Design of porous scaffolds for cartilage tissue engineering using a three-dimensional fiber-deposition technique

Rapid prototyping in tissue engineering: challenges and potential

Tissue engineering aims to produce patient-specific biological substitutes in an attempt to circumvent the limitations of existing clinical treatments for damaged tissue or organs. The main regenerative tissue engineering approach involves transplantation of cells onto scaffolds. The scaffold attempts to mimic the function of the natural extracellular matrix, providing a temporary template for the growth of target tissues. Scaffolds should have suitable architecture and strength to serve their intended function. This paper presents a comprehensive review of the fabrication methods, including conventional, mainly manual, techniques and advanced processing methods such as rapid prototyping (RP) techniques. The potential and challenges of scaffold-based technology are discussed from the perspective of RP technology.

Quantitative ultrasound can assess the regeneration process of tissue-engineered cartilage using a complex between adherent bone marrow cells and a three-dimensional scaffold

Articular cartilage (hyaline cartilage) defects resulting from traumatic injury or degenerative joint disease do not repair themselves spontaneously. Therefore, such defects may require novel regenerative strategies to restore biologically and biomechanically functional tissue. Recently, tissue engineering using a complex of cells and scaffold has emerged as a new approach for repairing cartilage defects and restoring cartilage function. With the advent of this new technology, accurate methods for evaluating articular cartilage have become important. In particular, in vivo evaluation is essential for determining the best treatment. However, without a biopsy, which causes damage, articular cartilage cannot be accurately evaluated in a clinical context. We have developed a novel system for evaluating articular cartilage, in which the acoustic properties of the cartilage are measured by introducing an ultrasonic probe during arthroscopy of the knee joint. The purpose of the current study was to determine the efficacy of this ultrasound system for evaluating tissue-engineered cartilage in an experimental model involving implantation of a cell/scaffold complex into rabbit knee joint defects. Ultrasonic echoes from the articular cartilage were converted into a wavelet map by wavelet transformation. On the wavelet map, the percentage maximum magnitude (the maximum magnitude of the measurement area of the operated knee divided by that of the intact cartilage of the opposite, nonoperated knee; %MM) was used as a quantitative index of cartilage regeneration. Using this index, the tissue-engineered cartilage was examined to elucidate the relations between ultrasonic analysis and biochemical and histological analyses. The %MM increased over the time course of the implant and all the hyaline-like cartilage samples from the histological findings had a high %MM. Correlations were observed between the %MM and the semiquantitative histologic grading scale scores from the histological findings. In the biochemical findings, the chondroitin sulfate content increased over the time course of the implant, whereas the hydroxyproline content remained constant. The chondroitin sulfate content showed a similarity to the results of the %MM values. Ultrasonic measurements were found to predict the regeneration process of the tissue-engineered cartilage as a minimally invasive method. Therefore, ultrasonic evaluation using a wavelet map can support the evaluation of tissue-engineered cartilage using cell/scaffold complexes.

The effect of PEGT/PBT scaffold architecture on the composition of tissue engineered cartilage

A highly interconnecting and accessible pore network has been suggested as one of a number of prerequisites in the design of scaffolds for tissue engineering. In the present study, two processing techniques, compression-molding/particulate-leaching (CM), and 3D fiber deposition (3DF), were used to develop porous scaffolds from biodegradable poly(ethylene glycol)-terephthalate/poly(butylene terephthalate) (PEGT/PBT) co-polymers with varying pore architectures. Three-dimensional micro-computed tomography (microCT) was used to characterize scaffold architectures and scaffolds were seeded with articular chondrocytes to evaluate tissue formation. Scaffold porosity ranged between 75% and 80%. Average pore size of tortuous CM scaffolds (182 microm) was lower than those of organized 3DF scaffolds (525 microm). The weight ratio of glycosaminoglycans (GAG)/DNA, as a measure of cartilage-like tissue formation, did not change after 14 days of culture whereas, following subcutaneous implantation, GAG/DNA increased significantly and was significantly higher in 3DF constructs than in CM constructs, whilst collagen type II was present within both constructs. In conclusion, 3DF PEGT/PBT scaffolds create an environment in vivo that enhances cartilaginous matrix deposition and hold particular promise for treatment of articular cartilage defects.

