Self-powered microscale pumps based on analyte-initiated depolymerization reactions

Polymer multilayer films regulate macroscopic fluid flow and power microfluidic devices via supramolecular interactions

Self-powered supramolecular micropumps could potentially provide a solution for powerless microfluidic devices where the fluid flow can be manipulated via modulating non-covalent interactions. An attempt has been made to fabricate thin-film-based micropumps by depositing a β-cyclodextrin ('host') functionalized polymer on a glass slide via layer-by-layer assembly. These supramolecular micropumps turned on the fluid flow upon addition of 'guest' molecules to the multilayer films. The flow velocity was tuned using the concentration of the guest molecules as well as the number of host layers inside the multilayer films. Numerical modelling reveals that the solutal buoyancy, which originates from host-guest complexation, is primarily responsible for the fluid flow. In view of its potential application in self-powered devices, the thin-film-based micropump was integrated into a microfluidic device to show molecular and colloidal transport over long distances.

A self-cleaning surface based on UV-activatable, AgCl micropumps for bacterial killing and removal

We report a self-cleaning, bacterial killing surface by immobilization of AgCl microparticles on a surface, acting as chemical micropumps. The surface shows a high bacterial killing efficacy of attached bacteria and exhibits sustainable removal of bacteria as a result of UV-activatable micropumping originating from the photocatalytic reaction of AgCl microparticles. Our work provides an advance in the sustainable use of bacterial contact-killing surfaces stricto sensu through removal of dead bacteria and debris that may shield contact-killing sites.

From radial to unidirectional water pumping in zeta-potential modulated Nafion nanostructures

Chemically propelled micropumps are promising wireless systems to autonomously drive fluid flows for many applications. However, many of these systems are activated by nocuous chemical fuels, cannot operate at high salt concentrations, or have difficulty for controlling flow directionality. In this work we report on a self-driven polymer micropump fueled by salt which can trigger both radial and unidirectional fluid flows. The micropump is based on the cation-exchanger Nafion, which produces chemical gradients and local electric fields capable to trigger interfacial electroosmotic flows. Unidirectional pumping is predicted by simulations and achieved experimentally by nanostructuring Nafion into microarrays with a fine tune modulation of surrounding surface zeta potentials. Nafion micropumps work in a wide range of salt concentrations, are reusable, and can be fueled by different salt cations. We demonstrate that they work with the common water-contaminant cadmium, using the own capture of this ion as fuel to drive fluid pumping. Thus, this system has potential for efficient and fast water purification strategies for environmental remediation. Unidirectional Nafion pumps also hold promise for effective analyte delivery or preconcentration for (bio)sensing assays.

Chemically propelled micropumps are wireless fluid flow driving systems with many potential applications. Here, the authors report a self-driven reusable Nafion micropump fueled by different salt cations in a wide range of concentrations that triggers both radial and unidirectional flows, showing efficient water remediation capabilities.

Synthesis and depolymerization of self-immolative poly(disulfide)s with saturated aliphatic backbones

Self-immolative polymers (SIPs) are a class of degradable stimuli-responsive polymers, which, upon removal of labile end-caps, depolymerize selectively and stepwise to small molecules.

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

Fluid actuation and buoyancy driven oscillation by enzyme-immobilized microfluidic microcapsules

Mimicking microorganism's locomotion and actuation under fluid is difficult to realize. To better comprehend the motility in non-living matter, self-propelled synthetic systems are being developed as a fast-growing area of research. Inspired by the self-powered enzyme micropumps where the enzyme catalysis was harnessed to create motion, herein, enzyme-immobilized microfluidic microcapsules (MCs) were used as a microscale engine to maneuver the fluid flow. The fluid actuation was tuned by various parameters such as substrate concentration, reaction rate, diameter of MCs and the population of the MCs inside the flow chamber. The same MCs, when suspended in a solution, showed buoyancy driven motility by creating oxygen bubbles via an enzymatic reaction and the velocity of the MCs was directly dependent on the number of nucleated oxygen bubbles generated on the MC surface.

