Mucus is a viscoelastic and adhesive gel that protects the lung airways, gastrointestinal (GI) tract, vagina, eye and other mucosal surfaces. Most foreign particulates, including conventional particlebased drug delivery systems, are efficiently trapped in human mucus layers by steric obstruction and/ or adhesion. Trapped particles are typically removed from the mucosal tissue within seconds to a few hours depending on anatomical location, thereby strongly limiting the duration of sustained drug delivery locally. A number of debilitating diseases could be treated more effectively and with fewer side effects if drugs and genes could be more efficiently delivered to the underlying mucosal tissues in a controlled manner. This review first describes the tenacious mucus barrier properties that have precluded the efficient penetration of therapeutic particles. It then reviews the design and development of new mucus-penetrating particles that may avoid rapid mucus clearance mechanisms, and thereby provide targeted or sustained drug delivery for localized therapies in mucosal tissues.

Nanoparticles larger than the reported mesh-pore size range (10 -200 nm) in mucus have been thought to be much too large to undergo rapid diffusional transport through mucus barriers. However, large nanoparticles are preferred for higher drug encapsulation efficiency and the ability to provide sustained delivery of a wider array of drugs. We used high-speed multiple-particle tracking to quantify transport rates of individual polymeric particles of various sizes and surface chemistries in samples of fresh human cervicovaginal mucus. Both the mucin concentration and viscoelastic properties of these cervicovaginal samples are similar to those in many other human mucus secretions. Unexpectedly, we found that large nanoparticles, 500 and 200 nm in diameter, if coated with polyethylene glycol, diffused through mucus with an effective diffusion coefficient (D eff) only 4-and 6-fold lower than that for the same particles in water (at time scale ‫؍‬ 1 s). In contrast, for smaller but otherwise identical 100-nm coated particles, D eff was 200-fold lower in mucus than in water. For uncoated particles 100 -500 nm in diameter, D eff was 2,400-to 40,000-fold lower in mucus than in water. Much larger fractions of the 100-nm particles were immobilized or otherwise hindered by mucus than the large 200-to 500-nm particles. Thus, in contrast to the prevailing belief, these results demonstrate that large nanoparticles, if properly coated, can rapidly penetrate physiological human mucus, and they offer the prospect that large nanoparticles can be used for mucosal drug delivery. drug delivery ͉ mucosal tissues ͉ particle tracking ͉ PEG

Mucus is a complex biological material that lubricates and protects the human lungs, gastrointestinal (GI) tract, vagina, eyes, and other moist mucosal surfaces. Mucus serves as a physical barrier against foreign particles, including toxins, pathogens, and environmental ultrafine particles, while allowing rapid passage of selected gases, ions, nutrients, and many proteins. Its selective barrier properties are precisely regulated at the biochemical level across vastly different length scales. At the macroscale, mucus behaves as a non-Newtonian gel, distinguished from classical solids and liquids by its response to shear rate and shear stress, while, at the nanoscale, it behaves as a low viscosity fluid. Advances in the rheological characterization of mucus from the macroscopic to nanoscopic levels have contributed critical understanding to mucus physiology, disease pathology, and the development of drug delivery systems designed for use at mucosal surfaces. This article reviews the biochemistry that governs mucus rheology, the macro- and microrheology of human and laboratory animal mucus, rheological techniques applied to mucus, and the importance of an improved understanding of the physical properties of mucus to advancing the field of drug and gene delivery.

Graphene was deposited on a transparent and flexible substrate, and tensile strain up to ϳ0.8% was loaded by stretching the substrate in one direction. Raman spectra of strained graphene show significant red shifts of 2D and G band (؊27.8 and ؊14.2 cm ؊1 per 1% strain, respectively) because of the elongation of the carbon؊carbon bonds. This indicates that uniaxial strain has been successfully applied on graphene. We also proposed that, by applying uniaxial strain on graphene, tunable band gap at K point can be realized. First-principle calculations predicted a band-gap opening of ϳ300 meV for graphene under 1% uniaxial tensile strain. The strained graphene provides an alternative way to experimentally tune the band gap of graphene, which would be more efficient and more controllable than other methods that are used to open the band gap in graphene. Moreover, our results suggest that the flexible substrate is ready for such a strain process, and Raman spectroscopy can be used as an ultrasensitive method to determine the strain.