Three-dimensional fiber-deposited PEOT/PBT copolymer scaffolds for tissue engineering: Influence of porosity, molecular network mesh size, and swelling in aqueous media on dynamic mechanical properties

Among novel scaffold fabrication techniques, 3D fiber deposition (3DF) has recently emerged as a means to fabricate well-defined and custom-made scaffolds for tissue regeneration, with 100% interconnected pores. The mechanical behavior of these constructs is dependent not only on different three-dimensional architectural and geometric features, but also on the intrinsic chemical properties of the material used. These affect the mechanics of the solid material and eventually of 3D porous constructs derived from them. For instance, poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) block copolymers are known to have mechanical properties, depending on the PEOT/PBT weight ratio in block form and on the molecular weight of the initial poly(ethylene glycol) (PEG) blocks. These differences are enhanced even more by their different swelling properties in aqueous media. Therefore, this article examines the influence of copolymer compositions in terms of their swelling on dynamic mechanical properties of solid material and porous 3DF scaffolds. The molecular weight of the starting PEG blocks used in the copolymer synthesis varied from 300 to 1000 g/mol. The PEOT/PBT weight ratio in the blocks used varied from 55/45 to 80/20. This corresponded to an increase of the swelling ratio Q from 1.06 to 2.46, and of the mesh size xi from approximately 9 Angstrom to approximately 47 Angstrom. With increased swelling, dynamic mechanical analysis (DMA) revealed a decrease in elastic response and an increase of viscoelasticity. Thus, by coupling structural and chemical characteristics, the viscoelastic properties of PEOT/PBT 3DF scaffolds may be fine tuned to achieve mechanical requirements for a variety of engineered tissues. Ultimately, the combination of 3DF and DMA may be useful to validate the hypothesis that mimicking the biomechanical behavior of a specific tissue for its optimal replacement is an important issue for at least some tissue-engineering applications.

The effect of PEGT/PBT scaffold architecture on oxygen gradients in tissue engineered cartilaginous constructs

Repair of articular cartilage defects using tissue engineered constructs composed of a scaffold and cultured autologous cells holds promise for future treatments. However, nutrient limitation (e.g. oxygen) has been suggested as a cause of the onset of chondrogenesis solely within the peripheral boundaries of larger constructs. In the present study, oxygen gradients were evaluated by microelectrode measurements in two porous polyethylene glycol terephthalate/polybutylene terephthalate (PEGT/PBT) scaffold architectures, a compression-molded and particle-leached sponge (CM) and a 3D-deposited fiber (3DF) scaffold. During the first 14 days in vitro, gradients intensified, after which a gradual decrease of the gradients was observed in vitro. In vivo, however, gradients changed instantly and became less pronounced. Although similar gradients were observed regardless of scaffold type, significantly more cells were present in the center of 3DF constructs after 2 weeks of in vivo culture. Our results stress the importance of a rationally designed scaffold for tissue-engineering applications. Organized structures, such as the 3DF PEGT/PBT polymer scaffolds, offer possibilities for regulation of nutrient supply and, therefore, hold promise for clinical approaches for cartilage repair.

Scaffold-based tissue engineering: rationale for computer-aided design and solid free-form fabrication systems

One of the milestones in tissue engineering has been the development of 3D scaffolds that guide cells to form functional tissue. Recently, mouldless manufacturing techniques, known as solid free-form fabrication (SFF), or rapid prototyping, have been successfully used to fabricate complex scaffolds. Similarly, to achieve simultaneous addition of cells during the scaffold fabrication, novel robotic assembly and automated 3D cell encapsulation techniques are being developed. As a result of these technologies, tissue-engineered constructs can be prepared that contain a controlled spatial distribution of cells and growth factors, as well as engineered gradients of scaffold materials with a predicted microstructure. Here, we review the application, advancement and future directions of SFF techniques in the design and creation of scaffolds for use in clinically driven tissue engineering.