Soft nanotechnology involves both understanding the behavior of soft matter and using these components to build useful nanoscale structures and devices. However, molecular scale properties such as Brownian motion, diffusion, surface forces, and conformational flexibility dominate the chemistry and physics in soft nanotechnology, and therefore the design rules for generating functional structures from soft, self-assembled materials are still developing. Biological motors illustrate how wet nanoscale machines differ from their macroscopic counterparts. These molecular machines convert chemical energy into mechanical motion through an isothermal process: chemical reactions generate chemical potential and diffusion of ions, leading to conformational changes in proteins and the production of mechanical force. Because the actuation steps form a thermodynamic cycle that is reversible, the application of mechanical forces can also generate a chemical potential. This reverse process of mechanotransduction is the underlying sensing and signaling mechanism for a wide range of physiological phenomena such as hearing, touch, and growth of bone. Many of the biological systems that respond to mechanical stimuli do this via complex stress-activated ion channels or remodeling of the actin cytoskeleton. These biological actuation and mechanosensing processes are rather different from nano- and microelectromechanical systems (NEMS and MEMS) produced via semiconductor fabrication technologies. In our group, we are working to emulate biological mechanotransduction by systematically developing building blocks based on polymer brushes. In this soft nanotechnology approach to mechanotransduction, the chemical building blocks are polymer chains whose conformational changes and actuation can be investigated at a very basic level in polymer brushes, particularly polyelectrolyte brushes. Because these polymer brushes are easily accessible synthetically with control over parameters such as composition, chain length, and chain density, brushes provide a robust platform to study the coupling of mechanical forces with conformational changes of the chains. This Account provides an overview of our recent research in the design of mechanosensitive polymer brushes starting with the demonstration of nanoactuators and leading to our first attempts toward the creation of artificial mechanotransduction elements. As the brushes collapse in response to external triggers such as pH and ion concentration, polyelectrolyte brushes provide stimuli-responsive films. These collapse transitions lead to the generation of mechanical forces, and by reversing the chain of events, we designed a mechanically responsive film with a chemical output. Having reported an initial proof-of-principle experiment, we think that the stage is set for the preparation of more elaborate mechanosensitive surfaces.

Gentle touch: Mechanoresponsive polyelectrolyte brushes show a strong correlation between local fluorescence intensity and local pressures (see picture; PDMS=polydimethylsiloxane). The response of the surface to mechanical stimuli was completely reversible and provided a sensitivity under 10 kPa, similar to human skin.

Thin hydrogel films of the thermoresponsive polymer poly(N‐isopropylacrylamide) (pNIPAm) were prepared by electrochemically triggered reversible addition‐fragmentation chain transfer (RAFT) polymerization. Two different RAFT agents were employed, which work in either acidic or basic solution. In both cases, addition of RAFT agents had an influence on the thickness and the surface morphology of the films. At low concentration, the polymerization efficiency increased. At high concentration, the efficiency decreased at acidic pH, while it remained constant under basic conditions. Neither RAFT agent displayed electrochemical activity on its own, but they did modify the electrochemical behavior of the initiator. The addition of RAFT agent strongly enhances the homogeneity of the hydrogel surfaces, which presumably is due to a reduced amount of microgel formation.

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Strong cationic poly­(2-(methacryloyloxy)­ethyltrimethylammonium chloride) (PMETAC) brushes are widely employed as platform for studying fundamental physicochemical properties of permanently charged polymers at interfaces. We report here a detailed analysis of the structure, interfacial and bulk charging of PMETAC brushes over a broad range of pH values and salt concentrations (pH 2.5–9.5, 0.01–10 mM KCl electrolyte). Streaming current and surface conductivity measurements were quantitatively analyzed on the basis of a recently developed theory for electrohydrodynamics at diffuse soft interfaces under lateral flow conditions taking into account ion pairing at low salt concentrations. In addition, the effects of different chaotropic anions on brush charge and thickness were deciphered from interpretation of surface conductivity data collected in 1 mM KNO3, KI, and KClO4 electrolytes. In combination, confrontation between theory and experiments reveals the existence of a PMETAC segment density gradient at the brush/solution interphase that is independent of pH and KCl concentrations above 0.1 mM. With decreasing salt content at pH 6 from 0.1 mM to 0.01 mM, the corresponding nonmonotonous variation of streaming current indicates electrostatically driven interfacial swelling of the brush, while the accompanied variation in surface conductivity reveals a dramatic decrease in the net density of structural charges within the bulk material. The latter feature is assigned to the occurrence of pairing between chloride anions from background electrolyte and quaternary ammonium groups supported by PMETAC chains. The decrease of the net charge density in the bulk of the brush is shown to be more pronounced with increasing anion hydrophobicity, i.e., upon promoting the ion-pairing process that subsequently leads to a shrinking of the brush volume. The study illustrates the complementary information derived from surface conductivity and streaming current analysis: the former reflects electrohydrodynamic processes in the bulk of the brush, whereas the latter primarily mirrors processes at the polymer/solution interphase. It further demonstrates that electrokinetic and surface conductivity are valuable tools for demonstrating and monitoring ion-pairing processes useful to reversibly tune macroscopic properties such as brush swelling or stiffness.