Graphene has attracted a lot of interest for fundamental studies as well as for potential applications. Till now, micromechanical cleavage (MC) of graphite has been used to produce high-quality graphene sheets on different substrates. Clear understanding of the substrate effect is important for the potential device fabrication of graphene. Here we report the results of the Raman studies of micromechanically cleaved monolayer graphene on standard SiO 2 (300 nm)/Si, single crystal quartz, Si, glass, polydimethylsiloxane (PDMS), and NiFe. Our data suggests that the Raman features of monolayer graphene are independent of the substrate used; in other words, the effect of substrate on the atomic/electronic structures of graphene is negligible for graphene made by MC. On the other hand, epitaxial monolayer graphene (EMG) on SiC substrate is also investigated. Significant blueshift of Raman bands is observed, which is attributed to the interaction of the graphene sheet with the substrate, resulting in the change of lattice constant and also the electronic structure.

Outsmarting the barrier: Conventional poly(ethylene glycol) (PEG) coated particles (green with light purple border) are immobilized in human mucus networks by adhesive interactions (red) with the mucus mesh (black lines). A dense, low molecular weight PEG coating (blue) uniquely endows nanoparticles with a mucoinert surface that enables their rapid mucus penetration (trajectory indicated by grey line).

The mechanisms by which mucus helps prevent viruses from infecting mucosal surfaces are not well understood. We engineered non-mucoadhesive nanoparticles of various sizes and used them as probes to determine the spacing between mucin fibers (pore sizes) in fresh undiluted human cervicovaginal mucus (CVM) obtained from volunteers with healthy vaginal microflora. We found that most pores in CVM have diameters significantly larger than human viruses (average pore size 340 ± 70 nm; range approximately 50–1800 nm). This mesh structure is substantially more open than the 15–100-nm spacing expected assuming mucus consists primarily of a random array of individual mucin fibers. Addition of a nonionic detergent to CVM caused the average pore size to decrease to 130 ± 50 nm. This suggests hydrophobic interactions between lipid-coated “naked” protein regions on mucins normally cause mucin fibers to self-condense and/or bundle with other fibers, creating mucin “cables” at least three times thicker than individual mucin fibers. Although the native mesh structure is not tight enough to trap most viruses, we found that herpes simplex virus (approximately 180 nm) was strongly trapped in CVM, moving at least 8,000-fold slower than non-mucoadhesive 200-nm nanoparticles. This work provides an accurate measurement of the pore structure of fresh, hydrated ex vivo CVM and demonstrates that mucoadhesion, rather than steric obstruction, may be a critical protective mechanism against a major sexually transmitted virus and perhaps other viruses.

Incomplete coverage and short duration of action limit the effectiveness of vaginally administered drugs, including microbicides for preventing sexually transmitted infections. We investigated vaginal distribution, retention, and safety of nanoparticles with surfaces modified to enhance transport through mucus. We show that mucus-penetrating particles (MPPs) provide uniform distribution over the vaginal epithelium, whereas conventional nanoparticles (CPs) that are mucoadhesive are aggregated by mouse vaginal mucus, leading to poor distribution. Moreover, when delivered hypotonically, MPPs were transported advectively (versus diffusively) through mucus deep into vaginal folds (rugae) within minutes. By penetrating into the deepest mucus layers, more MPPs were retained in the vaginal tract after 6 h compared to CPs. After 24 h, when delivered in a conventional vaginal gel, patches of a model drug remained on the vaginal epithelium, whereas the epithelium was coated with drug delivered by MPP. We then developed MPPs composed of acyclovir monophosphate (ACVp). When administered prior to vaginal herpes simplex virus 2 (HSV-2) challenge, ACVp-MPPs protected 53% of mice, compared to only 16% protected by soluble drug. Overall, MPPs improved vaginal drug distribution and retention, provided more effective protection against vaginal viral challenge than soluble drug, and were non-toxic when administered daily for one week.