Pillar[5]arene microcapsules turn on fluid flow in the presence of paraquat

We report the fabrication of pillar[5]arene (P[5]A) stabilized MCs via the self-assembly and crosslinking of P[5]A nanoaggregates at the liquid–liquid interface. These P[5]A MCs microengines turn on fluid flow in the presence of paraquat (PQ) due to host–guest molecular recognition.

Enhanced Light-Harvesting Efficiency and Adaptation: A Review on Visible-Light-Driven Micro/Nanomotors

As visible light accounts for a larger proportion of solar energy and is harmless to living organisms, it has the potential to be the energy source of micro/nanomotors, which transform visible-light energy into mechanical motion, for different applications, especially in environmental remediation. However, how to precisely control the motion of visible-light-driven micro/nanomotors (VLD-MNMs) and efficiently utilize the weak visible-light photon energy to acquire rapid motion are significant challenges. This review summarizes the most critical aspects, involving photoactive materials, propulsion mechanisms, control methods, and applications of VLD-MNMs, and discusses strategies to systematically enhance the energy-harvesting efficiency and adaptation. At first, the photoactive materials have been divided into inorganic and organic photoactive materials and comprehensively discussed. Then, different propulsion mechanisms of the current VLD-MNMs are presented to explain the improvement in the actuation force, speed, and environmental adaptability. In addition, considering the characteristics of easy control of VLD-MNMs, we summarized the direction, speed, and cluster control methods of VLD-MNMs for different application requirements. Subsequently, the potential applications of VLD-MNMs, e.g., in environmental remediation, micropumps, cargo delivery, and sensing in microscale, are presented. Finally, discussions and suggestions for future directions to enhance the energy-harvesting efficiency and adaptation of VLD-MNMs are provided.

Depolymerization of Bottlebrush Polypentenamers and Their Macromolecular Metamorphosis

The depolymerization of bottlebrush (BB) polymers with varying lengths of polycyclopentene (PCP) backbone and polystyrene (PS) grafts is investigated. In all cases, ring closing metathesis (RCM) depolymerization of the PCP BB backbone appears to occur through an end-to-end depolymerization mechanism as evidenced by size exclusion chromatography. Investigation on the RCM depolymerization of linear PCP reveals a more random chain degradation process. Quantitative depolymerization occurs under thermodynamic conditions (higher temperature and dilution) that drives RCM into cyclopentenes (CPs), each bearing one of the original PS grafts from the BB. Catalyst screening reveals Grubbs' third (G3) and second (G2) generation catalyst depolymerize BBs significantly faster than Grubbs' first generation (G1) and Hoveyda-Grubbs' second generation (HG2) catalyst under identical conditions while solvent (toluene versus CHCl₃) plays a less significant role. The length of the BB backbone and PS side chains also play a minor role in depolymerization kinetics, which is discussed. The ability to completely deconstruct these BB architectures into linear grafts provides definitive insights toward the ATRP "grafting-from" mechanism originally used to construct the BBs. Core-shell BB block copolymers (BBCPs) are shown to quantitatively depolymerize into linear diblock polymer grafts. Finally, the complete depolymerization of BBs into α-cyclopentenyl-PS allows further transformation to other architectures, such as 3-arm stars, through thiol-ene coupling onto the CP end group. These unique materials open the door to stimuli-responsive reassembly of BBs and BBCPs into new morphologies driven by macromolecular metamorphosis.

Motile Micropump Based on Synthetic Micromotors for Dynamic Micropatterning

Micropump systems show great potential on the micropatterning process as a result of remarkable performance and functionality. However, existing micropumps cannot be employed as direct writing tools to perform the complex micropatterning process because of their lacking motility and controllability. Here, we propose a motile micropump system based on the combination of a water-driven ZnO/Ni/polystyrene Janus micromotor with a traditional immobilized micropump. This novel motile micropump system can translate the trajectory of Janus micromotors into predefined micropatterns by pumping away passive silica particles around the micromotor under the effect of diffusiophoresis. The resolution and efficiency of the micropatterning process can be regulated by controlling the diameters of Janus micromotors. Diverse surface micropatterns can be fabricated though remote magnetic control of the motile micropump system. Such ability to transform the versatile motile micropump into predetermined surface micropatterns creates new opportunities for mask-free micropatterning.

