The effect of processing variables on morphological and mechanical properties of supercritical CO2 foamed scaffolds for tissue engineering

Biomimetic collagen/elastin meshes for ventral hernia repair in a rat model

Ventral hernia repair remains a major clinical need. Herein, we formulated a type I collagen/elastin crosslinked blend (CollE) for the fabrication of biomimetic meshes for ventral hernia repair. To evaluate the effect of architecture on the performance of the implants, CollE was formulated both as flat sheets (CollE Sheets) and porous scaffolds (CollE Scaffolds). The morphology, hydrophylicity and in vitro degradation were assessed by SEM, water contact angle and differential scanning calorimetry, respectively. The stiffness of the meshes was determined using a constant stretch rate uniaxial tensile test, and compared to that of native tissue. CollE Sheets and Scaffolds were tested in vitro with human bone marrow-derived mesenchymal stem cells (h-BM-MSC), and finally implanted in a rat ventral hernia model. Neovascularization and tissue regeneration within the implants was evaluated at 6weeks, by histology, immunofluorescence, and q-PCR. It was found that CollE Sheets and Scaffolds were not only biomechanically sturdy enough to provide immediate repair of the hernia defect, but also promoted tissue restoration in only 6weeks. In fact, the presence of elastin enhanced the neovascularization in both sheets and scaffolds. Overall, CollE Scaffolds displayed mechanical properties more closely resembling those of native tissue, and induced higher gene expression of the entire marker genes tested, associated with de novo matrix deposition, angiogenesis, adipogenesis and skeletal muscles, compared to CollE Sheets. Altogether, this data suggests that the improved mechanical properties and bioactivity of CollE Sheets and Scaffolds make them valuable candidates for applications of ventral hernia repair.

STATEMENT OF SIGNIFICANCE

Due to the elevated annual number of ventral hernia repair in the US, the lack of successful grafts, the design of innovative biomimetic meshes has become a prime focus in tissue engineering, to promote the repair of the abdominal wall, avoid recurrence. Our meshes (CollE Sheets and Scaffolds) not only showed promising mechanical performance, but also allowed for an efficient neovascularization, resulting in new adipose and muscle tissue formation within the implant, in only 6weeks. In addition, our meshes allowed for the use of the same surgical procedure utilized in clinical practice, with the commercially available grafts. This study represents a significant step in the design of bioactive acellular off-the-shelf biomimetic meshes for ventral hernia repair.

Fabrication of Porous Materials from Natural/Synthetic Biopolymers and Their Composites

Biopolymers and their applications have been widely studied in recent years. Replacing the oil based polymer materials with biopolymers in a sustainable manner might give not only a competitive advantage but, in addition, they possess unique properties which cannot be emulated by conventional polymers. This review covers the fabrication of porous materials from natural biopolymers (cellulose, chitosan, collagen), synthetic biopolymers (poly(lactic acid), poly(lactic-co-glycolic acid)) and their composite materials. Properties of biopolymers strongly depend on the polymer structure and are of great importance when fabricating the polymer into intended applications. Biopolymers find a large spectrum of application in the medical field. Other fields such as packaging, technical, environmental, agricultural and food are also gaining importance. The introduction of porosity into a biomaterial broadens the scope of applications. There are many techniques used to fabricate porous polymers. Fabrication methods, including the basic and conventional techniques to the more recent ones, are reviewed. Advantages and limitations of each method are discussed in detail. Special emphasis is placed on the pore characteristics of biomaterials used for various applications. This review can aid in furthering our understanding of the fabrication methods and about controlling the porosity and microarchitecture of porous biopolymer materials.

Analytical relationships for prediction of the mechanical properties of additively manufactured porous biomaterials

Recent developments in additive manufacturing techniques have motivated an increasing number of researchers to study regular porous biomaterials that are based on repeating unit cells. The physical and mechanical properties of such porous biomaterials have therefore received increasing attention during recent years. One of the areas that have revived is analytical study of the mechanical behavior of regular porous biomaterials with the aim of deriving analytical relationships that could predict the relative density and mechanical properties of porous biomaterials, given the design and dimensions of their repeating unit cells. In this article, we review the analytical relationships that have been presented in the literature for predicting the relative density, elastic modulus, Poisson's ratio, yield stress, and buckling limit of regular porous structures based on various types of unit cells. The reviewed analytical relationships are used to compare the mechanical properties of porous biomaterials based on different types of unit cells. The major areas where the analytical relationships have improved during the recent years are discussed and suggestions are made for future research directions. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 3164–3174, 2016.

