Search

Mechanical stimulation (mechanotherapy) can promote skeletal muscle repair, but a lack of reproducible protocols and mechanistic understanding of the relation between mechanical cues and tissue regeneration limit progress in this field. To address these gaps, we developed a robotic device equipped with real-time force control and compatible with ultrasound imaging for tissue strain analysis. We investigated the hypothesis that specific mechanical loading improves tissue repair by modulating inflammatory responses that regulate skeletal muscle regeneration. We report that cyclic compressive loading within a specific range of forces substantially improves functional recovery of severely injured muscle in mice. This improvement is attributable in part to rapid clearance of neutrophil populations and neutrophil-mediated factors, which otherwise may impede myogenesis. Insights from this work will help advance therapeutic strategies for tissue regeneration broadly.

Substrate stiffness has been recognized as an important regulator of cell fate and function, but an understanding of the full extent of processes affected by stiffness is lacking as its transcriptome-wide effects have not been mapped. This limited understanding has restricted the contexts in which engineers can employ stiffness as an engineering design parameter. To address these limitations, we performed RNA-seq on mesenchymal stem cells (MSCs) cultured in alginate hydrogels over a range of moduli to broadly map the transcriptome-wide changes associated with stiffness sensing. We found a large number of stiffness-sensitive genes, and that many genes respond to stiffness in nonlinear ways. Informed by these differential expression results, we explored a hypothesis related to current MSC clinical activity, and found that stiffness can regulate the expression of MSC immunomodulatory markers in response to cytokine stimulation. Overall, these results reveal previously unknown features of MSC stiffness response and demonstrate the value of coupling –omics approaches with biophysical experiments.

Variations in a multitude of material microenvironmental properties have been observed across tissues in vivo, and these have profound effects on cell phenotype. Phenomenological experiments have suggested that certain of these features of the physical microenvironment, such as stiffness, could sensitize cells to other features; meanwhile, mechanistic studies have detailed a number of biophysical mechanisms for this sensing. However, the broad molecular consequences of these potentially complex and nonlinear interactions bridging from biophysical sensing to phenotype have not been systematically characterized, limiting the overall understanding and rational deployment of these biophysical cues. Here, we explore these interactions by employing a 3D cell culture system that allows for the independent control of culture substrate stiffness, stress relaxation, and adhesion ligand density to systematically explore the transcriptional programs affected by distinct combinations of biophysical parameters using RNA-seq. In mouse mesenchymal stem cells and human cortical neuron progenitors, we find dramatic coupling among these substrate properties, and that the relative contribution of each property to changes in gene expression varies with cell type. Motivated by the bioinformatic analysis, the stiffness of hydrogels encapsulating mouse mesenchymal stem cells was found to regulate the secretion of a wide range of cytokines, and to accordingly influence hematopoietic stem cell differentiation in a Transwell coculture model. These results give insights into how biophysical features are integrated by cells across distinct tissues and offer strategies to synthetic biologists and bioengineers for designing responses to a cell’s biophysical environment.

Natural extracellular matrices (ECMs) are viscoelastic and exhibit stress relaxation. However, hydrogels used as synthetic ECMs for three-dimensional (3D) culture are typically elastic. Here, we report a materials approach to tune the rate of stress relaxation of hydrogels for 3D culture, independently of the hydrogel's initial elastic modulus, degradation, and cell-adhesion-ligand density. We find that cell spreading, proliferation, and osteogenic differentiation of mesenchymal stem cells (MSCs) are all enhanced in cells cultured in gels with faster relaxation. Strikingly, MSCs form a mineralized, collagen-1-rich matrix similar to bone in rapidly relaxing hydrogels with an initial elastic modulus of 17 kPa. We also show that the effects of stress relaxation are mediated by adhesion-ligand binding, actomyosin contractility and mechanical clustering of adhesion ligands. Our findings highlight stress relaxation as a key characteristic of cell-ECM interactions and as an important design parameter of biomaterials for cell culture.

The effectiveness of stem-cell therapies has been hampered by cell death and limited control over fate1. These problems can be partially circumvented by using macroporous biomaterials that improve the survival of transplanted stem cells and provide molecular cues to direct cell phenotype2–4. Stem cell behavior can also be controlled in vitro by manipulating the elasticity of both porous and non-porous materials5–7, yet translation to therapeutic processes in vivo remains elusive. Here, by developing injectable, void-forming hydrogels that decouple pore formation from elasticity, we show that mesenchymal stem cell (MSC) osteogenesis in vitro, and cell deployment in vitro and in vivo, can be controlled by modifying, respectively, the hydrogel's elastic modulus or its chemistry. When the hydrogels were used to transplant MSCs, the hydrogel's elasticity regulated bone regeneration, with optimal bone formation at 60 kPa. Our findings show that biophysical cues can be harnessed to direct therapeutic stem-cell behaviors in situ.