Self-Amplified Depolymerization of Oligo(thiourethanes) for the Release of COS/H2S

Herein we report the self-amplified depolymerization of an aryl oligo(thiourethane) (OTU) for the release of COS/H₂S. The OTU was synthesized via polyaddition of 4-isothiocyanatobenzyl alcohol and end-capped with an aryl azide. The aryl azide chain-end was reduced by tris(2-carboxyethyl)phosphine or H₂S to the corresponding aniline, resulting in depolymerization (i.e., self-immolation) and the release of COS/H₂S. Depolymerization was monitored by ¹H NMR and UV-Vis spectroscopy, and the released COS was converted into H₂S by the ubiquitous enzyme carbonic anhydrase in aqueous media.

Functionalizable, Side Chain-Immolative Poly(benzyl ether)s

Herein, we report a poly(benzyl ether)-based self-immolative polymer (SIP) with pendant pyridine disulfide groups. Cleavage of the side-chain disulfides leads to the formation of phenolates, which initiate depolymerization from the side chain. Due to the higher density of the disulfide groups compared to that of the chain-end-capping group, which normally is responsible for initiating depolymerization of SIPs, the side chain-immolative polymer (ScIP) can be readily degraded in the solid state where the mobility of polymer chains is substantially limited. The ScIP was also further modified through the thiol-disulfide exchange reaction to prepare ScIP-g-poly(ethylene glycol) graft polymers and organogels, which were also able to undergo complete reductive self-immolative degradation.

Optimal Sr-Doped Free TiO2@SrTiO3 Heterostructured Nanowire Arrays for High-Efficiency Self-Powered Photoelectrochemical UV Photodetector Applications

Due to their high performance, photoelectrochemical ultraviolet (UV) photodetectors have attracted much attention, but the recombination of photogenerated electrons at the interface of photoanode/electrolyte limited further improvement of photoelectrochemical UV photodetectors (PEC UVPDs). Modification of TiO2 photoanode by SrTiO3 could improve the performance of UVPD, because the energy barrier that is established at the TiO2–SrTiO3 interface could accelerate the separation of the photogenerated electrons-holes pair. However, the recombination center that is caused by the preparation of TiO2@SrTiO3 core-shell heterostructured nanostructure decreases the performance of PEC UVPDs, which is still an important problem that hindered its application in PEC UVPDs. In this paper, we presented a Sr-doped free TiO2@SrTiO3 core-shell heterostructured nanowire arrays as a photoanode for the self-powered PEC UVPD. This will not only accelerate the separation of the photogenerated electrons-holes pair, but it will also reduce the recombination of photogenerated electron-hole pairs in the photoanode. The intrinsic effect of SrTiO3 reaction time on the J variations of UVPDs is investigated in detail. An impressive responsivity of 0.358 A W−1 was achieved at 360 nm for the UVPD based on TiO2@SrTiO3 core-shell heterostructured nanowire arrays, which heretofore is a considerably high photoresponsivity for self-powered photoelectrochemical UVPDs. Additionally, this UVPD also exhibits a high on/off ratio, fast response time, excellent visible-blind characteristic, and linear optical signal response.

Modulating the Depolymerization of Self-Immolative Brush Polymers with Poly(benzyl ether) Backbones

We have synthesized a series of stimuli-responsive brush polymers by grafting azide-terminated side chains onto a self-immolative, alkyne-bearing poly(benzyl ether) backbone, which is prepared by anionic polymerization of quinone methide-based monomers. Upon exposure to a decapping reagent (Pd(0) or F^-), these brush polymers undergo an irreversible degradation cascade from head to tail to yield individual side chains. It is observed that several factors affect the depolymerization kinetics, including solvent polarity, type of counterion, the rate of the decapping chemistry, and interestingly, the rigidity of the side chains.

Self-immolative polymers with potent and selective antibacterial activity by hydrophilic side chain grafting

Self-immolative polymers, which exert potent antibacterial activity with low hemolytic toxicity to red blood cells, are triggered to unzip into small molecules by a chemical stimulus.

Tuning the hydrophobic cores of self-immolative polyglyoxylate assemblies

Amphiphilic block copolymers containing different self-immolative polyglyoxylates were synthesized and self-assembled to provide drug carriers with variable celecoxib loading capacities and release rates, as well as different in vitro toxicities.

