Background:Intra-articular injections of autologous platelet-rich plasma (PRP) have been shown to reduce joint pain in many patients with mild to moderate knee osteoarthritis (OA) (PRP responders). However, for unclear reasons, 20% to 30% of patients experience little to no improvement in symptoms (PRP nonresponders). Understanding the mechanisms underlying PRP's action is crucial to address this limitation. In vitro and animal studies suggest that the benefits of PRP in knee OA are partly attributable to exosomes secreted by platelets and other cells. Purpose/Hypothesis:This study aimed to (1) determine the effects of intra-articular PRP injections in patients with symptomatic knee OA on pain and physical function, and (2) evaluate whether PRP exosome concentration and size were associated with therapeutic efficacy. Responsiveness to PRP therapy in patients with knee OA may be related to exosome concentration and size. Study Design:Case-control study; Level of evidence, 3. Methods:A total of 60 patients seeking nonsurgical treatment of symptomatic mild to moderate knee OA-Kellgren-Lawrence (KL) grades 1 to 3-were prospectively enrolled in the study. Baseline descriptive and health data were collected, including patient-reported pain and functional outcomes at baseline, 6 weeks, and 12 weeks after injection. PRP responders were defined as patients with ≥20% reduction in the Numerical Rating Scale (NRS) of joint pain at 12 weeks after treatment. Exosome size and concentration and platelet concentration were measured in each sample. Logistic regression was used to explore relationships between responder status and patient factors-age, body mass index (BMI), sex, and KL grade. The mean exosome size and concentration were also compared between responders and nonresponders using an independent t-test. Results:Of the 58 patients included in the analyses, 43 (73%) reported a 20% reduction in NRS scores. The mean exosome size among a subset of patients who reported unacceptable pain symptoms before treatment (n = 43) was 80.8 ± 2.7 nm for responders and 78.0 ± 1.5 nm for nonresponders, a statistically significant difference (P = .005). There was no significant difference between responders and nonresponders in terms of platelet or exosome concentration. Age, BMI, sex, and KL grade were not significantly associated with the odds of PRP treatment response (all P > .05). Conclusion:PRP-derived exosome size appears to be associated with improvements in pain at 12 weeks after injection.

The activation of chondrogenic progenitor cells (CPCs) in articular cartilage during a traumatic injury is vital for cartilage regeneration. Although our understanding of the mechanisms underlying CPC chondrogenic activation remains incomplete, there is evidence that exosomal microRNAs (miRNAs or miRs) are involved in tissue healing due to their regulating role of posttranscriptional gene expressions. In this study, we profiled enriched and differential expression of miRNAs in exosomes derived from bovine joint cells (CPCs, chondrocytes, and synoviocytes) via Next Generation Sequencing analysis and validated the potential therapeutic effects of candidate exosomal miRNAs for cartilage regeneration. For CPC-based cartilage regeneration, we tested the impact of administering miR-107, miR-140, and miR-148a on CPCs because we found that these miRNAs were highly and differentially expressed in chondrocytes-derived exosomes (CC-Exo). We found that: (1) miR-140 induced chondrogenic gene expression including SRY-box transcription factor 9, collagen type 2A1, and aggrecan, and (2) miR-107 suppressed catabolic gene expression including matrix metalloproteinase 3, a disintegrin and metalloproteinase with thrombospondin motifs 5, and nitric oxide synthase 2. Our findings indicate that transfection of CPCs with specific chondrogenic miRNAs present in CC-Exo have the potential to promote CPC-based cartilage regeneration and could be an important component of posttraumatic osteoarthritis prevention. Impact Statement Chondrocytes, chondrogenic progenitor cells (CPCs), and synoviocytes secrete exosomal microRNAs (miRNAs) that contribute to joint health and disease. These miRNAs could also have important implications for improving cartilage repair and regeneration. In this study, we identified candidate miRNAs that were enriched in chondrocytes-derived exosomes and found that these miRNAs induced chondrogenic gene expression or suppressed catabolic gene expression in a CPC monolayer culture system. These findings suggest that miRNA-based cartilage repair strategies could be developed to regenerate damaged and diseased cartilage.

