Computer-aided cartilage tissue-engineering : a numerical evaluation of the influence of inhomogeneities, collagen architecture and temporal culture effects

Hyaline articular cartilage has a crucial role in the distribution of joint mechanical loads and smooth movement of bones. Because of its poor healing capacity, cartilage damage is progressive and may lead to osteoarthritis (OA). Replacing damaged cartilage with tissue engineered (TE) cartilage is promising for the treatment of OA. Despite improvements, however, the inferior mechanical property of today’s TE cartilage is one of the main reasons, which causes unsatisfactory mid- to long-term outcome in clinical applications. Insufficient mechanical stiffness, likely the results of insufficient collagen content in the extracellular matrix (ECM), lack of physiological collagen architecture (i.e. vertical fibers in the deep zone and horizontal fibers in the superficial zone) and inhomogeneous distribution of ECM are among the main different characteristics of current TE cartilage, compared to those of native cartilage. In addition, the production process of TE cartilage requires cells to synthesis ECM within a few weeks. This is quite different compared to the physiological process of native cartilage formation, which is on the order of months/years. It is unknown how such characteristics of TE cartilage may influence its mechanical performance. For a better fundamental understanding of the role of these aspects, the objective of this thesis was to evaluate (1) the influence of cellular-and tissue-scale inhomogeneities in ECM distribution on the mechanical environment of chondrocytes in agarose TE constructs during and at the end of culture, (2) the relative importance of mechanical stiffness, collagen content and collagen network architecture for the post-implantation mechanical performance of TE cartilage, (3) current and newly proposed mechanical stimulations to stimulate formation of a physiological collagen architecture in TE cartilage and (4) the effects of temporal ECM deposition on the mechanical behavior of TE cartilage. A finite element approach with a composition-based non-linear fiber-reinforced poroviscoelastic swelling material model was used. It was shown that inhomogeneities in ECM distributions reduce overall construct stiffness and may significantly alter the tissue-level mechanical environment in the construct as well as the micromechanical environment of chondrocytes at the sites of inhomogeneties, in both free-swelling and under mechanical loading culture conditions. To assess post-implantation mechanical conditions of TE constructs under physiological loading, an axisymmetirc model of a medial tibia plateau was used in which a cylindrical part of the mesh at the central region of the model represented a TE implant. Results indicated that adverse implant composition and ultrastructure would lead to post-implantation excessive mechanical loads, with collagen network architecture being the most critical variable. To explore which mechanical stimulations would be likely to trigger a physiological collagen architecture, strain fields as a result of unconfined compression, and a novel loading regime of sliding indentation with an without lateral tissue compression were evaluated. Results suggested that sliding indentation is likely to stimulate formation of an appropriate superficial zone with parallel fibers. Adding lateral compression may stimulate formation of a deep zone with perpendicularly aligned fibers. Finally, a numerical framework was developed to evaluate the effects of temporal ECM deposition on the mechanical behavior of TE cartilage. The effects of differences in the rate of proteoglycans and collagen synthesis during cartilage TE were quantitatively evaluated. The range of predicted construct stiffness was compared to those reported in the literature. It was shown that alterations in the synthesis rates of proteoglycans and collagen would significantly change the mechanical behavior of TE cartilage. This indicated that even with similar final ECM contents, constructs would have different mechanical properties depending on the history of development. The insights provided in this dissertation should be considered in future cartilage tissue-engineering studies, as they may indicate directions that would result in the development of mechanically superior TE cartilage with enhanced long-term survival. Furthermore, in the present study we showed how numerical simulations can be used to assist in evaluating and designing loading protocols for tissue engineering and provide useful insights for experimental studies to discriminate promising protocols from those with poor potential, which is a step forward towards successful tissue-engineering of cartilage

A multiscale framework for evaluating three-dimensional cell mechanics in fibril-reinforced poroelastic tissues with anatomical cell distribution – Analysis of chondrocyte deformation behavior in mechanically loaded articular cartilage

Characterization of the mechanical environment of cells in collagenous biological tissues during different daily activities is crucial for understanding the role of mechanics on cell biosynthesis and tissue health. However, current imaging methods are limited in characterizing very fast deformations of cells. This could be achieved with computational multiscale modeling, but current models accommodating collagen fibril networks and poroelastic ground matrix have included only a single cell. In this study, a workflow was developed for generating a three-dimensional multiscale model with imaging-based anatomical cell distributions and their micro-environment (pericellular and extracellular matrix). Fibril-reinforced poroelastic material models with (FRPES) and without (FRPE) swelling were implemented into the model and simulations were performed for evaluating cell deformations before and after experimental loading conducted for rabbit knee joint cartilage. We observed that the cells experienced considerably different deformation based on their location in all models. Both FRPE and FRPES models were able to predict the trends in the changes in cell deformations, although the average and median magnitudes differed between the model predictions and experiments. However, the FRPES model results were generally closer to the experimental results. Current findings suggest that morphological properties of cells are affected by the variations in the tissue properties between the samples and variations within the sample caused by the measurement geometry, local structure and composition. Thus, it would be important to consider the anatomical distribution and location of multiple cells along with the structure of fibril networks if cell deformation metrics are evaluated in collagenous tissues. (C) 2020 The Author(s). Published by Elsevier Ltd

Maximum shear strain-based algorithm can predict proteoglycan loss in damaged articular cartilage