Biomimetic antimicrobial polymers: recent advances in molecular design

The increasing prevalence of antibiotic-resistant bacterial infections, coupled with the decline in the number of new antibiotic drug approvals, has created a therapeutic gap that portends an emergent public health crisis.

Three-Dimensional Bacterial Behavior near Dynamic Surfaces Formed by Degradable Polymers

Understanding the behavior of bacteria near biodegradable surfaces is critical for the development of biomedical and antibiofouling materials. By using digital holographic microscopy (DHM), we investigated the three-dimensional (3D) behavior of Escherichia coli and Pseudomonas sp. in lipase-containing aquatic environments near dynamic surfaces constructed by biodegradable poly(ε-caprolactone) (PCL)-based polymers in real time. As the enzymatic degradation rate increases, the percentage of near-surface subdiffusive bacteria and consequently, the irreversible adhesion decreases. Atomic force microscopy (AFM) measurements reveal that the adhesion force between bacteria and the surfaces decreases with an increasing degradation rate. In addition, the degradation products elicit a negative chemotactic response in E. coli, further driving them away from the dynamic surfaces through more frequent tumbling motion. Our study clearly demonstrates that bacterial adhesion can be reduced on dynamic surfaces formed by degradable polymers.

A new function for thermal phase transition-based polymer actuators: autonomous motion on a surface of constant temperature

It is very challenging to make materials capable of autonomous oscillation known in many living systems (such as the heartbeat). Herein, we describe an approach to creating a thermo-mechano-thermal feedback loop for thermal phase transition-based polymer actuators, which leads to hour-long, autonomous motion on a substrate surface of constant temperature. We investigated the variables that determine the amplitude and period of the motion, and demonstrated exemplary physical work powered by direct thermomechanical energy conversion. Such continuous motion of a solid polymer driven by thermal energy without the need for temperature up/down switching is unprecedented, and the validated feedback loop can be implemented into other thermal phase transition-based polymer actuators.

Microfluidic pumping by micromolar salt concentrations

An ion-exchange-resin-based microfluidic pump is introduced that utilizes trace amounts of ions to generate fluid flows. We show experimentally that our pump operates in almost deionized water for periods exceeding 24 h and induces fluid flows of μm s^-1 over hundreds of μm. This flow displays a far-field, power-law decay which is characteristic of two-dimensional (2D) flow when the system is strongly confined and of three-dimensional (3D) flow when it is not. Using theory and numerical calculations we demonstrate that our observations are consistent with electroosmotic pumping driven by μmol L^-1 ion concentrations in the sample cell that serve as 'fuel' to the pump. Our study thus reveals that trace amounts of charge carriers can produce surprisingly strong fluid flows; an insight that should benefit the design of a new class of microfluidic pumps that operate at very low fuel concentrations.

Origins of concentration gradients for diffusiophoresis

Fluid transport that is driven by gradients of pressure, gravity, or electro-magnetic potential is well-known and studied in many fields. A subtler type of transport, called diffusiophoresis, occurs in a gradient of chemical concentration, either electrolyte or non-electrolyte. Diffusiophoresis works by driving a slip velocity at the fluid-solid interface. Although the mechanism is well-known, the diffusiophoresis mechanism is often considered to be an esoteric laboratory phenomenon. However, in this article we show that concentration gradients can develop in a surprisingly wide variety of physical phenomena - imposed gradients, asymmetric reactions, dissolution, crystallization, evaporation, mixing, sedimentation, and others - so that diffusiophoresis is in fact a very common transport mechanism, in both natural and artificial systems. We anticipate that in georeservoir extractions, physiological systems, drying operations, laboratory and industrial separations, crystallization operations, membrane processes, and many other situations, diffusiophoresis is already occurring - often without being recognized - and that opportunities exist for designing this transport to great advantage.

