Fabrication of Pliable Biodegradable Polymer foams to Engineer Soft Tissues

Natural Polymer-Based Micronanostructured Scaffolds for Bone Tissue Engineering

Although bone tissue allografts and autografts aremoften used as a regenerative tissue during the bone healing, their availability, donor site morbidity, and immune response to grafted tissue are limiting factors their more common usage. Tissue engineered implants, such as acellular or cellular polymeric structures, can be an alternative solution. A variety of scaffold fabrication techniques including electrospinning, particulate leaching, particle sintering, and more recently 3D printing have been used to create scaffolds with interconnected pores and mechanical properties for tissue regeneration. Simply combining particle sintering and molecular self-assembly to create porous microstructures with imbued nanofibers to produce micronanostructures for tissue regeneration applications. Natural polymers like polysaccharides, proteins and peptides of plant or animal origin have gained significant attention due to their assured biocompatibility in tissue regeneration. However, majority of these polymers are water soluble and structures derived from them are in the form of hydrogels and require additional stabilization via cross-linking. For bone healing applications scaffolds are required to be strong, and support attachment, proliferation and differentiation of osteoprogenitors into osteoblasts. Our ongoing work utilizes plant polysaccharide cellulose derivatives and collagen to create mechanically stable and bioactive micronanostructured scaffold for bone tissue engineering. Scaffold microstructure is essentially solvent sintered cellulose acetate (CA) microspheres in the form of a negative template for trabecular bone with defined pore and mechanical properties. Collagen nanostructures are imbued into the 3D environment of CA scaffolds using collagen molecular self-assembly principles. The resultant CA-collagen micronanostructures provide the benefits of combined polymers and serve as an alternative material platform to many FDA approved polyesters. Our ongoing studies and published work confirm improved osteoprogenitor adhesion, proliferation, migration, differentiation, extracellular matrix (ECM) secretion in promoting bone healing. In this chapter we will provide a detailed protocol on the creation of micronanostructured CA-collagen scaffolds and their characterization for bone tissue engineering using human mesenchymal stem cells.

The ECM: To Scaffold, or Not to Scaffold, That Is the Question

The extracellular matrix (ECM) has pleiotropic effects, ranging from cell adhesion to cell survival. In tissue engineering, the use of ECM and ECM-like scaffolds has separated the field into two distinct areas-scaffold-based and scaffold-free. Scaffold-free techniques are used in creating reproducible cell aggregates which have massive potential for high-throughput, reproducible drug screening and disease modeling. Though, the lack of ECM prevents certain cells from surviving and proliferating. Thus, tissue engineers use scaffolds to mimic the native ECM and produce organotypic models which show more reliability in disease modeling. However, scaffold-based techniques come at a trade-off of reproducibility and throughput. To bridge the tissue engineering dichotomy, we posit that finding novel ways to incorporate the ECM in scaffold-free cultures can synergize these two disparate techniques.

Auxetic Structures for Tissue Engineering Scaffolds and Biomedical Devices

An auxetic structure utilizing a negative Poisson's ratio, which can expand transversally when axially expanded under tensional force, has not yet been studied in the tissue engineering and biomedical area. However, the recent advent of new technologies, such as additive manufacturing or 3D printing, has showed prospective results aimed at producing three-dimensional structures. Auxetic structures are fabricated by additive manufacturing, soft lithography, machining technology, compressed foaming, and textile fabrication using various biomaterials, including poly(ethylene glycol diacrylate), polyurethane, poly(lactic-glycolic acid), chitosan, hydroxyapatite, and using a hard material such as a silicon wafer. After fabricating the scaffold with an auxetic effect, researchers have cultured fibroblasts, osteoblasts, chondrocytes, myoblasts, and various stem cells, including mesenchymal stem cells, bone marrow stem cells, and embryonic stem cells. Additionally, they have shown new possibilities as scaffolds through tissue engineering by cell proliferation, migration, alignment, differentiation, and target tissue regeneration. In addition, auxetic structures and their unique deformation characteristics have been explored in several biomedical devices, including implants, stents, and surgical screws. Although still in the early stages, the auxetic structure, which can create mechanical properties tailored to natural tissue by changing the internal architecture of the structure, is expected to show an improved tissue reconstruction ability. In addition, continuous research at the cellular level using the auxetic micro and nano-environment could provide a breakthrough for tissue reconstruction.

Long-term glycemic control and prevention of diabetes complications in vivo using oleic acid-grafted-chitosan‑zinc-insulin complexes incorporated in thermosensitive copolymer

Daily injections for basal insulin therapy are far from ideal resulting in hypo/hyperglycemic episodes associated with fatal complications in type-1 diabetes patients. Here we report a delivery system that provides controlled release of insulin closely mimicking physiological basal insulin requirement for an extended period following a single subcutaneous injection. Stability of insulin was significantly improved by formation of zinc-insulin hexamers, further stabilized by electrostatic complex formation with chitosan polymer. Insulin complexes were homogenously incorporated into PLA-PEG-PLA, a biodegradable thermogel copolymer, that instantaneously forms a subcutaneous gel-depot following injection. Chitosan polymer was hydrophobically modified using oleic acid prior to complex formation with insulin to enable distribution of oleic acid-grafted-chitosan‑zinc-insulin complexes into the hydrophobic core of PLA-PEG-PLA thermogel-copolymer micelles. In vivo, daily administration of marketed long-acting insulin, glargine, resulted in fluctuating blood glucose levels between 91 and 443 mg/dL in type 1 diabetic rats. However, single administration of thermogel copolymeric formulation successfully demonstrated slow diffusion of insulin complexes maintaining peak-free basal insulin level of 21 mU/L for 91 days. Sustained release of basal insulin also correlated with efficient glycemic control (blood glucose <120 mg/dL), prevention of diabetic ketoacidosis and absence of cataract development, unlike other treatment groups. Moreover, there was no sign of inflammation, tissue damage, or collagen deposition around depot site, suggesting exceptional biocompatibility of the formulation for long-term use.