Biological processes are exquisitely well controlled on a spatial and temporal scale, and this has driven interest in drug delivery devices that can be altered on-demand to adapt the release profile in real time. However, systems designed to release payload in response to extracorporeal or environmental cues typically exhibit considerable leakiness in drug release. We hypothesized that a more complete On/Off switch could be achieved with physical entrapment of nanoparticles within hydrogels, exploiting steric hindrance to reduce baseline release, and that the microarchitecture of the system could be reversibly adapted using ultrasound to enable switchable release. To test this, the release of PEGylated gold-nanoparticles from ionically crosslinked alginate hydrogels was first examined and demonstrated a dramatic increase in release rate in response to ultrasound. Bone morphogenetic protein-2 (BMP-2) conjugated gold nanoparticles could also be released from hydrogels with ultrasound, and maintained bioactivity following alginate encapsulation and ultrasound release. This approach to increasing control over local bioagent delivery should afford researchers and clinicians the ability to mimic and drive natural temporal responses. Natural biological processes (e.g., embryological development, bone generation and angiogenesis) are intricately controlled in the temporal and spatial domain, and systems that enable this type of signaling control could provide powerful research and clinical tools. One successful strategy to obtain spatial control is polymer-based drug delivery, as these allow local delivery at a specific anatomic site. These delivery systems are engineered to exhibit temporal control by sustaining release of bioagents over a defined period.[1] Despite the success and clinical translation of some of these strategies, the advantages of more precise release initiation or intermittent release profiles is becoming clear both in pathologic[2, 3] and tissue engineering applications.[4] In addition, the majority of monolithic polymeric systems exhibit an initial burst release.[5] A high initial drug concentration may be undesirable, and may also be wasteful as this coincides with the timing of the initial inflammatory response – a potentially harsh environment. Systems that can be instructed to deliver their payload on-demand are favorable in many situations. For example, delayed delivery of BMP-2 can enhance fracture healing, when compared with immediate delivery.[6] Furthermore, increased control of a delivery system may allow a reduction in the bioagent payload, which could improve safety while reducing cost. Drug delivery devices can alter the drug release rate by taking information from their environment (e.g., temperature, pH)[7] or from non-invasive, externally modulated energy sources such as heat[8], magnetic[9], electrical[10], light[11] or by wirelessly communicating with implanted microchips.[12] Ultrasound, which is commonly employed in the clinic for diagnostic and therapeutic purposes, has previously been demonstrated to accelerate release of bioactive agents from biomaterials.[13, 14] These systems typically alter their structure permanently (i.e., ultrasound destruction of the material), which results in a more permanent increase in release rate. However, inspired by sonophoresis[15], self-healing ionically crosslinked alginate hydrogels that return to a baseline release rate following the removal of the ultrasound stimulus were recently demonstrated.[2] A common limitation of all these systems is that, similar to most polymeric controlled drug delivery strategies, there can be relatively high baseline release rate from the material. There are many reports of responsive nanoparticles[16] that can respond to stimuli such as those listed above or that are embedded within matrices to effect a change on the matrix, which in turn releases a drug payload; however, we are unaware of reports that specifically deliver bioactive nanoparticles in response to a stimulus. This project was based on the hypothesis that incorporation of nanoparticles into an ultrasound responsive hydrogel would largely eliminate baseline release due to steric hindrance, and that release of the nanoparticles could be triggered in response to ultrasound. The pore size of alginate hydrogels is typically in the range of several nm[17], which was expected to lead to physical entrapment of nanoparticles larger than 10 nm. This system can additionally exploit the favorable physicochemical properties of nanoparticles, including their ability to co-deliver agents and their ability to enhance bioactivity.[18, 19] This approach could also overcome the challenge of localizing nanoparticles at defect sites, as the hydrogel depot can be physically placed in the desired anatomic location. The first aim of the study was to explore, using a model nanoparticle, the release rate of gold nanoparticles (AuNPs) in response to ultrasound. Next, BMP-2 was selected as a model therapeutic due to its clinical use and prior demonstrations of its enhanced efficacy when delivered in a delayed manner in a femoral fracture critical sized defect model in rats.[6] BMP-2 was conjugated to the gold nanoparticles and the ability of these particles to be released from the hydrogels in response to ultrasound, in a bioactive form, was analyzed in vitro.

