive properties and in vitro cell proliferation studies using human fetal osteoblast cells suggest the promising nature of such scaffolds. The significant degree of freedom offered by the TSESW process should be an additional enabler in the quest toward the mimicry of the complex elegance of the native tissues.

Scaffold Sheet Design Strategy for Soft Tissue Engineering

Creating heterogeneous tissue constructs with an even cell distribution and robust mechanical strength remain important challenges to the success of in vivo tissue engineering. To address these issues, we are developing a scaffold sheet tissue engineering strategy consisting of thin (∼200 μm), strong, elastic, and porous crosslinked urethane-doped polyester (CUPE) scaffold sheets that are bonded together chemically or through cell culture. Suture retention of the tissue constructs (four sheets) fabricated by the scaffold sheet tissue engineering strategy is close to the surgical requirement (1.8 N) rendering their potential for immediate implantation without a need for long cell culture times. Cell culture results using 3T3 fibroblasts show that the scaffold sheets are bonded into a tissue construct via the extracellular matrix produced by the cells after 2 weeks of in vitro cell culture.

Cell culture on MEMS platforms: a review

Microfabricated systems provide an excellent platform for the culture of cells, and are an extremely useful tool for the investigation of cellular responses to various stimuli. Advantages offered over traditional methods include cost-effectiveness, controllability, low volume, high resolution, and sensitivity. Both biocompatible and bio-incompatible materials have been developed for use in these applications. Biocompatible materials such as PMMA or PLGA can be used directly for cell culture. However, for bio-incompatible materials such as silicon or PDMS, additional steps need to be taken to render these materials more suitable for cell adhesion and maintenance. This review describes multiple surface modification strategies to improve the biocompatibility of MEMS materials. Basic concepts of cell-biomaterial interactions, such as protein adsorption and cell adhesion are covered. Finally, the applications of these MEMS materials in Tissue Engineering are presented.

Applications of microscale technologies for regenerative dentistry

While widespread advances in tissue engineering have occurred over the past decade, many challenges remain in the context of tissue engineering and regeneration of the tooth. For example, although tooth development is the result of repeated temporal and spatial interactions between cells of ectoderm and mesoderm origin, most current tooth engineering systems cannot recreate such developmental processes. In this regard, microscale approaches that spatially pattern and support the development of different cell types in close proximity can be used to regulate the cellular microenvironment and, as such, are promising approaches for tooth development. Microscale technologies also present alternatives to conventional tissue engineering approaches in terms of scaffolds and the ability to direct stem cells. Furthermore, microscale techniques can be used to miniaturize many in vitro techniques and to facilitate high-throughput experimentation. In this review, we discuss the emerging microscale technologies for the in vitro evaluation of dental cells, dental tissue engineering, and tooth regeneration.

Fabrication and in Vitro Evaluation of Calcium Phosphate Combined with Chitosan Fibers for Scaffold Structures

A rapid prototyping and rapid tool technique-based method was developed to fabricate chitosan fiber calcium phosphate cement composites (CF/CPC) for bone tissue engineering scaffold applications. The products were characterized and the in vitro performance with canine bone marrow stem cells (BMCs) on CF/CPC scaffold with controlled fiber structures evaluated. The X-ray diffraction analysis showed that about 91% of the inorganic part of the CF/CPC scaffold was hydroxyapatite (HA) and the variation in CF had little effect on the percentage of HA content. The results from in vitro study demonstrated that the interconnected macropores rapidly formed inside the CF/CPC scaffolds and that the patterns were related to the fiber structures used. The differences in the fiber structures altered the morphology of the BMCs without affecting the proliferation of the BMCs.

BioMEMS -Advancing the Frontiers of Medicine

Biological and medical application of micro-electro-mechanical-systems (MEMS) is currently seen as an area of high potential impact. Integration of biology and microtechnology has resulted in the development of a number of platforms for improving biomedical and pharmaceutical technologies. This review provides a general overview of the applications and the opportunities presented by MEMS in medicine by classifying these platforms according to their applications in the medical field.

Skin tissue engineering for tissue repair and regeneration

Tissue-engineered skin is a significant advance in the field of wound healing. It has mainly been developed because of limitations associated with the use of autografts and allografts where the donor site suffers from pain, infection, and scarring. Recently, tissue-engineered skin replacements have been finding widespread application, especially in the case of burns, where the major limiting factor is the availability of autologous skin. The development of a bioartificial skin facilitates the treatment of patients with deep burns and various skin-related disorders. The present review gives a comprehensive overview of the developments and future prospects of scaffolds as skin substitutes for tissue repair and regeneration.

Novel dense CO2 technique for beta-galactosidase immobilization in polystyrene microchannels

In this study we design new fabrication techniques and demonstrate the potential of using dense CO2 for facilitating crucial steps in the fabrication of polymeric lab-on-a-chip microdevices by embedding biomolecules at temperatures well below the polymer's glass transition temperature (T(g)). These new techniques are environmentally friendly and done without the use of a clean room. Carbon dioxide at 40 degrees C and between 4.48 and 6.89 MPa was used to immobilize the biologically active molecule, beta-galactosidase (beta-gal), on the surface of polystyrene microchannels. To our knowledge, this is the first time dense CO2 has been used to directly immobilize an enzyme in a microchannel. beta-gal activity was maintained and shown via a fluorescent reaction product, after enzyme immobilization and microchannel capping by the designed fabrication steps at 40 degrees C and pressures up to 6.89 MPa.

Biodegradable micro-osmotic pump for long-term and controlled release of basic fibroblast growth factor

Microelectromechanical system (MEMS) technology not only provides the possibility of integration of multiple functions but also enables more precise control of dosing of therapeutic agents when the therapeutic window is very limited. Local delivery of basic fibroblast growth factor (bFGF) over a specific dose and time course is critical for mesenchymal tissue regeneration. However, bFGF is degraded quickly in vivo and difficulty of controlling the dose level impedes its effective use in angiogenesis and tissue regeneration. We constructed biodegradable micro-osmotic pumps based on MEMS technology for long-term controlled release of bFGF. The devices were constructed by micro-molding and thermal assembly of 85/15 poly(L-lactide-co-glycolide) sheets. The release of bFGF was regulated at 40 ng/day for four weeks; bioactivity was assessed by monitoring the growth of 3T3 fibroblasts. The proposed devices can be further miniaturized and used for the delivery of multiple therapeutic agents at the individual releasing schedules.