In Vitro and in Vivo Optimization of Phase Sensitive Smart Polymer for Controlled Delivery of Rivastigmine for Treatment of Alzheimer's Disease

PURPOSE

Alzheimer's disease is a neurodegenerative disorder, and most common form of dementia afflicting over 35 million people worldwide. Rivastigmine is a widely used therapeutic for ameliorating clinical manifestations of Alzheimer's disease. However, current treatments require frequent dosing either orally or via transdermal patch that lead to compliance issues and administration errors risking serious adverse effects. Our objective was to develop a smart polymer based delivery system for controlled release of rivastigmine over an extended period following a single subcutaneous injection.

METHODS

Rivastigmine release was optimized by tailoring critical factors including polymer concentration, polymer composition, drug concentration, solvent composition, and drug hydrophobicity (rivastigmine tartrate vs base). Optimized in vitro formulation was evaluated in vivo for safety and efficacy.

RESULTS

Formulation prepared using PLGA (50:50) at 5% w/v in 95:5 benzyl benzoate: benzoic acid demonstrated desirable controlled drug release characteristics in vitro. The formulation demonstrated sustained release of rivastigmine tartrate for 7 days in vivo with promising biocompatibility and acetylcholinesterase inhibition efficacy for 14 days.

CONCLUSION

The results exemplify an easily injectable controlled release formulation of rivastigmine prepared using phase-sensitive smart polymer. The optimized formulation significantly increases the dosing interval, and can potentially improve patient compliance as well as quality of life of patients living with Alzheimer's disease.

Smart Thermosensitive Copolymer Incorporating Chitosan-Zinc-Insulin Electrostatic Complexes for Controlled Delivery of Insulin: Effect of Chitosan Chain Length

This work was designed to optimize thermosensitive copolymeric depot-based system for delivering insulin at a controlled rate for a prolonged period following a single subcutaneous injection. Intrinsic ability of insulin to form hexamers in the presence of zinc and electrostatic complexes with chitosan (CS) were explored for improving stability and release characteristics of insulin through the copolymeric depot. CS-zinc-insulin complexes were prepared using CS of different chain lengths (5, 30, 50, 200 kDa). Effect of different chain lengths of CS on the thermal stability, binding constant, and release profile of insulin was determined. Increasing chain length of CS demonstrated increasing thermal stability of insulin. However, higher chain length of CS adversely affected the release profile of insulin. Hydrolytic degradation analysis showed rapid degradation of copolymer in formulation containing higher chain length of CS (200 kDa)-zinc-insulin complexes, implying formation of bigger pores and channels in copolymeric matrix during initial release in this system. However, formulation containing smaller chain length of CS (5 kDa)-zinc-insulin complexes demonstrated slow copolymer degradation and sustained insulin release profile. Additionally, CS-zinc-insulin complexes were effective in preserving stability of insulin during the entire duration of release and storage.

Surface-modified polymers for cardiac tissue engineering

Cardiovascular disease (CVD), leading to myocardial infarction and heart failure, is one of the major causes of death worldwide. The physiological system cannot significantly regenerate the capabilities of a damaged heart. The current treatment involves pharmacological and surgical interventions; however, less invasive and more cost-effective approaches are sought. Such new approaches are developed to induce tissue regeneration following injury. Hence, regenerative medicine plays a key role in treating CVD. Recently, the extrinsic stimulation of cardiac regeneration has involved the use of potential polymers to stimulate stem cells toward the differentiation of cardiomyocytes as a new therapeutic intervention in cardiac tissue engineering (CTE). The therapeutic potentiality of natural or synthetic polymers and cell surface interactive factors/polymer surface modifications for cardiac repair has been demonstrated in vitro and in vivo. This review will discuss the recent advances in CTE using polymers and cell surface interactive factors that interact strongly with stem cells to trigger the molecular aspects of the differentiation or formulation of cardiomyocytes for the functional repair of heart injuries or cardiac defects.