A covalently crosslinked methacrylated (MA)-alginate cryogel vaccine has been previously shown to generate a potent response against murine melanoma, but is not mechanically robust and requires a large 16G needle for delivery. Here, covalent and ionic crosslinking of cryogels are combined with the hypothesis that this will result in a tough MA-alginate cryogel with improved injectability. All tough cryogels can be injected through a smaller, 18G needle without sustaining any damage, while covalently crosslinked-only cryogels break after injection. Cytosine-phosphodiester-guanine (CpG)-delivering tough cryogels effectively activate dendritic cells (DCs). Granulocyte macrophage colony-stimulating factor releasing tough cryogels recruit four times more DCs than blank gels by day 7 in vivo. The tough cryogel vaccine induces strong antigen-specific cytotoxic T-lymphocyte and humoral responses. These vaccines prevent tumor formation in 80% of mice inoculated with HER2/neu-overexpressing DD breast cancer cells. The MA-alginate tough cryogels provide a promising minimally invasive delivery platform for cancer vaccinations.

Biomaterials have dramatically increased in functionality and complexity, allowing unprecedented control over the cells that interact with them. From these engineering advances arises the prospect of improved biomaterial-based therapies, yet practical constraints favour simplicity. Tools from the biology community are enabling high-resolution and high-throughput bioassays that, if incorporated into a biomaterial design framework, could help achieve unprecedented functionality while minimizing the complexity of designs by identifying the most important material parameters and biological outputs. However, to avoid data explosions and to effectively match the information content of an assay with the goal of the experiment, material screens and bioassays must be arranged in specific ways. By borrowing methods to design experiments and workflows from the bioprocess engineering community, we outline a framework for the incorporation of next-generation bioassays into biomaterials design to effectively optimize function while minimizing complexity. This framework can inspire biomaterials designs that maximize functionality and translatability.

In lymph nodes, fibroblastic reticular cells (FRCs) form a collagen-based reticular network that supports migratory dendritic cells (DCs) and T cells and transports lymph. A hallmark of FRCs is their propensity to contract collagen, yet this function is poorly understood. Here we demonstrate that podoplanin (PDPN) regulates actomyosin contractility in FRCs. Under resting conditions, when FRCs are unlikely to encounter mature DCs expressing the PDPN receptor CLEC-2, PDPN endowed FRCs with contractile function and exerted tension within the reticulum. Upon inflammation, CLEC-2 on mature DCs potently attenuated PDPN-mediated contractility, which resulted in FRC relaxation and reduced tissue stiffness. Disrupting PDPN function altered the homeostasis and spacing of FRCs and T cells, which resulted in an expanded reticular network and enhanced immunity.

Although hydrogels now see widespread use in a host of applications, low fracture toughness and brittleness have limited their more broad use. As a recently described interpenetrating network (IPN) of alginate and polyacrylamide demonstrated a fracture toughness of ≈ 9000 J/m(2), we sought to explore the biocompatibility and maintenance of mechanical properties of these hydrogels in cell culture and in vivo conditions. These hydrogels can sustain a compressive strain of over 90% with minimal loss of Young's Modulus as well as minimal swelling for up to 50 days of soaking in culture conditions. Mouse mesenchymal stem cells exposed to the IPN gel-conditioned media maintain high viability, and although cells exposed to conditioned media demonstrate slight reductions in proliferation and metabolic activity (WST assay), these effects are abrogated in a dose-dependent manner. Implantation of these IPN hydrogels into subcutaneous tissue of rats for 8 weeks led to mild fibrotic encapsulation and minimal inflammatory response. These results suggest the further exploration of extremely tough alginate/PAAM IPN hydrogels as biomaterials.

The rate of stress relaxation of adhesion substrates potently regulates cell fate and function in vitro, and in this study the authors test whether it can regulate bone formation in vivo by implanting alginate gels with differing rates of stress-relaxation carrying human mesenchymal stem cells into rat calvarial defects. After three months, the rats that received fast-relaxing hydrogels (t1/2 ≈ 50 s) show significantly more new bone growth than those that received slow-relaxing, stiffness-matched hydrogels. Strikingly, substantial bone regeneration results from rapidly relaxing hydrogels even in the absence of transplanted cells. Histological analysis reveals that the new bone formed with rapidly relaxing hydrogels is mature and accompanied by extensive matrix remodeling and hydrogel disappearance. This tissue invasion is found to be prominent after just two weeks and the ability of stress relaxation to modulate cell invasion is confirmed with in vitro analysis. These results suggest that substrate stress relaxation can mediate scaffold remodeling and thus tissue formation, giving tissue engineers a new parameter for optimizing bone regeneration.

