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DNA origami has been used as biotemplates for growing a range of inorganic materials to create novel organic–inorganic hybrid nanomaterials. Recently, the solution-based silicification of DNA has been used to grow thin silica shells on DNA origami. However, the silicification reaction is sensitive to the reaction conditions and often results in uncontrolled DNA origami aggregation, especially when growth of thicker silica layers is desired. Here, we investigated how site-specifically placed polynucleotide brushes influence the silicification of DNA origami. Our experiments showed that long DNA brushes, in the form of single- or double-stranded DNA, significantly suppress the aggregation of DNA origami during the silicification process. Furthermore, we found that double-stranded DNA brushes selectively promote silica growth on DNA origami surfaces. These observations were supported and explained by coarse-grained molecular dynamics simulations. This work provides new insights into our understanding of the silicification process on DNA and provides a powerful toolset for the development of novel DNA-based organic–inorganic nanomaterials.

Synovial fluid lubricates articular joints by forming a hydrated layer between the cartilage surfaces. In degenerative joint diseases like osteoarthritis (OA), the synovial fluid is compromised, which leads to less effective innate lubrication and exacerbated cartilage degeneration. Studies over the years have led to the development of partially or fully synthetic biolubricants to reduce the coefficient of friction with cartilage in knee joints. Cartilage-adhering, hydrated lubricants are particularly important to provide cartilage lubrication and chondroprotection under high normal load and slow speed. Here, we report the development of a hyaluronic acid (HA)-based lubricant functionalized with cationic branched poly-l-lysine (BPL) molecules that bind to cartilage via electrostatic interactions. We surmised that the electrostatic interactions between the BPL-modified HA molecules (HA-BPL) and the cartilage facilitate localization of the HA molecules to the cartilage surface. The number of BPL molecules on the HA backbone was varied to determine the optimal grafting density for cartilage binding and HA localization. Collectively, our results show that our HA-BPL molecules adhered readily to cartilage and were effective as a lubricant in cartilage-on-cartilage shear measurements where the modified HA molecules significantly reduce the coefficient of friction compared to phosphate-buffered saline or HA alone. This proof-of-concept study shows how the incorporation of cartilage adhering moieties, such as cationic molecules, can be used to enhance cartilage binding and lubrication properties of HA.

Diblock copolymers are valued for their ability to form thin films with nanoscale features that typically reflect those of their microphase-separated structures in concentrated solution. Here, we show that such self-assembled structures can be easily formed with diblock copolymers composed of thermally responsive polypeptides, such as resilin-like polypeptides (RLP) and elastin-like polypeptides (ELP), by exploiting the inverse temperature transition behavior of ELPs in aqueous media. Specifically, we examine the self-assembly of a series of RLP-b-ELP diblock copolypeptides in concentrated aqueous solution (30 and 50 wt %) by small-angle X-ray scattering (SAXS). By systematically varying RLP block length and temperature (10–45 °C), we observed microphase separation into hexagonally packed cylinders and lamellae. By analyzing the observed order–order transitions (OOT) and order–disorder transitions (ODT), we determined that self-assembly in this system is primarily driven by polymer–solvent interactions. While these thermally responsive polymers showed clear ODTs and OOTs at certain temperatures, temperature only had a weak effect on the spacing of the resulting nanostructures. In contrast, we found that nanostructure spacing was far more sensitive to RLP block length. Finally, we used atomic force microscopy (AFM) to demonstrate that spin casting RLP-b-ELP diblock copolypeptides also produce nanostructured thin films with spacings that correlate with those in concentrated solution.

Sensors based on two-dimensional (2D) field-effect transistors (FETs) are extremely sensitive and can detect charged analytes with attomolar limits of detection (LOD). Despite some impressive LODs, the operating mechanisms and factors that determine the signal-to-noise ratio in 2D FET-based sensors remain poorly understood. These uncertainties, coupled with an expansive design space for sensor layout and analyte positioning, result in a field with many reported highlights but limited collective progress. Here, we provide insight into sensing mechanisms of 2D molybdenum disulfide (MoS2) FETs by realizing precise control over the position and charge of an analyte using a customized atomic force microscope (AFM), with the AFM tip acting as an analyte. The sensitivity of the MoS2FET channel is revealed to be nonuniform, manifesting sensitive hotspots with locations that are stable over time. When the charge of the analyte is varied, an asymmetry is observed in the device drain–current response, with analytes acting to turn the device off leading to a 2.5× increase in the signal-to-noise ratio (SNR). We developed a numerical model, applicable to all FET-based charge-detection sensors, that confirms our experimental observation and suggests an underlying mechanism. Further, extensive characterization of a set of different MoS2FETs under various analyte conditions, coupled with the numerical model, led to the identification of three distinct SNRs that peak with dependence on the layout and operating conditions used for a sensor. These findings reveal the important role of analyte position and coverage in determining the optimal operating bias conditions for maximal sensitivity in 2D FET-based sensors, which provides key insights for future sensor design and control.

