No abstract

Enzymes show two distinct transport behaviors in the presence of their substrates in solution. First, their diffusivity enhances with an increasing substrate concentration. In addition, enzymes perform directional motion toward regions with a high substrate concentration, termed as chemotaxis. While a variety of enzymes has been shown to undergo chemotaxis, there remains a lack of quantitative understanding of the phenomenon. Here, we derive a general expression for the active movement of an enzyme in a concentration gradient of its substrate. The proposed model takes into account both the substrate-binding and catalytic turnover step, as well as the enhanced diffusion of the enzyme. We have experimentally measured the chemotaxis of a fast and a slow enzyme: urease under catalytic conditions and hexokinase for both full catalysis and for simple noncatalytic substrate binding. There is good agreement between the proposed model and the experiments. The model is general, has no adjustable parameters, and only requires three experimentally defined constants to quantify chemotaxis: enzyme–substrate binding affinity (K d), Michaelis–Menten constant (K M), and level of diffusion enhancement in the associated substrate (α).

Nature has designed multifaceted cellular structures to support life. Cells contain a vast array of enzymes that collectively perform essential tasks by harnessing energy from chemical reactions. Despite the complexity, intra- and intercellular motility at low Reynolds numbers remain the epicenter of life. In the past decade, detailed investigations on enzymes that are freely dispersed in solution have revealed concentration-dependent enhanced diffusion and chemotactic behavior during catalysis. Theoretical calculations and simulations have determined the magnitude of the impulsive force per turnover; however, an unequivocal consensus regarding the mechanism of enhanced diffusion has not been reached. Furthermore, this mechanical force can be transferred from the active enzymes to inert particles or surrounding fluid, thereby providing a platform for the design of biomimetic systems. Understanding the factors governing enzyme motion would help us to understand organization principles for dissipative self-assembly and the fabrication of molecular machines. The purpose of this article is to review the different classes of enzyme motility and discuss the possible mechanisms as gleaned from experimental observations and theoretical modeling. Finally, we focus on the relevance of enzyme motion in biology and its role in future applications. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 12 is March 10, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

No abstract

The active delivery of drugs to disease sites in response to specific biomarkers is a holy grail in theranostics. If successful, it would greatly diminish the therapeutic dosage and reduce collateral cytotoxicity. In this context, the development of nano and micromotors that are able to harvest local energy to move directionally is an important breakthrough. However, serious hurdles remain before such active systems can be employed in vivo in therapeutic applications. Such motors and their energy sources must be safe and biocompatible, they should be able to move through complex body fluids, and have the ability to reach specific cellular targets. Given the complexity in the design and deployment of nano and micromotors, it is also critically important to show that they are significantly superior to inactive “smart” nanoparticles in theranostics. Furthermore, receiving regulatory approval requires the ability to scale‐up the production of nano and micromotors with uniformity in structure, function, and activity. In this essay, the limitations of the current nano and micromotors and the issues that need to be resolved before such motors are likely to find theranostic applications are discussed.

In this paper, the fabrication and characterization of multi‐drug‐loaded microparticles are demonstrated for topical glaucoma therapy. Specifically, latanoprost (“LAT”) and dexamethasone (“DEX”) are loaded in monodisperse microparticles (diameter ≈150 μm) of a biodegradable polymer–poly (lactic‐co‐glycolic) acid (PLGA)—using capillary microfluidics coupled with solvent evaporation. Both individual (LAT in PLGA and DEX in PLGA) and combined (LAT and DEX in PLGA) microparticle formulations are demonstrated. The morphology, size distribution and in vitro release kinetics are studied, and in vitro mucoadhesion of the formulated microparticles is also assessed. In addition, discussion is placed in how precise knowledge of the particle composition enabled by the microfluidic fabrication method and in vitro release rate measurements allow for facile topical formulation design and dose optimization. Such precision‐fabricated, multi‐drug loaded, sustained‐release microparticles are envisioned to serve as a promising platform for topical administration of ocular drugs. This could potentially reduce the frequency of eyedrop‐based drug administration from several times a day to merely once a day (or less), thus greatly facilitating patient compliance and adherence to a strict therapeutic drug regimen.

We present a simple, bottom up method for the structural design of solid microparticles containing crystalline drug and excipient using microfluidic droplet-based processing. In a model system comprising 5-methyl-2-[(2-nitrophenyl)­amino]-3-thiophenecarbonitrile (ROY) as the drug and ethyl cellulose (EC) as the excipient, we demonstrate a diversity of particle structures, with exquisite control over the structural outcome at the single-particle level. Within microfluidic droplets containing drug and excipient, tuning droplet composition and solvent removal rates allows us to controllably access structural diversity via an interplay of three physical processes (liquid–liquid phase separation, drug crystallization, and polymer vitrification) occurring during solvent removal. Specifically, we demonstrate two levels of structural controla coarse “macro” particle structure and a finer “micro” structure. Further, we elucidate the key mechanistic elements responsible for the observed structural diversity using a combination of systematic experiments, thermodynamic arguments based on a three-component phase diagram, and dissipative particle dynamics simulations. We validate our method with two different excipient and drug combinationsROY and poly­(lactic-co-glycolic acid), and EC and carbamazepine (CBZ). Finally, we present preliminary investigations of in vitro drug release from two different types of CBZ–EC particles, highlighting how structural control allows the design of drug release profiles.

Topically administered ocular drug delivery systems typically face severe bioavailability challenges because of the natural protective mechanisms of eyes. The rational design of drug delivery systems that are able to persist on corneal surfaces for sustained drug release is critical to tackle this problem. In this study, we fabricated monodisperse chitosan-coated PLGA microparticles with tailored diameters from 5 to 120 μm by capillary microfluidic techniques and conducted detailed investigations of their mucoadhesion to artificial mucin-coated substrates. AFM force spectroscopy revealed strong instant adhesion to mucins, whereas the adhesion force, rupture length, and adhesion energy were positively correlated to the particle diameter and contact time. Particle detachment tests under shear flow in a microfluidic mucin-coated flow cell were in accord with the AFM measurements and revealed that microparticles smaller than 25 μm exhibited strong persistence in the flow cell, withstanding high shear rates up to 28,750 s −1 which are equivalent to the harshest in vivo ocular conditions. A simple scaling analysis connects the AFM and detachment tests, and reveals the existence of a threshold diameter below which mucoadhesion performance essentially saturatesan important insight in managing the opposing design criteria of enhanced mucoadhesion and slow, sustained drug delivery. Our findings thus pave the way for the rational design of mucoadhesive microparticulate ocular drug delivery systems that are capable of enhancing the bioavailability of topically applied drugs to eyes, as well as to other tissues whose epithelial surfaces contain mucosae.