Post-traumatic osteoarthritis (PTOA) is a common disease, where the mechanical integrity of articular cartilage is compromised. PTOA can be a result of chondral defects formed due to injurious loading. One of the first changes around defects is proteoglycan depletion. Since there are no methods to restore injured cartilage fully back to its healthy state, preventing the onset and progression of the disease is advisable. However, this is problematic if the disease progression cannot be predicted. Thus, we developed an algorithm to predict proteoglycan loss of injured cartilage by decreasing the fixed charge density (FCD) concentration. We tested several mechanisms based on the local strains or stresses in the tissue for the FCD loss. By choosing the degeneration threshold suggested for inducing chondrocyte apoptosis and cartilage matrix damage, the algorithm driven by the maximum shear strain showed the most substantial FCD losses around the lesion. This is consistent with experimental findings in the literature. We also observed that by using coordinate system-independent strain measures and selecting the degeneration threshold in an ad hoc manner, all the resulting FCD distributions would appear qualitatively similar, i.e., the greatest FCD losses are found at the tissue adjacent to the lesion. The proposed strain-based FCD degeneration algorithm shows a great potential for predicting the progression of PTOA via biomechanical stimuli. This could allow identification of high-risk defects with an increased risk of PTOA progression.</p

Viscoelastic response of cells and the role of actin cytoskeletal remodelling.

PhDThe mechanical properties of living cells provide useful information on cellular structure and function. In the present study a micropipette aspiration technique was developed to investigate the viscoelastic parameters of isolated articular chondrocytes. The Standard Linear Solid (SLS) and the Boltzmann Standard Linear Solid (BSLS) models were used to compute the instantaneous and equilibrium moduli and viscosity based on the response to an aspiration pressure of 7 cm of water. The modulus and viscosity of the chondrocytes increased with decreasing pressure rate. For example, the median equilibrium moduli obtained using the BSLS model increased from 0.19 kPa at 5.48 cmH2O/s to 0.62 kPa at 0.35 cmH2O/s. Cell deformation during micropipette aspiration was associated with an increase in cell volume and remodelling of the cortical actin visualised using GFP-actin. Interestingly, GFP-actin transfection inhibited the increase in cell moduli observed at the slower aspiration rate. Thus actin remodelling appears to be necessary for the pressure rate-dependent behaviour. A hypothesis is proposed explaining the role of actin remodelling and interaction with the membrane in regulating cell mechanics. Further studies investigated a mechanical injury model of cartilage explants which resulted in significant increases in all three viscoelastic parameters. Treatment with IL-1β also increased the instantaneous moduli of cells treated in explants but there was no difference in equilibrium moduli or viscosity. IL-1β treatment in monolayer had no effect on cell mechanics suggesting that previously reported changes in actin associated with IL-1β may be lost during cell isolation or trypsinisation. Separate studies demonstrated increases in chondrocyte moduli and viscosity during passage indicating changes in cell structure-function associated with de-differentiation in monolayer. In conclusion, this study has developed an optimised micropipette aspiration technique which was successfully used to quantify chondrocyte viscoelastic behaviour and to elucidate the underlying role of actin dynamics and response to pathological stimuli and in vitro culture.EPSR

Study of behaviour of Biomechanical System in indented Articular Cartilage on a cellular level

The study of biomechanical systems is of great interest to researches due to diverse applications in the medical sector. This study focuses on design and implementation of a mechanical device, a novel dual axis construct simulator (DACS) for in vitro studies on immortalised chondrocytes. DACS will help with experimental measurements of mechanical properties of Articular Cartilage (AC) on a cellular level and the relationship among cellular, pericellular and extracellular deformation in AC. Details provided in this research mainly focuses on software and hardware development processes and challenges involved. There will a brief introduction about biomechanical behaviour of the cartilage and DACS impact on future studies. This will be followed by description of LCMPilot, an experimental software and challenges involved to develop and control DACS in an automated setup

Biological perspectives and current biofabrication strategies in osteochondral tissue engineering

From Springer Nature via Jisc Publications RouterHistory: received 2019-09-26, accepted 2020-06-29, registration 2020-06-29, pub-electronic 2020-07-09, online 2020-07-09, pub-print 2020-12Publication status: PublishedFunder: Engineering and Physical Sciences Research Council; doi: http://dx.doi.org/10.13039/501100000266; Grant(s): EP/L014904/1Funder: Fundação para a Ciência e a Tecnologia; doi: http://dx.doi.org/10.13039/501100001871; Grant(s): PTDC/MEC-GIN/29232/2017, 0245_IBEROS_1_EAbstract: Articular cartilage and the underlying subchondral bone are crucial in human movement and when damaged through disease or trauma impacts severely on quality of life. Cartilage has a limited regenerative capacity due to its avascular composition and current therapeutic interventions have limited efficacy. With a rapidly ageing population globally, the numbers of patients requiring therapy for osteochondral disorders is rising, leading to increasing pressures on healthcare systems. Research into novel therapies using tissue engineering has become a priority. However, rational design of biomimetic and clinically effective tissue constructs requires basic understanding of osteochondral biological composition, structure, and mechanical properties. Furthermore, consideration of material design, scaffold architecture, and biofabrication strategies, is needed to assist in the development of tissue engineering therapies enabling successful translation into the clinical arena. This review provides a starting point for any researcher investigating tissue engineering for osteochondral applications. An overview of biological properties of osteochondral tissue, current clinical practices, the role of tissue engineering and biofabrication, and key challenges associated with new treatments is provided. Developing precisely engineered tissue constructs with mechanical and phenotypic stability is the goal. Future work should focus on multi-stimulatory environments, long-term studies to determine phenotypic alterations and tissue formation, and the development of novel bioreactor systems that can more accurately resemble the in vivo environment