A family of poly­(glycerol sebacate) (PGS) analogues were synthesized by Candida antarctica lipase B (CALB) catalysis to tailor biomaterial properties. Different fractions of glycerol (G) units in PGS were replaced by 1,8-octanediol (O) units. Poly­(glycerol-1,8-octanediol-sebacate), PGOS, synthesized by CALB catalysis with a 1:3 molar ratio of G to O units has M n and M w values of 9500 and 92,000, respectively. PGS undergoes fiber fusion during electrospinning, and cross-linked PGS rapidly resorbs when implanted. By decreasing the molar ratio of glycerol-to-octanediol from 1:1 to 1:4, the peak melting temperature (T m) increased from 27 to 47 °C. PGOS with 1:3 G to O units was electrospun into nanofibers without the need for a second component. The copolymer is semicrystalline and, when cross-linked, undergoes slow in vitro mass loss (3.5 ± 1.0% in 31 days) at pH 7.4 and 37 °C. Furthermore, PGOS cross-linked films have an elastic modulus of 106.1 ± 18.6 MPa, which is more than 100 times that of cross-linked PGS. New PGOS polymers showed tunable molecular weights, better thermal properties, and excellent electrospinnability. This work expanded PGS analogues’ function, making these suitable biodegradable polymers for various biomedical applications.

Electrospinning is a robust material fabrication method allowing for fine control of mechanical, chemical, and functional properties in scaffold manufacturing. Electrospun fiber scaffolds have gained prominence for their potential in a variety of applications such as tissue engineering and textile manufacturing, yet none have assessed the impact of solvent retention in fibers on the scaffold’s mechanical properties. In this study, we hypothesized that retained electrospinning solvent acts as a plasticizer, and gradual solvent evaporation, by storing fibers in ambient air, will cause significant increases in electrospun fiber scaffold brittleness and stiffness, and a significant decrease in scaffold toughness. Thermogravimetric analysis indicated solvent retention in PGA, PLCL, and PET fibers, and not in PU and PCL fibers. Differential scanning calorimetry revealed that polymers that were electrospun below their glass transition temperature (Tg) retained solvent and polymers electrospun above Tg did not. Young’s moduli increased and yield strain decreased for solventretaining PGA, PLCL, and PET fiber scaffolds as solvent evaporated from the scaffolds over a period of 14 days. Toughness and failure strain decreased for PGA and PET scaffolds as solvent evaporated. No significant differences were observed in the mechanical properties of PU and PCL scaffolds that did not retain solvent. These observations highlight the need to consider solvent retention following electrospinning and its potential effects on scaffold mechanical properties.

Demand for materials that mechanically replicate native tissue has driven development and characterization of various new biomaterials. However, a consequence of materials and characterization technique diversity is a lack of consensus within the field, with no clear way to compare values measured via different modalities. This likely contributes to the difficulty in replicating findings across the research community; recent evidence suggests that different modalities do not yield the same mechanical measurements within a material, and direct comparisons cannot be made across different testing platforms. Herein, we examine whether “material properties” are characterization modality-specific by analyzing the elastic moduli determined by five typical biomaterial mechanical characterization techniques: unconfined-compression, tensiometry, rheometry, and micro-indentation at the macroscopic level, and microscopically using nanoindentation. These analyses were performed in two different polymeric gels frequently used for biological applications, polydimethylsiloxane (PDMS) and agarose. Each was fabricated to span a range of moduli, from physiologic to supraphysiologic values. All five techniques identified the same overall trend within each material group, supporting their ability to appreciate relative moduli differences. However, significant differences were found across modalities, illustrating a difference in absolute moduli values, and thereby precluding direct comparison of measurements from different characterization modalities. These observed differences may depend on material compliance, viscoelasticity, and microstructure. While determining the underlying mechanism(s) of these differences was beyond the scope of this work, these results demonstrate how each modality affects the measured moduli of the same material, and the sensitivity of each modality to changes in sample material composition.