Tissue repair is often impaired in pathological states, highlighting the need for innovative wound-healing technologies. This study introduces composite hyaluronic acid gas-entrapping materials (GEMs) delivering carbon monoxide (CO) to promote wound healing in pigs. These composite materials facilitate burst release followed by sustained release of CO over 48 h. In a porcine full-thickness wound model, CO-GEMs significantly accelerated wound closure compared to the standard-of-care dressing (Tegaderm). Wound area closure with CO-GEMs was 68.6% vs 56.8% on day 14, 41.0% vs 25.1% on day 28, and 26.9% vs 11.8% on day 42, effectively reducing healing time by 14 days. Histological analysis revealed increased epithelialization and neovascularization with reduced inflammation. These findings demonstrate the potential of CO-GEMs as a topical therapeutic to enhance tissue repair in clinically relevant models, supporting further testing for wound-healing applications.

Arthrofibrosis is defined as the excessive accumulation of connective tissue in and around joints, which interferes with the range of motion required for activities of daily living. Although joint stiffness can be restored by surgical interventions such as adhesion lysis, arthroscopic debridement, and capsular release, arthrofibrosis tends to redevelop in the months following the surgery. Thus, there is a critical and urgent need to develop a non-invasive, pharmacological therapy to prevent or resolve arthrofibrosis. A subclass of small extracellular vesicles called exosomes convey bioactive regulators like micro ribonucleic acids (miRNAs/miRs), which can function as anti- and pro-fibrotic agents. Currently, there is no research on miRNA-based therapeutic potentials for treating arthrofibrosis. Previous research and clinical observations on fibrosis across organ systems suggest that there are commonalities in pathogenic mechanisms that can be targeted in arthrofibrosis therapy. In this study, we collated and critically analyzed the existing literature on exosomal miRNAs in organ fibrosis to discover potential candidates for diagnosing, preventing, and/or treating arthrofibrosis. Fifty-six articles were finally selected and categorized by anti- and pro-fibrotic candidates of miRNAs. Notably, let-7, miR-26, miR-29, miR-146, miR-148/-152, miR-214, miR-223, and miR-21 emerged as prominent candidates that should be investigated further for effectiveness in arthrofibrosis therapy.

Pulmonary fibrosis is characterized by excessive deposition of extracellular matrix (ECM), stiffening of the lung tissue, and impaired gas exchange. Our current understanding of fibrogenesis generally focuses on the individual roles of mechanical and biochemical stimuli in driving disease progression. However, many mechano-chemical pathways are interrelated, so dissecting the interactive effects of mechanical and biochemical signals is an important knowledge gap. To address this gap, we investigated lung fibroblast behavior on static and cyclically strained photopolymerizable hydrogels consisting of different ratios of methacrylated gelatin, methacrylated hyaluronan, and non-methacrylated gelatin to create substrates with tunable stiffness and chemistry, representative of both healthy and fibrotic lung ECM properties. We observed that higher stiffness gels amplified the impact of strain, resulting in distinct differences in expression of MMP1, CTGF, Rho/ROCK, and ECM deposition genes. Substrates with hyaluronan demonstrated a capacity to modulate strain-induced fibrogenic responses, suggesting a buffering effect of hyaluronan on fibrotic disease progression. Overall, our results highlight mechanotransductive changes in gene expression in response to substrate composition, stiffness, and cyclic mechanical strain. Through the controlled study of mechanical and biochemical cues, our findings contribute to a deeper understanding of the pathogenesis of pulmonary fibrosis.

The human body represents a collection of interacting systems that range in scale from nanometers to meters. Investigations from a systems perspective focus on how the parts work together to enact changes across spatial scales, and further our understanding of how systems function and fail. Here, we highlight systems approaches presented at the 2022 Summer Biomechanics, Bio-engineering, and Biotransport Conference in the areas of solid mechanics; fluid mechanics; tissue and cellular engineering; biotransport; and design, dynamics, and rehabilitation; and biomechanics education. Systems approaches are yielding new insights into human biology by leveraging state-of-the-art tools, which could ultimately lead to more informed design of therapies and medical devices for preventing and treating disease as well as rehabilitating patients using strategies that are uniquely optimized for each patient. Educational approaches can also be designed to foster a foundation of systems-level thinking.