Characterization and investigation of the deformation behavior of porous magnesium scaffolds with entangled architectured pore channels

We report a kind of porous magnesium with entangled architectured pore structure for potential applications in biomedical implant. The pore size, spatial structure and Young׳s modulus of the as-prepared porous Mg are suitable for bone tissue engineering applications. Particularly, with regard to the load-bearing conditions, a new analytical model is employed to investigate its structure and mechanical response under compressive stress based on Gibson-Ashby model. It is found that there are three types of stress-strain behaviors in the large range of porosity from 20% to 80%. When the porosity is larger than an upper critical value, the porous magnesium exhibits densifying behavior with buckling deformation mechanism. When the porosity is smaller than a lower critical value, the porous magnesium exhibits shearing behavior with cracking along the maximum shear stress. Between the two critical porosities, both the buckling deformation and shearing behavior coexist. The upper critical porosity is experimentally determined to be 60% for 270μm pore size and 62% for 400μm pore size, while the lower critical porosity is 40% for 270μm pore size and 42% for 400μm pore size. A new analytical model could be used to accurately predict the mechanical response of the porous magnesium. No matter the calculated critical porosity or yielding stress in a large range of porosity by using the new model are well consistent with the experimental values. All these results could help to provide valuable data for developing the present porous magnesium for potential bio applications.

Low viscosity-PLGA scaffolds by compressed CO2foaming for growth factor delivery

Foaming technology using supercritical and compressed fluids has emerged as a promising solution in regenerative medicine for manufacturing porous polymeric scaffolds.

Preparation and biological performance of poly(vinyl alcohol)/hydroxyapatite porous composites used for cartilage repair

PVA/HA porous materials with proper cell structure and biological properties which were suitable for cartilage repair were prepared via thermoplastic foaming technique by using water/ethanol as co-blowing agent.

A Dual Role of Graphene Oxide Sheet Deposition on Titanate Nanowire Scaffolds for Osteo-implantation: Mechanical Hardener and Surface Activity Regulator

Scaffold biomaterials with open pores and channels are favourable for cell growth and tissue regeneration, however the inherent poor mechanical strength and low surface activity limit their applications as load-bearing bone grafts with satisfactory osseointegration. In this study, macro-porous graphene oxide (GO) modified titanate nanowire scaffolds with desirable surface chemistry and tunable mechanical properties were prepared through a simple hydrothermal process followed by electrochemical deposition of GO nanosheets. The interconnected and porous structure of the GO/titanate nanowire scaffolds provides a large surface area for cellular attachment and migration and displays a high compressive strength of approximately 81.1 MPa and a tunable Young’s modulus over the range of 12.4–41.0 GPa, which satisfies site-specific requirements for implantation. Surface chemistry of the scaffolds was modulated by the introduction of GO, which endows the scaffolds flexibility in attaching and patterning bioactive groups (such as -OH, -COOH and -NH₂). In vitro cell culture tests suggest that the GO/titanate nanowire scaffolds act as a promising biomaterial candidate, in particular the one terminated with -OH groups, which demonstrates improved cell viability, and proliferation, differentiation and osteogenic activities.

Cell Structure, Density and Impact Strength of Cellulose Acetate Foamed with Supercritical Carbon Dioxide

This paper investigated the effects of processing conditions on the cell structure, density and impact strength of cellulose acetate (CA) foamed with supercritical carbon dioxide (SC-CO2). The scanning electron microscope (SEM) images show that there is the open-cell structure obviously when the foaming temperature exceeds 210°C, and the increasing foaming temperature improves the amount of open cells. The diameters of external cells range from 0.50 μm to 0.85 μm, and the diameters of internal cells are around 0.20 μm. The presence of cosolvents is beneficial to form open-cell structure, which is ascribed to more amount of CO2 dissolved into the CA matrix. Densities of foamed CA are measured with the method of volumetric flask, and the density ranges between 0.69 g·cm-3 and 1.02 g·cm-3, which are lower than that of the original sample (1.27 g·cm-3). The density decreases with increasing the saturation temperature, the saturation time or the foaming temperature. And densities with ethanol are much lower than those with acetone. Both of the impact strengths and specific impact strengths of foamed CA, higher than those of the original sample, increase firstly and decrease subsequently with increasing the foaming temperature. The impact strength of CA foaming at 230°C is 1.4 times higher than that of the original sample, and the specific impact strength increases by 84%.

Supercritical CO2 foaming of thermoplastic materials derived from maize: proof-of-concept use in mammalian cell culture applications

BACKGROUND

Foams are high porosity and low density materials. In nature, they are a common architecture. Some of their relevant technological applications include heat and sound insulation, lightweight materials, and tissue engineering scaffolds. Foams derived from natural polymers are particularly attractive for tissue culture due to their biodegradability and bio-compatibility. Here, the foaming potential of an extensive list of materials was assayed, including slabs elaborated from whole flour, the starch component only, or the protein fraction only of maize seeds.