PCL-PGLA Composite Tubular Scaffold Preparation and Biocompatibility Investigation

The objective of this paper was to fabricate a biodegradable tubular scaffold for small diameter (d < 6 mm) blood vessel tissue engineering. The tube scaffold needed a porous wall for cell attachment, proliferation and tissue regeneration with its degradation. A novel method given in this paper was to coat a porous layer of poly (∊-caprolactone) (PCL) on the outside of a poly (glycolic-colactic acid) (PGLA with GA: LA = 90:10) fiber braided tube to give a PCL-PGLA composite. The PGLA tube was fabricated using a braiding machine by inserting a Teflon tube with the desired diameter in center of the 20 spindles, which are the carriers of PGLA fibers. Changing the diameter of the Teflon tube can vary the inner diameter of a braided PGLA tube. Thermally induced phase separation method was used for PCL solution coating on the surface of the PGLA braided tube. Controlling the polymer concentration, non-solvent addition and quenching temperature generated the pore structures, with pore sizes ranging from 10–30 μm. The fibroblast cells were seeded on the tubular scaffold and cultured in vitro for the biocompatibility investigation. Histology results showed that the fibroblast cells proliferated on the interconnected pore of the PCL porous layer in 1 week.

Polystyrene/multi-wall carbon nanotube composite and its foam assisted by ultrasound vibration

Polystyrene/multi-wall carbon nanotube composite with an interconnected honeycomb-like structure was prepared by firstly coating the surface of the polystyrene pellets with multi-wall carbon nanotube, and sequentially welded through an ultrasound vibration technique. The mechanical and morphological properties of as-prepared composite were investigated in various measurements. It was found that an aggregative and honeycomb-like morphology of multi-wall carbon nanotube existed in the polystyrene/multi-wall carbon nanotube composite according to the polarized optical microscopic and scanning electron microscopic results; the ultrasound vibration could benefit to the performance of flexural strength. Furthermore, different composite foams were studied in this work, employing supercritical carbon dioxide as a blowing agent. Compared to other foams prepared by the conventional methods, the compressive strength of the foams derived from as-described novel method, was significantly improved. Also, being ascribed to this interconnected structure by coating carbon nanotube on polystyrene pellets, good electrical conductivity of 0.05–0.11 S/m was achieved in the novel composite foams.

Polystyrene foams with inter-connected carbon particulate network

New polystyrene (PS)/carbon nanofiber (CNF) and PS/graphite foams with an inter-connected honeycomb-like carbon particulate network of CNF or graphite were prepared by first coating the surface of polymer pellets with either CNF or graphite and then conducting batch foaming using carbon dioxide (CO2) as a blowing agent. It was found that the inter-connected honeycomb-like carbon particulate network could significantly reduce the compression yielding of conventional PS foams. With 1 wt% of CNFs or graphite, the PS foams with inter-connected honeycomb-like carbon particulate network were 5–9 times more electrically conductive than foams made of compounded PS nanocomposite with the same carbon particle loading. In addition, the PS foams with inter-connected honeycomb-like carbon particulate network were more thermally conductive and revealed significantly improved thermal stability comparing to foams made of compounded polymer nanocomposites.

Alternative conduits for microvascular anastomoses

Thrombotic events in vascular substitutes are the main cause of obliteration of most microvascular prostheses and subsequent failure of microvascular anastomoses. The development of new biomaterials for vascular replacement aims to obtain an ideal graft for microvascular surgery. Completely bioresorbable vascular prostheses with the capacity to induce regeneration and growth of a new vascular segment seem to overcome the limitations of contemporary artificial prostheses, mostly made of artificial materials and lacking the capacity to grow and be remodeled. Autologous vessels are currently the most used material for small-diameter arterial replacement. Immune acceptance is a major advantage offered by this technique, but the time required is a limitation in emergency surgery. The need for a prosthetic graft that would have the same properties as a small-diameter conduit has led investigators to pursue many avenues in vascular biology. This article details the development of microvascular synthetic prostheses, clarifying the current status and the future aims.

Bioactivation of porous polyurethane scaffolds using fluorinated RGD surface modifiers

Biomaterial scaffolds for tissue engineering require appropriate cell adhesion, proliferation, and infiltration into their three-dimensional (3D) porous structures. Surface modification techniques have the potential to enhance cell infiltration into synthetic scaffolds while retaining bulk material properties intact. The objective of this work was to assess the potential of achieving a uniform surface modification in 3D porous constructs through the blending of surface-modifying additives known as bioactive fluorinated surface modifiers (BFSMs) with a base polyurethane material. By coupling RGD peptides to the fluorinated surface modifiers to form RGD-BFSMs, the BFSMs can act as a vehicle for the delivery of RGD moieties to the surface without direct covalent attachment to the polymer substrate. Fluorescent RGD-BFSMs were shown to migrate to the polymer-air interfaces within the porous scaffolds by two-photon confocal microscopy. A-10 rat aortic smooth muscle cells were cultured for 4 weeks on nonmodified and RGD-BFSM-modified porous scaffolds, and cell adhesion, proliferation, and viability were quantified at different depths. RGD-BFSM-modified scaffolds showed significantly greater cell numbers within deeper regions of the scaffolds, and this difference became more pronounced over time. This study demonstrates an effective approach to promote cell adhesion and infiltration within thick (approximately 0.5 cm) porous synthetic scaffolds by providing a uniform distribution of adhesive peptide throughout the scaffolds without the use of covalent surface reaction chemistry.