Skin undergoes mechanical alterations due to changes in the composition and structure of the collagenous dermis with aging. Previous studies have conflicting findings, with both increased and decreased stiffness reported for aging skin. The underlying structure-function relationships that drive age-related changes are complex and difficult to study individually. One potential contributor to these variations is the accumulation of nonenzymatic crosslinks within collagen fibers, which affect dermal collagen remodeling and mechanical properties. Specifically, these crosslinks make individual fibers stiffer in their plastic loading region and lead to increased fragmentation of the collagenous network. To better understand the influence of these changes, we investigated the impact of nonenzymatic crosslink changes on the dermal microstructure using discrete fiber networks representative of the dermal microstructure. Our findings suggest that stiffening the plastic region of collagen's mechanical response has minimal effects on network-level stiffness and failure stresses. Conversely, simulating fragmentation through a loss of connectivity substantially reduces network stiffness and failure stress, while increasing stretch ratios at failure.

Adipose tissue plays a crucial role in metabolic syndrome, autoimmune diseases, and many cancers. Because of adipose’s role in so many aspects of human health, there is a critical need for in vitro models that replicate adipose architecture and function. Traditional monolayer models, despite their convenience, are limited, showing heterogeneity and functional differences compared to 3D models. While monolayer cultures struggle with detachment and inefficient differentiation, healthy adipocytes in 3D culture accumulate large lipid droplets, secrete adiponectin, and produce low levels of inflammatory cytokines. The shift from monolayer models to more complex 3D models aims to better replicate the physiology of healthy adipose tissue in culture. This study introduces a simple and accessible protocol for generating adipose organoids using a scaffold-free spheroid model. The method, utilizing either 96-well spheroid plates or agarose micromolds, demonstrates increased throughput, uniformity, and ease of handling compared to previous techniques. This protocol allows for diverse applications, including drug testing, toxin screening, tissue engineering, and co-culturing. The choice between the two methods depends on the experimental goals, with the 96-well plate providing individualized control and the micromold offering scale advantages. The outlined protocol covers isolation, expansion, and characterization of stromal vascular fraction cells, followed by detailed steps for spheroid formation and optional downstream analyses.

The mechanical properties of skin change during aging but the relationships between structure and mechanical function remain poorly understood. Previous work has shown that young skin exhibits a substantial decrease in tissue volume, a large macro-scale Poisson's ratio, and an increase in micro-scale collagen fiber alignment during mechanical stretch. In this study, label-free multiphoton microscopy was used to quantify how the microstructure and fiber kinematics of aged mouse skin affect its mechanical function. In an unloaded state, aged skin was found to have less collagen alignment and more non-enzymatic collagen fiber crosslinks. Skin samples were then loaded in uniaxial tension and aged skin exhibited a lower mechanical stiffness compared to young skin. Aged tissue also demonstrated less volume reduction and a lower macro-scale Poisson's ratio at 10% uniaxial strain, but not at 20% strain. The magnitude of 3D fiber realignment in the direction of loading was not different between age groups, and the amount of realignment in young and aged skin was less than expected based on theoretical fiber kinematics affine to the local deformation. These findings provide key insights on how the collagen fiber microstructure changes with age, and how those changes affect the mechanical function of skin, findings which may help guide wound healing or anti-aging treatments.