Tendon, ligament, and skeletal muscle are highly-organized tissues that largely rely on a hierarchical collagenous matrix to withstand high tensile loads experienced in activities of daily life. This critical biomechanical role predisposes these tissues to injury, and current treatments fail to recapitulate the biomechanical function of native tissue. This has prompted researchers to pursue engineering functional tissue replacements, or dysfunction/disease/development models, by emulating in vivo stimuli within in vitro tissue engineering platforms; specifically mechanical stimulation, as well as active contraction in skeletal muscle. Mechanical loading is critical for matrix production and organization in the development, maturation, and maintenance of native tendon, ligament, and skeletal muscle, as well as their interfaces. Tissue engineers seek to harness these mechanobiological benefits using bioreactors to apply both static and dynamic mechanical stimulation to tissue constructs, and induce active contraction in engineered skeletal muscle. The vast majority of engineering approaches in these tissues are scaffold-based, providing interim structure and support to engineered constructs, and sufficient integrity to withstand mechanical loading. Alternatively, some recent studies have employed developmentally-inspired scaffold-free techniques, relying on cellular self-assembly and matrix production to form tissue constructs. Whether utilizing a scaffold or not, incorporation of mechanobiological stimuli has been shown to improve the composition, structure, and biomechanical function of engineered tendon, ligament, and skeletal muscle. Together, these findings highlight the importance of mechanobiology and suggest how it can be leveraged to engineer these tissues and their interfaces, and to create functional multi-tissue constructs.

This study examined poly(glycerol-1,8-octanediol-sebacate) (PGOS) copolymers with low-level substitution of O (1,8-octanediol) for G (glycerol) units (G/O ratios 0.

No abstract

Bioreactors are commonly used to apply biophysically-relevant stimulations to tissue-engineered constructs in order to explore how these stimuli influence tissue development, healing, and homeostasis, and they offer great flexibility because key features of the stimuli (e.g., duty cycle, frequency, amplitude, duration) can be controlled to elicit a desired cellular response. However, most bioreactors that apply mechanical and electrical stimulations do so to a scaffold after the construct has developed, preventing study of the influence these stimuli have on early construct development. To enable such exploration, there is a need for a bioreactor that allows the direct application of mechanical and electrical stimulation to constructs as they develop. Herein, we develop and calibrate a bioreactor, based on our previously established modified Flexcell system, to deliver precise mechanical and electrical stimulation, either independently or in combination, to developing scaffold-free tissue constructs. Linear calibration curves were established, then used to apply precise dynamic mechanical and electrical stimulations, over a range of physiologically relevant strains and voltages respectively. Following calibration, applied mechanical and electrical stimulations were not statistically different from their desired target values, and were consistent whether delivered independently or in combination. Concurrent delivery of mechanical and electrical stimulation resulted in a negligible change in mechanical (< 2%) and electrical (<1%) values, compared to their independently-delivered values. With this calibrated bioreactor, we can apply precise, controlled, reproducible mechanical and electrical stimulations, alone or in combination, to scaffold-free, tissue engineered constructs as they develop.

Bioreactors are commonly used to apply biophysically-relevant stimulations to tissue-engineered constructs in order to explore how these stimuli influence tissue development, healing, and homeostasis. These bioreactors offer great flexibility as key features of the stimuli (e.g., duty cycle, frequency, amplitude, duration) can be controlled to elicit a desired cellular response. Controlled delivery of mechanical and/or electrical stimulation has been shown to improve the structure and function of engineered tissue constructs, compared to unstimulated controls, for applications in various musculoskeletal soft tissues. However, most bioreactors that apply mechanical and electrical stimulations, do so to a scaffold after the construct has developed, preventing study of the influence these stimuli have on early construct development. Thus, there is a need for a bioreactor that allows the direct application of mechanical and electrical stimulation to constructs as they develop, to enable such exploration and to better mimic key aspects of tissue development. Hence, the objective of this study was to develop and calibrate a bioreactor to deliver precise mechanical and electrical stimulation, either independently or in combination, to developing scaffold-free tissue constructs. Standard Flexcell Tissue Train plates were modified with stainless steel loading pins and stimulating electrodes to integrate direct mechanical and electrical stimulation, respectively, into our established scaffold-free, single-fiber engineering platform. Linear calibration curves were established, then used to apply precise dynamic mechanical and electrical stimulations, over a range of physiologically relevant strains (0.50, 0.70, 0.75, 1.0, 1.5%) and voltages (1.5, 3.5 V), respectively. Once calibrated, applied mechanical and electrical stimulations were not statistically different from their desired target values, and were consistent whether delivered independently or in combination. Indeed, concurrent delivery of mechanical and electrical stimulation resulted in a negligible change in mechanical (< 2%) and electrical (<1%) values, from their independently-delivered values. With this calibrated bioreactor, we can apply precise, controlled, reproducible mechanical and electrical stimulations, alone or in combination, to scaffold-free, tissue engineered constructs as they develop, to explore how these stimuli can be leveraged to advance and accelerate functional tissue engineering in a variety of musculoskeletal soft tissues.IMPACT STATEMENTAs the importance of biophysical stimulation in tissue engineering continues to be recognized and incorporated, this bioreactor provides a platform to further our understanding of the roles independent or combined mechanical and electrical stimulation have in tissue development and functional maturation, and may inform future tissue engineering approaches for clinical applications.