Tactical asset allocation (TAA) has evolved over the last two decades from a purely domestic, single-decision strategy to a complex, global, multistrategy approach known as “global TAA” or “global macro.” It produces uncorrelated alpha and is scalable. As a consequence, global macro is attracting pension funds and other institutional assets in increasing numbers.

This article asks whether it is possible to diversify investments through time—a hotly debated topic, and one that divides academics and practitioners perhaps like no other financial paradigm. In the late 1960s, Samuelson first argued strongly against the existence of time diversification. Since then, other academics have raised a theoretical challenge to the notion of time diversification. On the other hand, empirical evidence has hardly supported their view. To date, the question still remains wide open: Does time diversification exist? If so, what are the policy implications for asset allocation? This article addresses the gap between theory and empirical evidence by reexamining these issues using a time series framework. Three time series models are fitted with historical data on S&P 500 Index and T-bill and T-bond returns over the period 1802–2002. Although both T-bill and T-bond returns are found to evolve in different serial correlated stationary processes, stock returns are consistent with a mean-reversion process. The change in relative riskiness of stock over bill and bond returns as a result of different time diversification effects strongly supports the conventional recommendation on asset allocation policy with different investment horizons.

Selecting an investment manager is a matter of choosing the manager who can produce the best numbers in the future. But, the same set of numbers drawn from the past can often present two very different pictures. Changing the benchmark, changing the fiscal year, risk-adjusting the performance, can all make a bad product look good or a good product look bad. The quest for a single, simple measure of performance often leads to an overly simplistic view of the past, which can lead to poor choices for the future.

The choice between active and passive management is an important one, but often made on a foundation of false assumptions and flawed logic. A strong case can be made, at any time, for either. The basis for a sound choice is surprisingly simple. Ultimately, it hinges on views that 1) markets are efficient or inefficient, 2) some managers are able to identify inefficiencies in advance, or they are not, and 3) investors can identify the successful managers in advance, or they cannot. Recent past results are a very dangerous basis for this important choice. An array of questions and issues should be addressed in the choice between active and passive management, providing a roadmap for a sensible decision.

Studies of cellular mechanotransduction have converged upon the idea that cells sense extracellular matrix (ECM) elasticity by gauging resistance to the traction forces they exert on the ECM. However, these studies typically utilize purely elastic materials as substrates, whereas physiological ECMs are viscoelastic, and exhibit stress relaxation, so that cellular traction forces exerted by cells remodel the ECM. Here we investigate the influence of ECM stress relaxation on cell behaviour through computational modelling and cellular experiments. Surprisingly, both our computational model and experiments find that spreading for cells cultured on soft substrates that exhibit stress relaxation is greater than cells spreading on elastic substrates of the same modulus, but similar to that of cells spreading on stiffer elastic substrates. These findings challenge the current view of how cells sense and respond to the ECM.

The authors examine the effect of European Monetary Union on the opportunities for active managers to add value. EMU clearly reduces the opportunities for currency and fixed-income managers, but the loss of opportunity is minimized because of the high correlations of currency returns and fixed-income returns. The direction of EMU’s impact on equity managers is less clear. A reduction of barriers to trade should result in a more singular economy, thus reducing differences between country equity returns; however, free trade may also result in greater industry concentrations within specific countries.

A method of fabricating horizontally-aligned zinc-oxide (ZnO) nanowire (NW) arrays with full control over the width and length is demonstrated. SEM images reveal the hexagonal structure typical of zinc oxide NWs. Arrays of high-aspect ratio horizontal ZnO NWs are fabricated by making use of the lateral overgrowth from dot patterns created by electron beam lithography (EBL). An array of patterned wires are lifted off and transferred to a flexible PDMS substrate with possible applications in several key nanotechnology areas.

Biomolecular and physical cues of the extracellular matrix environment regulate collective cell dynamics and tissue patterning. Nonetheless, how the viscoelastic properties of the matrix regulate collective cell spatial and temporal organization is not fully understood. Here we show that the passive viscoelastic properties of the matrix encapsulating a spheroidal tissue of breast epithelial cells guide tissue proliferation in space and in time. Matrix viscoelasticity prompts symmetry breaking of the spheroid, leading to the formation of invading finger-like protrusions, YAP nuclear translocation and epithelial-to-mesenchymal transition both in vitro and in vivo in a Arp2/3-complex-dependent manner. Computational modelling of these observations allows us to establish a phase diagram relating morphological stability with matrix viscoelasticity, tissue viscosity, cell motility and cell division rate, which is experimentally validated by biochemical assays and in vitro experiments with an intestinal organoid. Altogether, this work highlights the role of stress relaxation mechanisms in tissue growth dynamics, a fundamental process in morphogenesis and oncogenesis.