Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa. These materials are playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering and 'smart' optical systems, as well as biosensors, microelectromechanical systems, coatings and textiles. We review recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks. We also provide a critical outline of emerging developments.

Emerging applications of stimuli-responsive polymer materials

We show that nanometer-scale pores in single-layer freestanding graphene can effectively filter NaCl salt from water. Using classical molecular dynamics, we report the desalination performance of such membranes as a function of pore size, chemical functionalization, and applied pressure. Our results indicate that the membrane’s ability to prevent the salt passage depends critically on pore diameter with adequately sized pores allowing for water flow while blocking ions. Further, an investigation into the role of chemical functional groups bonded to the edges of graphene pores suggests that commonly occurring hydroxyl groups can roughly double the water flux thanks to their hydrophilic character. The increase in water flux comes at the expense of less consistent salt rejection performance, which we attribute to the ability of hydroxyl functional groups to substitute for water molecules in the hydration shell of the ions. Overall, our results indicate that the water permeability of this material is several ...

Water desalination across nanoporous graphene

This chapter introduces the finite element method (FEM) as a tool for solution of classical electromagnetic problems. Although we discuss the main points in the application of the finite element method to electromagnetic design, including formulation and implementation, those who seek deeper understanding of the finite element method should consult some of the works listed in the bibliography section.

Introduction to the Finite Element Method

This paper describes the use of surface plasmon resonance (SPR) spectroscopy and self-assembled monolayers (SAMs) to determine the characteristics of functional groups that give surfaces the ability to resist the nonspecific adsorption of proteins from solution. Mixed SAMs presenting different functional groups were prepared for screening using a synthetic protocol based on the reaction of organic amines with a SAM terminated by interchain carboxylic anhydride groups. Surfaces that presented derivatives of oligo(sarcosine), N-acetylpiperazine, and permethylated sorbitol groups were particularly effective in resisting the adsorption of proteins. Incorporation of these groups into single-component SAMs resulted in surfaces that are comparable to (but slightly less good than) single-component SAMs that present oligo(ethylene glycol) in their ability to resist the adsorption of proteins. In the group of surfaces examined, those that resisted the adsorption of proteins had the following properties:  they were ...

A Survey of Structure−Property Relationships of Surfaces that Resist the Adsorption of Protein

Delivery and toxicity are critical issues facing nanomedicine research. Currently, there is limited understanding and connection between the physicochemical properties of a nanomaterial and its interactions with a physiological system. As a result, it remains unclear how to optimally synthesize and chemically modify nanomaterials for in vivo applications. It has been suggested that the physicochemical properties of a nanomaterial after synthesis, known as its “synthetic identity”, are not what a cell encounters in vivo. Adsorption of blood components and interactions with phagocytes can modify the size, aggregation state, and interfacial composition of a nanomaterial, giving it a distinct “biological identity”. Here, we investigate the role of size and surface chemistry in mediating serum protein adsorption to gold nanoparticles and their subsequent uptake by macrophages. Using label-free liquid chromatography tandem mass spectrometry, we find that over 70 different serum proteins are heterogeneously adso...

Nanoparticle size and surface chemistry determine serum protein adsorption and macrophage uptake.

Control of nonspecific protein adsorption is very important for the design of biocompatible and biomimetic materials as well as drug carriers. Grafted polymer layers can be used to prevent protein adsorption. We have studied the molecular factors that determine the equilibrium and kinetic control of protein adsorption by grafted polymer layers. We find that polymers that are not attracted to the surface are very effective for kinetic control but not very good for equilibrium reduction of protein adsorption. Polymers with attractions to the surface show exactly the opposite behavior. The implications for molecular design of biocompatible materials also are discussed in this paper.

https://www.pnas.org/content/pnas/97/16/9037.full.pdf

Kinetic and thermodynamic control of protein adsorption

Tethered Polymer Layers

Tethered polymer layers

The structural and thermodynamic properties of tethered polymer layers formed by spreading diblock copolymers at a solid surface or at a fluid-fluid interface are studied using a molecular mean-field theory. The role of the anchoring block in determining the properties of the tethered polymer layer is studied in detail. It is found that both the anchoring and the tethered blocks are very important in determining the phase behavior of the polymer layer. The structures of the coexisting phases, the phase boundaries and the stability of the layer are found to depend on the ratio of molecular weight between the two blocks, the polymer-interface (surface) interactions and the strength of the interactions between the two blocks. The different phase transitions found are related to experimental observations. The properties of the polymer layers at coexistence reflect the block that is the dominant driving force for phase separation. The ability of the tethered polymer layers, under different conditions, to control protein adsorption to surfaces is also studied. It is found that the most important factors determining the ability of a polymer layer to reduce the equilibrium amount of proteins adsorbed to a surface are the surface coverage of polymer and the surface-polymer interactions. The polymer chain length plays only a secondary role. For the kinetic control, however, it is found that the potential of mean-force, and thus the early stages of adsorption, depends strongly on polymer molecular weight. Further, it is found that the molecular factors determining the ability of the tethered polymer layer to reduce the equilibrium amount of protein adsorption are different than those that control the kinetic behavior. Comparisons with experimental observations are presented. The predictions of the theory are in very good agreement with the measured adsorption isotherms. Guidelines for building optimal surface protection for protein adsorption, both kinetic and thermodynamic, are discussed.

Tethered polymer layers: phase transitions and reduction of protein adsorption

A better understanding of protein aggregation is bound to translate into critical advances in several areas, including the treatment of misfolded protein disorders and the development of self-assembling biomaterials for novel commercial applications. Because of its ubiquity and clinical potential, albumin is one of the best-characterized models in protein aggregation research; but its properties in different conditions are not completely understood. Here, we carried out all-atom molecular dynamics simulations of albumin to understand how electrostatics can affect the conformation of a single albumin molecule just prior to self-assembly. We then analyzed the tertiary structure and solvent accessible surface area of albumin after electrostatically triggered partial denaturation. The data obtained from these single protein simulations allowed us to investigate the effect of electrostatic interactions between two proteins. The results of these simulations suggested that hydrophobic attractions and counterion binding may be strong enough to effectively overcome the electrostatic repulsions between the highly charged monomers. This work contributes to our general understanding of protein aggregation mechanisms, the importance of explicit consideration of free ions in protein solutions, provides critical new insights about the equilibrium conformation of albumin in its partially denatured state at low pH, and may spur significant progress in our efforts to develop biocompatible protein hydrogels driven by electrostatic partial denaturation.

Marcelo A. Carignano

Papers

Kinetic and thermodynamic control of protein adsorption

Tethered Polymer Layers

Tethered polymer layers

Tethered polymer layers: phase transitions and reduction of protein adsorption

Electrostatic Unfolding and Interactions of Albumin Driven by pH Changes: A Molecular Dynamics Study