Thermal model for optimization of vascular laser tissue soldering

Laser tissue soldering (LTS) is a promising technique for tissue fusion based on a heat-denaturation process of proteins. Thermal damage of the fused tissue during the laser procedure has always been an important and challenging problem. Particularly in LTS of arterial blood vessels strong heating of the endothelium should be avoided to minimize the risk of thrombosis. A precise knowledge of the temperature distribution within the vessel wall during laser irradiation is inevitable. The authors developed a finite element model (FEM) to simulate the temperature distribution within blood vessels during LTS. Temperature measurements were used to verify and calibrate the model. Different parameters such as laser power, solder absorption coefficient, thickness of the solder layer, cooling of the vessel and continuous vs. pulsed energy deposition were tested to elucidate their impact on the temperature distribution within the soldering joint in order to reduce the amount of further animal experiments. A pulsed irradiation with high laser power and high absorbing solder yields the best results.

Antithrombogenic modification of small-diameter microfibrous vascular grafts

OBJECTIVE

To develop small-diameter vascular grafts with a microstructure similar to native matrix fibers and with chemically modified microfibers to prevent thrombosis.

METHODS AND RESULTS

Microfibrous vascular grafts (1-mm internal diameter) were fabricated by electrospinning, and hirudin was conjugated to the poly (L-lactic acid) microfibers through an intermediate linker of poly(ethylene glycol). The modified microfibrous vascular grafts were able to reduce platelet adhesion/aggregation onto microfibrous scaffolds, and immobilized hirudin suppressed thrombin activity that may interact with the scaffolds. This 2-pronged approach to modify microfibrous vascular graft showed significantly improved patency (from 50% to 83%) and facilitated endothelialization, and the microfibrous structure of the vascular grafts allowed efficient graft remodeling and integration, with the improvement of mechanical property (elastic modulus) from 3.5 to 11.1 MPa after 6 months of implantation.

CONCLUSIONS

Microfibrous vascular grafts with antithrombogenic microfibers can be used as small-diameter grafts, with excellent patency and remodeling capability.

Biomaterials for vascular tissue engineering

Cardiovascular disease is the leading cause of mortality in the USA. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. While synthetic polymers have been extensively studied as substitutes in vascular engineering, they fall short of meeting the biological challenges at the blood-material interface. Various tissue engineering strategies have emerged to address these flaws and increase long-term patency of vascular grafts. Vascular cell seeding of scaffolds and the design of bioactive polymers for in situ arterial regeneration have yielded promising results. This article describes the advances made in biomaterials design to generate suitable materials that not only match the mechanical properties of native vasculature, but also promote cell growth, facilitate extracellular matrix production and inhibit thrombogenicity.

Polymeric materials for tissue engineering of arterial substitutes

Cardiovascular disease is the leading cause of mortality in the United States. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. Synthetic polymeric materials, while providing the appropriate mechanical strength, lack the compliance and biocompatibility that bioresorbable and naturally occurring protein polymers offer. Vascular tissue engineering approaches have emerged in order to meet the challenges of designing a vascular graft with long-term patency. In vitro culture techniques that have been explored with vascular cell seeding of polymeric scaffolds and the use of bioactive polymers for in situ arterial regeneration have yielded promising results. This review describes the development of polymeric materials in various tissue engineering strategies for the improvement in the mechanical and biological performance of an arterial substitute.

Biomaterials in orthopaedics

At present, strong requirements in orthopaedics are still to be met, both in bone and joint substitution and in the repair and regeneration of bone defects. In this framework, tremendous advances in the biomaterials field have been made in the last 50 years where materials intended for biomedical purposes have evolved through three different generations, namely first generation (bioinert materials), second generation (bioactive and biodegradable materials) and third generation (materials designed to stimulate specific responses at the molecular level). In this review, the evolution of different metals, ceramics and polymers most commonly used in orthopaedic applications is discussed, as well as the different approaches used to fulfil the challenges faced by this medical field.

Comparison of Carbon Nanofibers and Activated Carbon on Carbon Dioxide Foaming of Polystyrene

Carbon nanofibers (CNFs) and activated carbon (AC) are two forms of carbon with different shape and surface properties. These two carbon materials were used as additive/nucleation agents in the polystyrene extrusion foaming process. Carbon dioxide (CO2) was used as the blowing agent. Both additives showed better properties than talc in thermal insulation foams based on foam density, thermal conductivity, IR transmission, thermal stability, and compressive modulus. Overall, CNFs are better than AC due to the unique foam structure (`pseudo-bimodal' foam structure) of PS+ CNFs foams observed in this study.

2007 AIChE Alpha Chi Sigma Award: From Material to Tissue: Biomaterial Development, Scaffold Fabrication, and Tissue Engineering

The need for techniques to facilitate the regeneration of failing or destroyed tissues remains great with the aging of the worldwide population and the continued incidence of trauma and diseases such as cancer. A 16-year history in biomaterial scaffold development and tissue engineering is examined, beginning with the synthesis of novel materials and fabrication of 3D porous scaffolds. Exploring cell-scaffold interactions and subsequently cellular delivery using biomaterial carriers, we have developed a variety of techniques for bone and cartilage engineering. In addition to delivering cells, we have utilized growth factors, DNA, and peptides to improve the in vitro and in vivo regeneration of tissues. This review covers important developments and discoveries within our laboratory, and the increasing breadth in the scope of our work within the expanding field of tissue engineering is presented.