Elbow trauma can lead to joint contracture and reduced range of motion (ROM). Nonsurgical interventions can improve ROM, but in some cases capsule release surgery is required. Although surgery can improve ROM, it often does not restore full ROM. Thus, alternatives are needed. One approach is to target activated myofibroblasts, which are commonly associated with fibrotic tissue. Mechanical and biochemical cues drive a feedback loop that can result in normal or pathological healing. We hypothesize that this feedback loop exists in joint contracture and can be manipulated so that myofibroblast activity is reduced, normal healing is achieved, and ROM is improved. We previously demonstrated that blebbistatin can inhibit myofibroblast contractile forces and reduce collagen synthesis in vitro. Thus, the purpose of this study was to assess the use of blebbistatin in an animal model of elbow contracture, which was induced in 7 groups of 4 rats each (n = 28). All elbows were mechanically and histologically tested. The uninjured contralateral elbows of each rat were used as a control group. Capsule release surgery significantly improved (p < 0.01) outcomes 1 week after surgery compared to injury alone and was not significantly different from uninjured elbows. Three weeks after surgery, outcomes worsened, indicating joint stiffening consistent with what is observed clinically. The addition of blebbistatin did not significantly improve outcomes. Future work will investigate relationships among treatment, fibrotic tissue deposition, myofibroblast activity, and biomechanics to determine if blebbistatin is a useful adjunctive therapy for treating joint contracture.

Scarring from traumatic injury, burns, and other complications remains a significant problem that diminishes quality of life for millions of people worldwide. A common target for the development of new therapies to promote healing and reduce scarring are myofibroblasts because of their central role in pathological scarring. Recent work indicates that adipocyte lineage cells also contribute to the wound healing process, including clinical reports that indicate that the placement of autologous adipose micrografts at the surgical site improves the appearance and pliability of existing scars. To better understand how adipocyte lineage cells interact with fibroblasts to promote healing, we first utilized an in vitro model of wound healing to visualize fibroblast spheroid collagen deposition via time-lapse imaging. We then introduced pre-adipocyte and adipocyte spheroids to visualize pair-wise spheroid interactions and collagen deposition among all three cell types. Finally, we quantified differences in the extracellular matrix (ECM) proteins produced using liquid chromatography with tandem mass spectrometry (LC-MS/MS). We found that all three cell-types contribute to ECM deposition and that the composition of the ECM proteins, or matrisome, was significantly different depending on which cells were co-cultured together. By better understanding the interactions among these cell types, novel adipose-tissue-based therapeutic approaches can be developed to improve wound healing and reduce scar tissue.

Adipocytes regulate tissues through production of adipokines that can act both locally and systemically. Adipocytes also have been found to play a critical role in regulating the healing process. To better understand this role, we developed a three-dimensional human adipocyte spheroid system that has an adipokine profile similar to in vivo adipose tissues. Previously, we found that conditioned medium from these spheroids induces human dermal fibroblast conversion into highly contractile, collagen-producing myofibroblasts through a transforming growth factor beta-1 (TGF-β1) independent pathway. Here, we sought to identify how mature adipocytes signal to dermal fibroblasts through adipokines to induce myofibroblast conversion. By using molecular weight fractionation, heat inactivation and lipid depletion, we determined mature adipocytes secrete a factor that is 30-100 kDa, heat labile and lipid associated that induces myofibroblast conversion. We also show that the depletion of the adipokine adiponectin, which fits those physico-chemical parameters, eliminates the ability of adipocyte-conditioned media to induce fibroblast to myofibroblast conversion. Interestingly, native adiponectin secreted by cultured adipocytes consistently elicited a stronger level of α-smooth muscle actin expression than exogenously added adiponectin. Thus, adiponectin secreted by mature adipocytes induces fibroblast to myofibroblast conversion and may lead to a phenotype of myofibroblasts distinct from TGF-β1-induced myofibroblasts.