Chemistry pumps: a review of chemically powered micropumps

Lab-on-a-chip devices have over recent years attracted a significant amount of attention in both the academic circle and industry, due to their promise in delivering versatile functionalities with high throughput and low sample amount. Typically, mechanical or electrokinetic micropumps are used in the majority of lab-on-a-chip devices that require powered fluid flow, but the technical challenges and the requirement of external power associated with these pumping devices hinder further development and miniaturization of lab-on-a-chip devices. Self-powered micropumps, especially those powered by chemical reactions, have been recently designed and can potentially address some of these issues. In this review article, we provide a detailed introduction to four types of chemically powered micropumps, with particular focus on their respective structures, operating mechanisms and practical usefulness as well as limitations. We then discuss the various functionalities and controllability demonstrated by these micropumps, ending with a brief discussion of how they can be improved in the future. Due to the absence of external power sources, versatile activation methods and sensitivity to environmental cues, chemically powered micropumps could find potential applications in a wide range of lab-on-a-chip devices.

Art, auto-mechanics, and supramolecular chemistry. A merging of hobbies and career

While the strict definition of supramolecular chemistry is "chemistry beyond the molecule", meaning having a focus on non-covalent interactions, the field is primarily associated with the creation of synthetic receptors and self-assembly. For synthetic ease, the receptors and assemblies routinely possess a high degree of symmetry, which lends them an aspect of aesthetic beauty. Pictures of electron orbitals similarly can be seen as akin to works of art. This similarity was an early draw for me to the fields of supramolecular chemistry and molecular orbital theory, because I grew up in a household filled with art. In addition to art, my childhood was filled with repairing and constructing mechanical entities, such as internal combustion motors, where many components work together to achieve a function. Analogously, the field of supramolecular chemistry creates systems of high complexity that achieve functions or perform tasks. Therefore, in retrospect a career in supramolecular chemistry appears to be simply an extension of childhood hobbies involving art and auto-mechanics.

Hyperbranched Self-Immolative Polymers (hSIPs) for Programmed Payload Delivery and Ultrasensitive Detection

Upon stimuli-triggered single cleavage of capping moieties at the focal point and chain terminal, self-immolative dendrimers (SIDs) and linear self-immolative polymers (l-SIPs) undergo spontaneous domino-like radial fragmentation and cascade head-to-tail depolymerization, respectively. The nature of response selectivity and signal amplification has rendered them a unique type of stimuli-responsive materials. Moreover, novel design principles are required for further advancement in the field of self-immolative polymers (SIPs). Herein, we report the facile fabrication of water-dispersible SIPs with a new chain topology, hyperbranched self-immolative polymers (hSIPs), by utilizing one-pot AB2 polycondensation methodology and sequential postfunctionalization. The modular engineering of three categories of branching scaffolds, three types of stimuli-cleavable capping moieties at the focal point, and seven different types of peripheral functional groups and polymeric building blocks affords both structurally and functionally diverse hSIPs with chemically tunable amplified-release features. On the basis of the hSIP platform, we explored myriad functions including visible light-triggered intracellular release of peripheral conjugated drugs in a targeted and spatiotemporally controlled fashion, intracellular delivery and cytoplasmic reductive milieu-triggered plasmid DNA release via on/off multivalency switching, mitochondria-targeted fluorescent sensing of H2O2 with a detection limit down to ∼20 nM, and colorimetric H2O2 assay via triggered dispersion of gold nanoparticle aggregates. To further demonstrate the potency and generality of the hSIP platform, we further configure it into biosensor design for the ultrasensitive detection of pathologically relevant antigens (e.g., human carcinoembryonic antigen) by integrating with enzyme-mediated cycle amplification with positive feedback and enzyme-linked immunosorbent assay (ELISA).

Synthetic Nano- and Micromachines in Analytical Chemistry: Sensing, Migration, Capture, Delivery, and Separation

Synthetic nano- and microscale machines move autonomously in solution or drive fluid flows by converting sources of energy into mechanical work. Their sizes are comparable to analytes (sub-nano- to microscale), and they respond to signals from each other and their surroundings, leading to emergent collective behavior. These machines can potentially enable hitherto difficult analytical applications. In this article, we review the development of different classes of synthetic nano- and micromotors and pumps and indicate their possible applications in real-time in situ chemical sensing, on-demand directional transport, cargo capture and delivery, as well as analyte isolation and separation.