Composite fibrin scaffolds increase mechanical strength and preserve contractility of tissue engineered blood vessels

OBJECTIVES

We recently demonstrated that fibrin-based tissue engineered blood vessels (TEV) exhibited vascular reactivity, matrix remodeling and sufficient strength for implantation into the veins of an ovine animal model, where they remained patent for 15 weeks. Here we present an approach to improve the mechanical properties of fibrin-based TEV and examine the relationship between mechanical strength and smooth muscle cell (SMC) function.

MATERIALS AND METHODS

To this end, we prepared TEV that were composed of two layers: a cellular layer containing SMC embedded in fibrin hydrogel to provide contractility and matrix remodeling; and a second cell-free fibrin layer composed of high concentration fibrinogen to provide mechanical strength.

RESULTS

The ultimate tensile force of double-layered TEV increased with FBG concentration in the cell-free layer in a dose-dependent manner. Double-layered TEV exhibited burst pressure that was ten-fold higher than single-layered tissues but vascular reactivity remained high even though the cells were constricting an additional tissue layer.

CONCLUSION

These results showed that mechanical strength results largely from the biomaterial but contractility requires active cellular machinery. Consequently, they may suggest novel approaches for engineering biomaterials that satisfy the requirement for high mechanical strength while preserving SMC function.

Polyurethane/poly(lactic-co-glycolic) acid composite scaffolds fabricated by thermally induced phase separation

In this study, we present a novel composite scaffold fabricated using a thermally induced phase separation (TIPS) process from poly(lactic-co-glycolic) (PLGA) and biomedical polyurethane (PU). This processing method has been tuned to allow intimate (molecular) mixing of these two very different polymers, giving rise to a unique morphology that can be manipulated by controlling the phase separation behaviour of an initially homogenous polymer solution. Pure PLGA scaffolds possessed a smooth, directional fibrous sheet-like structure with pore sizes of 0.1-200mum, a porous Young's modulus of 93.5kPa and were relatively brittle to touch. Pure PU scaffolds had an isotropic emulsion-like structure, a porous Young's modulus of 15.7kPa and were much more elastic than the PLGA scaffolds. The composite PLGA/PU scaffold exhibits advantageous morphological, mechanical and cell adhesion and growth supporting properties, when compared with scaffolds fabricated from PLGA or PU alone. This novel method provides a mechanism for the formation of tailored bioactive scaffolds from nominally incompatible polymers, representing a significant step forward in scaffold processing for tissue-engineering applications.

Preparation of 3-D regenerated fibroin scaffolds with freeze drying method and freeze drying/foaming technique

Although three-dimensional fibroin scaffolds have been prepared with freeze drying method, the porosity and pore sizes still can not satisfy the requirement of tissue engineering. In this article, fibroin porous scaffold with high porosity and > 100 microm diameter interconnected pores was firstly prepared with freeze drying method through adjusting fibroin concentration. The morphology of different scaffolds lyophilized from different fibroin concentration was observed by SEM. A novel freeze drying improved method, freeze drying/foaming technique, was also devised to prepare fibroin scaffolds at different fibroin concentrations. Using the said method, the porosity and pore size of fibroin scaffolds prepared from 12% concentration were 85.8 +/- 4% and 109 +/- 20 microm respectively with yield strength up to 450 +/- 6 KPa while the porosity and pore size of fibroin scaffolds prepared from 8% concentration were 96.9 +/- 3.6% and 120 +/- 30 microm respectively with yield strength up to 30 +/- 1 KPa. The freeze drying/foaming technique produced scaffolds with a useful combination of high yield strength, interconnected pores, and pore sizes greater than 100 microm in diameter. Through adjusting fibroin concentration and thawing time, the porosity, pore sizes and mechanical properties could be controlled to satisfy the different requirements of tissue engineering. The results suggested that fibroin scaffolds prepared with the above methods could be formed for utility in biomaterial application.

Promotion of cell affinity of porous PLLA scaffolds by immobilization of RGD peptides via plasma treatment

In the present work, RGDS (Arg-Gly-Asp-Ser) was immobilized on PLLA scaffolds with plasma treatment. The amount of immobilization, determined by HPLC, was confirmed to be in the effective order. Results from the culture of rat osteosarcoma (ROS), osteoblastic-like cells, demonstrate that the immobilization of RGDS could effectively enhance the attachment of ROS cells on PLLA and increase the cell density in PLLA scaffolds. In addition, experiments of in vitro mineralization indicate that there were more cells and mineralization focci in the RGDS-immobilized scaffolds, suggesting a tendency to form bone-like tissues, compared with the unmodified PLLA scaffold. On the other hand, the PLLA scaffolds immobilized with RGES (Arg-Gly-Glu-Ser) were much less effective in promotion of ROS attachment, suggesting that the enhancement on cell attachment was mainly due to the recognition of RGDS by the adhesion receptors on the cell membrane. The results presented in this work demonstrate that RGDS could be successfully immobilized on PLLA scaffolds with plasma treatment and such modification can make PLLA scaffolds more suitable for culture of osteoblast-like cells and for generation of bone-like tissues.