METHODOLOGY/PRINCIPAL FINDINGS

We used supercritical CO2 to produce foams from thermoplasticized maize derived materials. Polyethylene-glycol, sorbitol/glycerol, or urea/formamide were used as plasticizers. We report expansion ratios, porosities, average pore sizes, pore morphologies, and pore size distributions for these materials. High porosity foams were obtained from zein thermoplasticized with polyethylene glycol, and from starch thermoplasticized with urea/formamide. Zein foams had a higher porosity than starch foams (88% and 85%, respectively) and a narrower and more evenly distributed pore size. Starch foams exhibited a wider span of pore sizes and a larger average pore size than zein (208.84 vs. 55.43 μm2, respectively). Proof-of-concept cell culture experiments confirmed that mouse fibroblasts (NIH 3T3) and two different prostate cancer cell lines (22RV1, DU145) attached to and proliferated on zein foams.

CONCLUSIONS/SIGNIFICANCE

We conducted screening and proof-of-concept experiments on the fabrication of foams from cereal-based bioplastics. We propose that a key indicator of foamability is the strain at break of the materials to be foamed (as calculated from stress vs. strain rate curves). Zein foams exhibit attractive properties (average pore size, pore size distribution, and porosity) for cell culture applications; we were able to establish and sustain mammalian cell cultures on zein foams for extended time periods.

Influence of hydroxyapatite on thermoplastic foaming performance of water-plasticized poly(vinyl alcohol)

Water states in the system are adjusted by changing HA content to obtain proper PVA/HA composite foams through thermoplastic processing.

Study of Lysozyme-Loaded Poly-L-Lactide (PLLA) Porous Microparticles in a Compressed CO₂ Antisolvent Process

Lysozyme (LSZ)-loaded poly-L-lactide (PLLA) porous microparticles (PMs) were successfully prepared by a compressed CO₂ antisolvent process in combination with a water-in-oil emulsion process using LSZ as a drug model and ammonium bicarbonate as a porogen. The effects of different drug loads (5.0%, 7.5% and 10.0%) on the surface morphology, particle size, porosity, tapped density and drug release profile of the harvested PMs were investigated. The results show that an increase in the amount of LSZ added led to an increase in drug load (DL) but a decrease in encapsulation efficiency. The resulting LSZ-loaded PLLA PMs (LSZ-PLLA PMs) exhibited a porous and uneven morphology, with a density less than 0.1 g·cm-3, a geometric mean diameter of 16.9-18.8 μm, an aerodynamic diameter less than 2.8 μm, a fine particle fraction (FPF) of 59.2%-66.8%, and a porosity of 78.2%-86.3%. According to the results of differential scanning calorimetry, the addition of LSZ improved the thermal stability of PLLA. The Fourier transform infrared spectroscopy analysis and circular dichroism spectroscopy measurement reveal that no significant changes occurred in the molecular structures of LSZ during the fabrication process, which was further confirmed by the evaluation of enzyme activity of LSZ. It is demonstrated that the emulsion-combined precipitation with compressed antisolvent (PCA) process could be a promising technology to develop biomacromolecular drug-loaded inhalable carrier for pulmonary drug delivery.

Tissue engineering and regenerative medicine as applied to the gastrointestinal tract

The gastrointestinal (GI) tract is a complex system characterized by multiple cell types with a determined architectural arrangement. Tissue engineering of the GI tract aims to reinstate the architecture and function of all structural layers. The key point for successful tissue regeneration includes the use of cells/biomaterials that elucidate minimal immune response after implantation. Different biomaterial choices and cell sources have been proposed to engineer the GI tract. This review summarizes the recent advances in bioengineering the GI tract with emphasis on cell sources and scaffolding biomaterials.

The future of carbon dioxide for polymer processing in tissue engineering

The use of CO2 for scaffold fabrication in tissue engineering was popularized in the mid-1990 s as a tool for producing polymeric foam scaffolds, but had fallen out of favor to some extent, in part due to challenges with pore interconnectivity. Pore interconnectivity issues have since been resolved by numerous dedicated studies that have collectively outlined how to control the appropriate parameters to achieve a pore structure desirable for tissue regeneration. In addition to CO2 foaming, several groups have leveraged CO2 as a swelling agent to impregnate scaffolds with drugs and other bioactive additives, and for encapsulation of plasmids within scaffolds for gene delivery. Moreover, in contrast to CO2 foaming, which typically relies on supercritical CO2 at very high pressures, CO2 at much lower pressures has also been used to sinter polymeric microspheres together in the presence of cells to create cell-seeded scaffolds in a single step. CO2 has a number of advantages for polymer processing in tissue engineering, including its ease of use, low cost, and the opportunity to circumvent the use of organic solvents. Building on these advantages, and especially now with the tremendous precedent that has paved the way in defining operating parameters, and making the technology accessible for new groups to adapt, we invite and encourage our colleagues in the field to leverage CO2 as a new tool to enhance their own respective unique capabilities.