Anatomy of Nanoscale Propulsion

Nature supports multifaceted forms of life. Despite the variety and complexity of these forms, motility remains the epicenter of life. The applicable laws of physics change upon going from macroscales to microscales and nanoscales, which are characterized by low Reynolds number (Re). We discuss motion at low Re in natural and synthetic systems, along with various propulsion mechanisms, including electrophoresis, electrolyte diffusiophoresis, and nonelectrolyte diffusiophoresis. We also describe the newly uncovered phenomena of motility in non-ATP-driven self-powered enzymes and the directional movement of these enzymes in response to substrate gradients. These enzymes can also be immobilized to function as fluid pumps in response to the presence of their substrates. Finally, we review emergent collective behavior arising from interacting motile species, and we discuss the possible biomedical applications of the synthetic nanobots and microbots.

Surface-accessible detection units in self-immolative polymers enable translation of selective molecular detection events into amplified responses in macroscopic, solid-state plastics

This Communication describes a strategy for incorporating detection units onto each repeating unit of self-immolative CDr polymers. This strategy enables macroscopic plastics to respond quickly to specific applied molecular signals that react with the plastic at the solid-liquid interface between the plastic and surrounding fluid. The response is a signal-induced depolymerization reaction that is continuous and complete from the site of the reacted detection unit to the end of the polymer. Thus, this strategy retains the ability of CDr polymers to provide amplified responses via depolymerization while simultaneously enhancing the rate of response of CDr-based macroscopic plastics to specific applied signals. Depolymerizable poly(benzyl ethers) were used to demonstrate the strategy and now are capable of depolymerizing in the context of rigid, solid-state polymeric materials.

The role of hydrazine in mixed fuels (H2O2/N2H4) for Au–Fe/Ni nanomotors

Hydrazine in mixed fuels facilitates the oxidation of H₂O₂ to oxygen bubbles that propel the Au–Fe/Ni nanomotors.

End-capped poly(4,5-dichlorophthalaldehyde): a stable self-immolative poly(aldehyde) for translating specific inputs into amplified outputs, both in solution and the solid state

Poly(4,5-dichlorophthalaldehyde) is a new self-immolative CD_r polymer that is substantially more stable than poly(phthalaldehyde), yet retains its favorable attributes.

Chemically powered micro- and nanomotors

Chemically powered micro- and nanomotors are small devices that are self-propelled by catalytic reactions in fluids. Taking inspiration from biomotors, scientists are aiming to find the best architecture for self-propulsion, understand the mechanisms of motion, and develop accurate control over the motion. Remotely guided nanomotors can transport cargo to desired targets, drill into biomaterials, sense their environment, mix or pump fluids, and clean polluted water. This Review summarizes the major advances in the growing field of catalytic nanomotors, which started ten years ago.

Multiresponsive Azobenzene End-Cap for Self-Immolative Polymers

Azobenzene was introduced as a new multiresponsive end-cap for self-immolative polymers. Using small-molecule model compounds, it was demonstrated that reducing agents including hydrazine and dithiothreitol could reduce the azobenzene to the corresponding hydrazobenzene, resulting in a 1,6-elimination reaction with the potential to initiate the depolymerization of self-immolative polycarbamates. An activated azobenzene derivative was then prepared, allowing for its incorporation as an end-cap for polycarbamates based on alternating N,N'-dimethylethylene diamine and 4-hydroxybenzyl alcohol. Upon treatment with hydrazine, depolymerization proceeded. To demonstrate the versatility of this end-cap, the chemistry was also applied to polycarbamates based on 4-aminobenzyl alcohol. In addition to allowing triggered depolymerization, the azobenzene end-cap also provides a visual signal upon triggering owing to the strong visible absorption of the azobenzene, which shifts to the UV range upon reduction. Furthermore, azobenzene is capable of undergoing trans-cis isomerization in response to UV light, providing multiple functions in a single end-cap.

Self-powered glucose-responsive micropumps

A self-powered polymeric micropump based on boronate chemistry is described. The pump is triggered by the presence of glucose in ambient conditions and induces convective fluid flows, with pumping velocity proportional to the glucose concentration. The pumping is due to buoyancy convection that originates from reaction-associated heat flux, as verified from experiments and finite difference modeling. As predicted, the fluid flow increases with increasing height of the chamber. In addition, pumping velocity is enhanced on replacing glucose with mannitol because of the enhanced exothermicity associated with the reaction of the latter.