Layer Manufacturing for in Vivo Devices

Traditional in vivo devices fabricated to be used as implantation devices included sutures, plates, pins, screws, and joint replacement implants. Also, akin to developments in regenerative medicine and drug delivery, there has been the pursuit of less conventional in vivo devices that demand complex architecture and composition, such as tissue scaffolds. Commercial means of fabricating traditional devices include machining and moulding processes. Such manufacturing techniques impose considerable lead times and geometrical limitations, and restrict the economic production of customized products. Attempts at the production of non-conventional devices have included particulate leaching, solvent casting, and phase transition. These techniques cannot provide the desired total control over internal architecture and compositional variation, which subsequently restricts the application of these products. Consequently, several parties are investigating the use of freeform layer manufacturing techniques to overcome these difficulties and provide viable in vivo devices of greater functionality. This paper identifies the concepts of rapid manufacturing (RM) and the development of biomanufacturing based on layer manufacturing techniques. Particular emphasis is placed on the development and experimentation of new materials for bio-RM, production techniques based on the layer manufacturing concept, and computer modelling of in vivo devices for RM techniques.

Mammalian cell survival and processing in supercritical CO(2)

We demonstrate that mammalian cells can survive for 5 min within high-pressure CO(2)(.) Cell survival was investigated by exposing a range of mammalian cell types to supercritical CO(2) (scCO(2)) (35 degrees C, 74 bar; 1 bar = 100 kPa) for increasing exposure and depressurization times. The myoblastic C2C12 cell line, 3T3 fibroblasts, chondrocytes, and hepatocytes all displayed appreciable but varying metabolic activity with exposure times up to 1 min. With depressurization times of 4 min, cell population metabolic activity was >/=70% of the control population. Based on survival data, we developed a single-step scCO(2) technique for the rapid production of biodegradable poly(dl-lactic acid) scaffolds containing mammalian cells. By using optimum cell-survival conditions, scCO(2) was used to process poly(dl-lactic acid) containing a cell suspension, and, upon pressure release, a polymer sponge containing viable mammalian cells was formed. Cell functionality was demonstrated by retention of an osteogenic response to bone morphogenetic protein-2 in C2C12 cells. A gene microarray analysis showed no statistically significant changes in gene expression across 4,418 genes by a single-class t test. A significance analysis of microarrays revealed only eight genes that were down-regulated based on a delta value of 1.0125 and a false detection rate of 0.

Small-Diameter Artificial Arteries Engineered In Vitro

Although the need for a functional arterial replacement is clear, the lower blood flow velocities of small-diameter arteries like the coronary artery have led to the failure of synthetic materials that are successful for large-diameter grafts. Although autologous vessels remain the standard for small diameter grafts, many patients do not have a vessel suitable for use because of vascular disease, amputation, or previous harvest. As a result, tissue engineering has emerged as a promising approach to address the shortcomings of current therapies. Investigators have explored the use of arterial tissue cells or differentiated stem cells combined with various types of natural and synthetic scaffolds to make tubular constructs and subject them to chemical and/or mechanical stimulation in an attempt to develop a functional small-diameter arterial replacement graft with varying degrees of success. Here, we review the progress in all these major facets of the field.

A novel processing method for injection-molded polyether–urethane scaffolds. Part 1: Processing

A large-scale scaffold processing method with injection molding has been successfully developed. Water was used as afoaming agent for the new technique. NaCl was used as a porogen to achieve an open-cell structure. Organic solvents, which are common foaming agents for polyurethane, where not used. Toxic remains in the polymer were therefore prevented. Pore size and porosity was adjustable through process parameters. A parameter study showed that an increase in injection pressure, plasticize speed, cylinder, and mold temperature raised the mean pore diameter. The porosity also could be mended by the cylinder and mold temperature, in addition to NaCl concentration. It was possible to produce scaffolds with a porosity of 64 +/- 3%, a pore size distribution from 30-450 microm, and a mean pore diameter of 270 +/- 90 microm. The interconnective pores were found to lie between 5 and 58 microm.

A “room-temperature” injection molding/particulate leaching approach for fabrication of biodegradable three-dimensional porous scaffolds

A "room-temperature" injection molding approach combined with particulate leaching (RTIM/PL) has been, for the first time, developed in this work to fabricate three-dimensional porous scaffolds composed of biodegradable polyesters for tissue engineering. In this approach, a "wet" composite of particulate/polymer/solvent was used in processing, and thus the injection was not performed at melting state. Appropriate viscosity and flowability were facilely obtained at a certain solvent content so that the composite was able to be injected into a mould under low pressure at room temperature, which was very beneficial for avoiding thermal degradation of polyesters. As a demonstration, tubular and ear-shaped porous scaffolds were fabricated from biodegradable poly(D,L-lactide-co-glycolide) (PLGA) by this technology. Porosities of the resulting scaffolds were as high as 94%. The pores were well interconnected. Besides the well-known characteristics of injection molding to be suitable for automatization of a fabrication process with high repeatability and precision, this RTIM/PL approach is much suitable for tailoring highly porous foams with its advantages flexible for shaping complicated scaffolds, free of thermal degradation and high-pressure machine, etc.