Specifically adsorbed bottlebrush coatings are found in nature as brush-like glycoproteins that decorate biointerfaces and provide antifouling, lubrication, or wear-protection. Although various synthetic strategies have been developed to mimic glycoprotein structure and function, the use of these mimics is still limited because of the current lack of understanding of their adsorption behavior and surface conformation. In this paper, we examine the adsorption behavior of PEG-based, biotinylated bottlebrushes with different backbone and bristle lengths to streptavidin model surfaces in phosphate-buffered saline. By using quartz crystal microbalance, localized surface plasmon resonance, and atomic force microscopy, we learn how bottlebrush dimensions impact their adsorption kinetics, surface conformation, mechanical properties, and antifouling properties. Our bottlebrushes qualitatively mirror the adsorption behavior of linear polymers and exhibit three kinetic regimes of adsorption: (I) a transport-limited regime, (II) a pause, and (III) a penetration-limited regime. Furthermore, we find that the bristle length more dramatically affects brush properties than the backbone length. Generally, larger bottlebrush dimensions lead to reduced molar adsorption, retarded kinetics, weaker antifouling, and softer brush coatings. Longer bristles also lead to less mass adsorption, while the opposite trend is observed for increasing backbone length. In summary, our findings aid the rational design of new bottlebrush coatings by elucidating how their dimensions impact adsorption, surface conformation, and the properties of the final coating.

The development of stretchable electronics requires the invention of compatible high-performance power sources, such as stretchable supercapacitors and batteries. In this work, two-dimensional (2D) titanium carbide (Ti3C2Tx) MXene is being explored for flexible and printed energy storage devices by fabrication of a robust, stretchable high-performance supercapacitor with reduced graphene oxide (RGO) to create a composite electrode. The Ti3C2Tx/RGO composite electrode combines the superior electrochemical and mechanical properties of Ti3C2Txand the mechanical robustness of RGO resulting from strong nanosheet interactions, larger nanoflake size, and mechanical flexibility. It is found that the Ti3C2Tx/RGO composite electrodes with 50 wt % RGO incorporated prove to mitigate cracks generated under large strains. The composite electrodes exhibit a large capacitance of 49 mF/cm2(∼490 F/cm3and ∼140 F/g) and good electrochemical and mechanical stability when subjected to cyclic uniaxial (300%) or biaxial (200% × 200%) strains. The as-assembled symmetric supercapacitor demonstrates a specific capacitance of 18.6 mF/cm2(∼90 F/cm3and ∼29 F/g) and a stretchability of up to 300%. The developed approach offers an alternative strategy to fabricate stretchable MXene-based energy storage devices and can be extended to other members of the large MXene family.

Understanding of how to integrate DNA molecules with graphene materials is important for the development of biosensors and biomolecular logic circuits. For some of these applications, controlling DNA structural conformation on the graphene substrate is critically important and can be achieved through the use of self-assembled monolayers. Here, we performed all-atom molecular dynamics simulations to understand how various 1-octadecylamine (ODA) coatings of the graphene surface affect the conformation of double-stranded DNA (dsDNA) on the surface. The simulation results demonstrated that dsDNA structures become more stable as ODA concentration increases due to the formation of DNA–ODA hydrogen bonds and reduction of DNA–surface interactions, which aid in retaining internal DNA interactions. Specifically, the interaction of ODA molecules with DNA prevents nucleobases from forming π–π stacking interactions with the surface. Some dsDNA conformations, such as sharp kinks or unwinding, can occur more frequently in DNA with A–T sequences due to weaker pairing interactions than with G–C sequences. Furthermore, our results conclude that both DNA sequence and ODA concentration play an essential role in experimentally observed conformational changes of DNA on the graphene surface.

