The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

Stem cells respond to nanoscale surface features, with changes in cell growth and differentiation mediated by alterations in cell adhesion. The interaction of nanotopographical features with integrin receptors in the cells' focal adhesions alters how the cells adhere to materials surfaces, and defines cell fate through changes in both cell biochemistry and cell morphology. In this Review, we discuss how cell adhesions interact with nanotopography, and we provide insight as to how materials scientists can exploit these interactions to direct stem cell fate and to understand how the behaviour of stem cells in their niche can be controlled. We expect knowledge gained from the study of cell-nanotopography interactions to accelerate the development of next-generation stem cell culture materials and implant interfaces, and to fuel discovery of stem cell therapeutics to support regenerative therapies.

Harnessing nanotopography and integrin–matrix interactions to influence stem cell fate

A cell undergoes many genetic and epigenetic changes as it transitions to malignancy. Malignant transformation is also accompanied by a progressive loss of tissue homeostasis and perturbations in tissue architecture that ultimately culminates in tumor cell invasion into the parenchyma and metastasis to distant organ sites. Increasingly, cancer biologists have begun to recognize that a critical component of this transformation journey involves marked alterations in the mechanical phenotype of the cell and its surrounding microenvironment. These mechanical differences include modifications in cell and tissue structure, adaptive force-induced changes in the environment, altered processing of micromechanical cues encoded in the extracellular matrix (ECM), and cell-directed remodeling of the extracellular stroma. Here, we review critical steps in this “force journey,” including mechanical contributions to tissue dysplasia, invasion of the ECM, and metastasis. We discuss the biophysical basis of this force journey and present recent advances in the measurement of cellular mechanical properties in vitro and in vivo. We end by describing examples of molecular mechanisms through which tumor cells sense, process and respond to mechanical forces in their environment. While our understanding of the mechanical components of tumor growth, survival and motility remains in its infancy, considerable work has already yielded valuable insight into the molecular basis of force-dependent tumor pathophysiology, which offers new directions in cancer chemotherapeutics.

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Mechanics, malignancy, and metastasis: the force journey of a tumor cell.

Xugang Guo,*,† Antonio Facchetti,*,‡,§ and Tobin J. Marks*,‡ †Department of Materials Science and Engineering, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States

Imide- and Amide-Functionalized Polymer Semiconductors

Mechanotransduction research has focused historically on how externally applied forces can affect cell signalling and function. A growing body of evidence suggests that contractile forces that are generated internally by the actomyosin cytoskeleton are also important in regulating cell behaviour, and suggest a broader role for mechanotransduction in biology. Although the molecular basis for these cellular forces in mechanotransduction is being pursued in cell culture, researchers are also beginning to appreciate their contribution to in vivo developmental processes. Here, we examine the role for mechanical forces and contractility in regulating cell and tissue structure and function during development.

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Mechanotransduction in development: a growing role for contractility

Cells respond to mechanical forces whether applied externally or generated internally via the cytoskeleton. To study the cellular response to forces separately, we applied external forces to cells via microfabricated magnetic posts containing cobalt nanowires interspersed among an array of elastomeric posts, which acted as independent sensors to cellular traction forces. A magnetic field induced torque in the nanowires, which deflected the magnetic posts and imparted force to individual adhesions of cells attached to the array. Using this system, we examined the cellular reaction to applied forces and found that applying a step force led to an increase in local focal adhesion size at the site of application but not at nearby nonmagnetic posts. Focal adhesion recruitment was enhanced further when cells were subjected to multiple force actuations within the same time interval. Recording the traction forces in response to such force stimulation revealed two responses: a sudden loss in contractility that occurred within the first minute of stimulation or a gradual decay in contractility over several minutes. For both types of responses, the subcellular distribution of loss in traction forces was not confined to locations near the actuated micropost, nor uniformly across the whole cell, but instead occurred at discrete locations along the cell periphery. Together, these data reveal an important dynamic biological relationship between external and internal forces and demonstrate the utility of this microfabricated system to explore this interaction.

/pdf/magnetic-microposts-as-an-approach-to-apply-forces-to-living-3gioooklo7.pdf

Magnetic microposts as an approach to apply forces to living cells

A highly soluble pyromellitic diimide-based polymer was obtained through imidization polymerization. The novel architecture features diimide subunits linked alternately at 3,6 and N,N′ positions. The polymer is highly transparent in the near-ultraviolet–visible regions. Smooth and uniform thin-films were obtained through spin-coating even after blending the polymer with PCBM in 1:9 polymer/PCBM weight ratio. While the polymer itself has modest electron mobility in typical bottom-gate top-contact OFETs, an electron mobility of 3 × 10–3 cm2 V–1 s–1 was achieved for the blend, which increased to 10–2 cm2 V–1 s–1 on exposure to propylamine. Thus, polyimides are demonstrated as promising binder materials for solution-processable n-channel semiconductor blends, of which very few examples are known.

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Synthesis and Characterization of a Pyromellitic Diimide-Based Polymer with C- and N-Main Chain links: Matrix for Solution-Processable n-Channel Field-effect Transistors

We used neutron reflectivity to complement X-ray reflectivity characterization of PCBM-based layers formed on poly(3-hexylthiophene) (P3HT). Single-layer analyses were used to provide reliable scattering length density values for bilayer fitting. Atomic force microscopy analyses showed trends similar to the reflectivity experiments when observing upper surfaces. Styrene polymers added to PCBM in small concentrations (ca. 10%) led to processing advantages while retaining substantial electron mobility, about 0.001 cm2/V s. The further introduction of a relatively heavy bromo substituent on the styrene rings greatly increased the film smoothness, as revealed by increases of the oscillation amplitudes in the reflectivity. In addition, the bromine heavy atom increased the X-ray reflectivity scattering length density of the upper layer. Finally, we present data consistent with PCBM becoming partially mixed with the P3HT as the PCBM is spin coated from a solution with poly(bromostyrene) to form an overlying film, consistent with predictions based on published phase diagrams of the P3HT-PCBM system.

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Stuart B. Kirschner

Papers

Magnetic microposts as an approach to apply forces to living cells

Synthesis and Characterization of a Pyromellitic Diimide-Based Polymer with C- and N-Main Chain links: Matrix for Solution-Processable n-Channel Field-effect Transistors

X-ray and neutron reflectivity and electronic properties of PCBM-poly(bromo)styrene blends and bilayers with poly(3-hexylthiophene)

Solution processable organic p–n junction bilayer vertical photodiodes