Long-term evaluation of porous poly(epsilon-caprolactone-co-L-lactide) as a bone-filling material

Porous poly(epsilon-caprolactone-co-L-lactide) (P(CL-co-LA, wt % ca. 5/95) sponges were prepared, coated biomimetically with CaP/apatite, and implanted with noncoated control sponges into rat femur cortical defects and dorsal subcutaneous space. The implants were inspected histologically at 2, 4, and 33 weeks after the operation. All implants were filled with fibrovascular tissue within 4 weeks. The femur implants were partially ossified with compact bone, which in the CaP-coated sponges was less mature and more fragmented. Approximately equal amounts of bone were observed in both types of implants. The polymer induced a mild inflammatory reaction with foreign body giant cells but no accumulation of fluid. Degradation of the polymer was slow; most of it was found intact at 33 weeks in histological samples. Nondegraded polymer seems to prevent complete ossification. Cultured osteoblasts proliferated well on apatite-coated material, whereas only a few cells were seen on noncoated material. Thus CaP/apatite coating helped the attachment of osteoblasts in cell cultures but did not offer any advantage in bone formation over noncoated material in vivo. We conclude that a shorter degradation time of P(CL-co-LA) is needed to create an optimal implant. Furthermore, in vivo experiments seem to be necessary for the estimation of osteopromotive properties of a biomaterial.

Solvent effects on the microstructure and properties of 75/25 poly(D,L-lactide-co-glycolide) tissue scaffolds

Poly(lactide-co-glycolide) (PLGA) is used in many biomedical applications because it is biodegradable, biocompatible, and FDA approved. PLGA can also be processed into porous tissue scaffolds, often through the use of organic solvents. A static light scattering experiment showed that 75/25 PLGA is well solvated in acetone and methylene chloride, but forms aggregates in chloroform. This led to an investigation of whether the mechanical properties of the scaffolds were affected by solvent choice. Porous 75/25 PLGA scaffolds were created with the use of the solvent casting/particulate leaching technique with three different solvents: acetone, chloroform, and methylene chloride. Compression testing resulted in stiffness values of 21.7 +/- 4.8 N/mm for acetone, 18.9 +/- 4.2 N/mm for chloroform, and 30.2 +/- 9.6 N/mm for methylene chloride. Permeability testing found values of 3.9 +/- 1.9 x 10(-12) m2 for acetone, 3.6 +/- 1.3 x 10(-12) m2 for chloroform, and 2.4 +/- 1.0 x 10(-12) m2 for methylene chloride. Additional work was conducted to uncouple polymer/solvent interactions from evaporation dynamics, both of which may affect the scaffold properties. The results suggest that solvent choice creates small but significant differences in scaffold properties, and that the rate of evaporation is more important in affecting scaffold microstructure than polymer/solvent interactions.

Preparation and physicochemical properties of compression-molded keratin films

The S-sulfo keratin was extracted from wool and was then spray-dried to give S-sulfo keratin powder. Differential scanning calorimetry analysis showed that the glass transition temperature of S-sulfo keratins became lowered with the increase of moisture content, while perfectly dried S-sulfo keratin powder did not give thermal transition in the temperature range 30-130 degrees C. The compression molding of the S-sulfo keratin powder supplemented with one-tenth weight of water afforded a plastic-like transparent proteinous film above the glass transition temperature. The film obtained from the powder without water addition or compression molded below glass transition temperature partly remained powdery. The film compression molded at 120 degrees C gave the maximum ultimate strength and Young's modulus, 27.8 +/- 2.9 and 1218 +/- 80 MPa, respectively. Obtained film was insoluble and slightly swelled in water, but, in the presence of reducing agent, the film significantly swelled at pH 7.0 and even dissolved at pH 9.0, suggesting the relevance of abundant disulfide linkage. The film supported the mammalian cell adhesion and proliferation, demonstrating the biocompatibility of S-sulfo keratin films.

Nanoscale clustering of RGD peptides at surfaces using comb polymers. 2. Surface segregation of comb polymers in polylactide

Part 1 of these studies described poly(methyl methacrylate-r-polyoxyethylene methacrylate) P(MMA-r-POEM) comb polymers that present Arg-Gly-Asp (RGD) peptides at a surface in nanoscale clusters on a protein-resistant background for control of cell adhesion. Here in part 2, we examine surface segregation of these peptide-modified and unmodified comb polymers blended with polylactide (PLA) as a self-assembly approach suitable for surface modification of porous tissue engineering scaffolds. Multiple thermodynamic driving forces for surface enrichment of the comb polymer are exploited by annealing PLA/P(MMA-r-POEM) blends above the glass transition of the blend components but below the melting point of PLA, while in contact with water. Predictions of the interfacial composition profiles of annealed blends were made using a self-consistent field (SCF) lattice model. The calculations predict strong enrichment of the comb in the top approximately 50 A of blends, and organization of comb molecules in quasi-2D conformations at the interface, similar to the apparent structure of pure comb surfaces in contact with water described in part 1. Experimentally, PLA/comb blend surfaces were characterized by contact angle measurements, XPS, quantification of ligand-cluster surface density and stability by AFM and fluorescent nanosphere labeling, and cell attachment assays. These data were consistent with SCF predictions, showing significant enrichment of the comb at water-annealed surfaces and RGD cluster densities consistent with 2D conformations for comb molecules in the surface layer. Bulk miscibility of the blends was verified by dynamic rheometry, small-angle neutron scattering, DSC and X-ray diffraction studies. Surface segregation of combs provided tunable cell adhesion on PLA through surface-localized nanoclusters of RGD atop a cell-resistant background.