In this special issue, we commemorate the near-decade-long service of JBME coeditors Drs. Beth Winkelstein and Victor Barocas. Both Beth and Victor, each ASME and Biomedical Engineering Society fellows, have been leaders in the field of biomechanics for decades. Beth has been a pioneer in the field of injury biomechanics, in particular at its intersection with spine and neck pain. Victor has made significant contributions to soft tissue biomechanics across a variety of biological systems, particularly in terms of computational multiscale mechanical models of fiber networks. They have each received numerous awards and accolades from national and international organizations for the quality of their work, but perhaps their greatest impact has been through their mentorship and support of the next generation of scientists and engineers, both in their own research laboratories and through their many service roles. As two former trainees of Beth and Victor, we each have tremendous gratitude for the mentorship and guidance that they have provided us. Along with dozens of their current and former trainees, our growth and career successes are a product of the time and energy that they have invested in us over the years.Beth and Victor have also had a transformative impact on JBME over the last decade. One of Beth and Victor's major accomplishments during their leadership of the journal was the expansion and diversification of the editorial board, which by the end of their tenure resulted in one of the most diverse, if not the most diverse, editorial boards in the ASME journal suite. Importantly, the board was diverse in many dimensions, with individuals from a wide range of both lived experiences and scientific interests. A diverse and engaged editorial board is now a major strength for the journal, and was a key part of Beth and Victor's success as editors. It is now part of their legacy as JBME continues to maintain an outstanding team of associate editors.Recognizing the editorial board as one of Beth and Victor's lasting contributions to JBME, this special issue includes invited papers by nine past Associate Editors who served their entire terms during Beth and Victor's editorship. As one might expect, the work is highly varied in topic but of uniformly high quality, spanning many different research areas that are within the scope of JBME. We can group this collection broadly into four themes: (1) Soft-tissue mechanics at moderate speed, (2) high-speed mechanics and injury, (3) vascular flows and fluid–structure interactions, and (4) the nervous system.On the topic of soft-tissue mechanics, Roberge et al. (Corr) performed an elegant and thorough study of the biomechanics of alginate microbeads, which are important as drug delivery and tissue engineering tools. Roberge shows that microbead mechanics can be quite different from bulk alginate mechanics, and they discuss the importance of this difference in microbead design. Vena et al. (Vena) combined classical bulge testing with confocal microscopy to measure the mechanical properties of bovine pericardium, including tissue anisotropy as demonstrated by the different principal curvatures of the bulge. Although the two papers deal with different length scales and different materials, they both demonstrate innovative experiments and their combination with theory that are hallmarks of JBME.Moving onto biomechanics at higher speeds, two papers address important foundational and applied problems in injury biomechanics. Siefert et al. (Stemper) performed characterization studies of soft tissues in the thorax at a high strain rate to provide important properties critically needed for accurate finite element modeling of penetrating injuries. This work not only provides valuable datasets but highlights important rate-dependent effects when compared to prior work. Miller et al. (Stitzel) uses continuous data collection during real-life exposures to define acceleration and vibration profiles of NASCAR drivers. Both of these studies advance important foundational knowledge about human injury and help refine modeling approaches of these events.Turning to vascular flows, three different papers address different challenging aspects of cardiovascular biofluid dynamics. Pham et al. (Marsden) demonstrate a novel tool—svMorph—to construct patient-specific model geometries for blood flow simulations, continuing the community's drive to increase the accessibility and efficiency of vascular flow simulations. Kim et al. (Figueroa) use fluid–structure interaction models to explore the hemodynamic consequences of an aortic dissection. The work demonstrates just how sophisticated computational biofluid-solid tools have become, accounting for the motion of the dissection flap during blood flow and exploring the outflow vessel obstruction that may result. Finally, Ponnaluri et al. (Manning) provide a thorough in vitro examination of the dynamics of transcatheter aortic valve replacements, demonstrating significant flaws with the standard testing techniques and highlighting the need for tight control on testing protocols for replacement heart valves.Finally, the role of the nervous system in biomechanics is the focus of two papers—one modeling GABAergic neurons and their roles in adjusting vascular resistance and the other defining the median nerve's spatial relationship to the transverse carpal ligament. David et al. present a neurovascular coupling model incorporating both somato-sensory neurons and GABAergic interneurons and investigate the role of nitric oxide in such systems. Li et al. utilize ultrasound techniques to define the three-dimensional geometry of the median nerve and its relationship to the ligament within the carpal tunnel, providing helpful information for those modeling this joint and its relationship to pain, and also validating two-dimensional approaches. Both studies are highly relevant to biomechanical investigations and provide meaningful insights for important physiological systems.In conclusion, we note that these papers are just a sample of the topics covered by JBME, both during Beth and Victor's editorship and continuing today under the editorship of Ross Ethier and Vicky Nguyen. Biomechanical engineering remains a vibrant and evolving field, but the essential features of JBME—scientific rigor, intellectual thoroughness, creativity, and a commitment to the improvement of public health—do not change. We thank Victor and Beth, the associate editors who contributed to this special issue, as well as those on the editorial board who were not able to contribute, for their dedication to the journal and its ideals.