Many approaches have been developed to characterize cell elasticity. Among these, atomic force microscopy (AFM) combined with modeling has been widely used to characterize cellular compliance. However, such approaches are often limited by the difficulties associated with using a specific instrument and by the complexity of analyzing the measured data. More recently, quantitative phase imaging (QPI) has been applied to characterize cellular stiffness by using an effective spring constant. This metric was further correlated to mass distribution (disorder strength) within the cell. However, these measurements are difficult to compare to AFM-derived measurements of Young’s modulus. Here, we describe, to our knowledge, a new way of analyzing QPI data to directly retrieve the shear modulus. Our approach enables label-free measurement of cellular mechanical properties that can be directly compared to values obtained from other rheological methods. To demonstrate the technique, we measured shear modulus and phase disorder strength using QPI, as well as Young’s modulus using AFM, across two breast cancer cell-line populations dosed with three different concentrations of cytochalasin D, an actin-depolymerizing toxin. Comparison of QPI-derived and AFM moduli shows good agreement between the two measures and further agrees with theory. Our results suggest that QPI is a powerful tool for cellular biophysics because it allows for optical quantitative measurements of cell mechanical properties.

Understanding the mechanical properties of polymer nanocomposite materials is essential for industrial use. Particularly, the determination of the polymer modulus at the nanofiller–polymer interphase is important for optimizing the interfacial mechanical properties. Nanoindentation via Atomic Force Microscopy (AFM) is well-established for measuring the modulus of the interphase region with nanoscale spatial resolution. However, indentation into heterogeneous materials presents a confounding issue often referred to as the “substrate effect”, i.e., the structural stress field caused by the rigid body is convoluted with the actual modulus of the interphase region. While finite element analysis (FEA)-based methods can be used to deconvolute the interphase modulus from measured apparent modulus–distance profiles, the experimental validation of this method is still needed. Here, we provide this validation using AFM nanoindentation on a layered model composite that consists of three layers with different moduli to recapitulate the properties of the matrix, the filler, and the interphase of real polymer nanocomposites. By systematically varying the thickness of the “artificial” interphase layer and the AFM probe radius, we obtain modulus–distance profiles over a wide range of indentation conditions. We validate a method to deconvolute the substrate effect using an empirically derived master curve obtained from FEA analysis. Furthermore, we showed that the effect of the artificial interphase on modulus– distance profiles can be distinguished only if the interphase layer is thick enough compared to the contact radius of the probe. Finally, we established an innovative and quantitative framework to predict the interphase thickness from mechanical nanoindentation measurements and discussed the lower, practical limit for interphase thickness determination. In summary, we provide a broadly applicable method to extract interphase mechanical properties of multiphase soft materials and practical guidelines for choosing optimal characterization conditions.

A powerful tool for controlling interfacial properties and molecular architecture relies on the tailored adsorption of stimuli-responsive block copolymers onto surfaces. Here, we use computational and experimental approaches to investigate the adsorption behavior of thermally responsive polypeptide block copolymers (elastin-like polypeptides, ELPs) onto silica surfaces, and to explore the effects of surface affinity and micellization on the adsorption kinetics and the resultant polypeptide layers. We demonstrate that genetic incorporation of a silica-binding peptide (silaffin R5) results in enhanced adsorption of these block copolymers onto silica surfaces as measured by quartz crystal microbalance and ellipsometry. We find that the silaffin peptide can also direct micelle adsorption, leading to close-packed micellar arrangements that are distinct from the sparse, patchy arrangements observed for ELP micelles lacking a silaffin tag, as evidenced by atomic force microscopy measurements. These experimental findings are consistent with results of dissipative particle dynamics simulations. Wettability measurements suggest that surface immobilization hampers the temperature-dependent conformational change of ELP micelles, while adsorbed ELP unimers (i.e., unmicellized block copolymers) retain their thermally responsive property at interfaces. These observations provide guidance on the use of ELP block copolymers as building blocks for fabricating smart surfaces and interfaces with programmable architecture and functionality.

Polymer brush coatings are frequently prepared by radical polymerization, a notoriously oxygen sensitive process. Glucose oxidase (GOx) can inexpensively enable radical polymerization in solution by enzymatically consuming oxygen as it oxidizes glucose. Here, we report the growth of polymeric brushes using GOx-assisted atom transfer radical polymerization (ATRP) from a surface while open to air. Specifically, we grew a set of biomedically relevant polymer brushes, including poly(oligo(ethylene glycol) methacrylate) (POEGMA), poly(2-dimethylaminoethyl methacrylate) (PDMAEMA), poly(sulfobetaine methacrylate) (PSBMA), and poly(2-(methylsulfinyl)ethyl acrylate (PMSEA). For each of these polymers, we monitored GOx-assisted and GOx-free ATRP reaction kinetics in real time using quartz crystal microbalance (QCM) and verified findings with localized surface plasmon resonance (LSPR). We modeled brush growth kinetics considering bimolecular termination. This model fit our data well (r2> 0.987 for all samples) and shows the addition of GOx increased effective kinetic chain lengths, propagation rates, and reproducibility. We tested the antifouling properties of the polymer brush coatings against human blood plasma and were surprised to find that coatings prepared with GOx repelled more plasma proteins in all cases than their GOx-free counterparts.