Fabrication of poly(alpha-hydroxy acid) foam scaffolds using multiple solvent systems

The present studies describe the fabrication and characterization of highly porous and interconnected poly(alpha-hydroxy acid) foam scaffolds produced using a phase separation multisolvent system, followed by a sublimation process. Fabrication parameters, including solvent composition, polymer concentration, freezing temperature, polymer type, and polymer molecular weight, were optimized to produce the desired foam microstructure. Analyses of selected samples with scanning electron microscopic images and mercury intrusion porosimetry indicated polymer foams with pore size ranges of 100-350 microm, a porosity >90%, and an interconnecting open-pore foam structure. Scaffold degradation profiles varied according to the type and molecular weight of the polymers. Cytocompatibility assays demonstrated that the preferred foam structures were nontoxic and osteoprecursor cells seeded into the scaffolds exhibited the ability to attach, propagate, and differentiate into a calcified structure.

Laser-tissue soldering with biodegradable polymer films in vitro: film surface morphology and hydration effects

BACKGROUND AND OBJECTIVE

Previous research introduced the concept of using biodegradable polymer film reinforcement of a liquid albumin solder for improvement of the tensile strength of repaired incisions in vitro. In this study, the effect of creating small pores in the PLGA films on the weld breaking strength is studied. Additionally, the effect of hydration on the strength of the reinforced welds is investigated.

STUDY DESIGN/MATERIALS AND METHODS

A 50%(w/v) bovine serum albumin solder with 0.5 mg/mL Indocyanine Green dye was used to repair an incision in bovine aorta. The solder was coagulated with an 806-nm CW diode laser. A poly(DL-lactic-co-glycolic acid) (PLGA) film was used to reinforce the solder (the controls had solder but no reinforcement). Breaking strengths were measured acutely and after hydration in saline for 1 and 2 days. The data were analyzed by ANOVA (P < 0.05) and multiple comparisons of means were performed using the Newman-Keuls test.

RESULTS

The creation of pores in the PLGA films qualitatively improved the film flexibility without having an apparent adverse effect on the breaking strength, while the actual technique of applying the film and solder had more of an effect. The acute maximum average breaking strengths of some of the film reinforced specimens (114.7 g-134.4 g) were significantly higher (P < 0.05) than the acute maximum average breaking strength of the unreinforced control specimens (68.3 g). Film reinforced specimens were shown to have a statistically significantly higher breaking strength than unreinforced controls after 1- and 2-day hydration.

CONCLUSIONS

Reinforcement of liquid albumin solders in laser-assisted incision repair appears to have advantages over conventional methods that do not reinforce the cohesive strength of the solder in terms of acute breaking strength and after immersion in moist environments for short periods of time. Using a film with the solder applied to one surface only may be advantageous over other techniques.

Biodegradable polymeric scaffolds for musculoskeletal tissue engineering

Biodegradable scaffolds have played an important role in a number of tissue engineering attempts over the past decade. The goal of this review article is to provide a brief overview of some of the important issues related to scaffolds fabricated from synthetic biodegradable polymers. Various types of such materials are available; some are commercialized and others are still in the laboratories. The properties of the most common of these polymers are discussed here. A variety of fabrication techniques were developed to fashion polymeric materials into porous scaffolds, and a selection of these is presented. The very important issue of scaffold architecture, including the topic of porosity and permeability, is discussed. Other areas such as cell growth on scaffolds, surface modification, scaffold mechanics, and the release of growths factors are also reviewed. A summary outlining the common themes in scaffold-related science that are found in the literature is presented.

Improved vascular tissue fusion using new light-activated surgical adhesive on a canine model

Newly developed light-activated surgical adhesives have been investigated as a substitute to traditional protein solders for vascular tissue fusion without the need for sutures. Canine femoral arteries (n = 14), femoral veins (n = 14), and carotid arteries (n = 10) were exposed, and a 0.3-0.6 cm longitudinal incision was made in the vessel walls. The surgical adhesive, composed of a poly(L-lactic-co-glycolic acid) scaffold doped with the traditional protein solder mix of bovine serum albumin and indocyanine green dye, was used to close the incisions in conjunction with an 805 nm diode laser. Blood flow was restored to the vessels immediately after the procedure and the incision sites were checked for patency. The new adhesives were flexible enough to be wrapped around the vessels while their solid nature avoided the problems associated with "runaway" of the less viscous liquid protein solders widely used by researchers. Assessment parameters included measurement of the ex vivo intraluminal bursting pressure 1-2 h after surgery, as well as histology. The acute intraluminal bursting pressures were significantly higher in the laser-solder group (>300 mmHg) compared to the suture control group (<150 mmHg) where four evenly spaced sutures were used to repair the vessel (n = 4). Histological analysis showed negligible evidence of collateral thermal damage to the underlying tissue in the laser-solder repair group. These initial results indicated that laser-assisted vascular repair using the new adhesives is safe, easy to perform, and contrary to conventional suturing, provides an immediate leak-free closure. In addition, the flexible and moldable nature of the new adhesives should allow them to be tailored to a wide range of tissue geometries, thus greatly improving the clinical applicability of laser-assisted tissue repair.