IntroductionWith technical advances, confocal and super-resolution microscopy have become powerful tools to dissect cellular pathophysiology. Cell attachment to glass surfaces compatible with advanced imaging is critical prerequisite but remains a considerable challenge for human beta cells. Recently, Phelps et al. reported that human beta cells plated on type IV collagen (Col IV) and cultured in neuronal medium preserve beta cell characteristics. MethodsWe examined human islet cells plated on two commercial sources of Col IV (C6745 and C5533) and type V collagen (Col V) for differences in cell morphology by confocal microscopy and secretory function by glucose-stimulated insulin secretion (GSIS). Collagens were authenticated by mass spectrometry and fluorescent collagen-binding adhesion protein CNA35. ResultsAll three preparations allowed attachment of beta cells with high nuclear localization of NKX6.1, indicating a well-differentiated status. All collagen preparations supported robust GSIS. However, the morphology of islet cells differed between the 3 preparations. C5533 showed preferable features as an imaging platform with the greatest cell spread and limited stacking of cells followed by Col V and C6745. A significant difference in attachment behavior of C6745 was attributed to the low collagen contents of this preparation indicating importance of authentication of coating material. Human islet cells plated on C5533 showed dynamic changes in mitochondria and lipid droplets (LDs) in response to an uncoupling agent 2-[2-[4-(trifluoromethoxy)phenyl]hydrazinylidene]-propanedinitrile (FCCP) or high glucose + oleic acid. DiscussionAn authenticated preparation of Col IV provides a simple platform to apply advanced imaging for studies of human islet cell function and morphology.

Tissue fibrosis is a major health issue that impacts millions of people and is costly to treat. However, few effective anti-fibrotic treatments are available. Due to their central role in fibrotic tissue deposition, fibroblasts and myofibroblasts are the target of many therapeutic strategies centered primarily on either inducing apoptosis or blocking mechanical or biochemical stimulation that leads to excessive collagen production. Part of the development of these drugs for clinical use involves in vitro prescreening. 2D screens, however, are not ideal for discovering mechanobiologically significant compounds that impact functions like force generation and other cell activities related to tissue remodeling that are highly dependent on the conditions of the microenvironment. Thus, higher fidelity models are needed to better simulate in vivo conditions and relate drug activity to quantifiable functional outcomes. To provide guidance on effective drug dosing strategies for mechanoresponsive drugs, we describe a custom force-bioreactor that uses a fibroblast-seeded fibrin gels as a relatively simple mimic of the provisional matrix of a healing wound. As cells generate traction forces, the volume of the gel reduces, and a calibrated and embedded Nitinol wire deflects in proportion to the generated forces over the course of 6 days while overhead images of the gel are acquired hourly. This system is a useful in vitro tool for quantifying myofibroblast dose-dependent responses to candidate biomolecules, such as blebbistatin. Administration of 50 μM blebbistatin reliably reduced fibroblast force generation approximately 40% and lasted at least 40 h, which in turn resulted in qualitatively less collagen production as determined via fluorescent labeling of collagen.

Skin is a complex tissue whose biomechanical properties are generally understood in terms of an incompressible material whose microstructure undergoes affine deformations. A growing number of experiments, however, have demonstrated that skin has a high Poisson's ratio, substantially decreases in volume during uniaxial tensile loading, and demonstrates collagen fiber kinematics that are not affine with local deformation. In order to better understand the mechanical basis for these properties, we constructed multiscale mechanical models (MSM) of mouse skin based on microstructural multiphoton microscopy imaging of the dermal microstructure acquired during mechanical testing. Three models that spanned the cases of highly aligned, moderately aligned, and nearly random fiber networks were examined and compared to the data acquired from uniaxially stretched skin. Our results demonstrate that MSMs consisting of networks of matched fiber organization can predict the biomechanical behavior of mouse skin, including the large decrease in tissue volume and nonaffine fiber kinematics observed under uniaxial tension.