Advances in biosensors and drug delivery are dependent on hydrogels that respond to external stimuli. In this work, we describe the preparation and characterization of photoresponsive hydrogels prepared by cross-linking of di-NHS ester of azobenzoic acid and four-armed, amine-terminated poly(ethylene glycol). The porous structure and composition of the hydrogels were confirmed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The reversible photoisomerization of the azobenzene-containing hydrogel cross-linkers in the gels was confirmed by absorption spectroscopy. Specifically, the photoisomerization of the cross-linkers between their transand cisconfigurations was observed by monitoring the absorbance of the hydrogels at the two characteristic peaks of azobenzene (π–π* at 330 nm and n–π* at 435 nm). The effect of photoisomerization on the hydrogel structure was investigated by microscopy. Ultraviolet (UV) irradiation-induced reduction in hydrogel size was observed, which may be a result of the inherently smaller footprint of the cisazobenzene conformation, as well as dipole–dipole interactions between the polar cisazobenzene and the polymer network. The UV-triggered reduction in hydrogel size was accompanied by enhanced release of the near-infrared fluorescent dye Alexa Fluor 750 (AF750). Enhanced release of AF750was observed in samples irradiated with UV versus dark control. Together, these data demonstrate the potential of these systems as reversible photoresponsive biomaterials.

Proteoglycans are important brush-like biomacromolecules, which serve a variety of functions in the human body. While protein–bottlebrush hybrids are promising proteoglycan mimics, many challenges still exist to robustly produce such polymers. In this paper, we report the modular synthesis of protein–brush hybrids containing elastin-like polypeptides (ELP) as model proteins by copper-catalyzed azide–alkyne cycloaddition. We exploit the recently discovered imidazole-1-sulfonyl azide (ISA) in a diazotransfer reaction to introduce an N-terminal azide onto an ELP. Next, we use a click reaction to couple the azido-ELP to an alkyne-terminated amine-rich polymer followed by a second diazotransfer step to produce an azide-rich backbone that serves as a scaffold. Finally, we used a second click reaction to graft alkyne-terminated poly(oligoethylene glycol methacrylate) (POEGMA) bristles to the azide-rich backbone to produce the final protein–bottlebrush hybrid. We demonstrate the effectiveness of this synthetic path at each step through careful characterization with 1H NMR, FTIR, GPC, and diagnostic test reactions on SDS-PAGE. Final reaction products could be consistently obtained for a variety of different molecular weight backbones with final total grafting efficiencies around 70%. The high-yielding reactions employed in this highly modular approach allow for the synthesis of protein–bottlebrush hybrids with different proteins and brush polymers. Additionally, the mild reaction conditions used have the potential to avoid damage to proteins during synthesis.

We synthesized long, nucleobase-modified, single-stranded DNA (ssDNA) using terminal deoxynucleotidyl transferase (TdT) enzymatic polymerization. Specifically, we investigated the effect of unnatural nucleobase size and incorporation density on ssDNA resistance to exo- and endonuclease degradation. We discovered that increasing the size and density of unnatural nucleobases enhances ssDNA resistance to degradation in the presence of exonuclease I, DNase I, and human serum. We also studied the mechanism of this resistance enhancement using molecular dynamics simulations. Our results show that the presence of unnatural nucleobases in ssDNA decreases local chain flexibility and hampers nuclease access to the ssDNA backbone, which hinders nuclease binding to ssDNA and slows its degradation. Our discoveries suggest that incorporating nucleobase-modified nucleotides into ssDNA, using enzymatic polymerization, is an easy and efficient strategy to prolong and tune the half-life of DNA-based materials in nucleases-containing environments.

A series of two-dimensional (2D) hybrid organic–inorganic perovskite (HOIP) crystals, based on acene alkylamine cations (i.e., phenylmethylammonium (PMA), 2-phenylethylammonium (PEA), 1-(2-naphthyl)methanammonium (NMA), and 2-(2-naphthyl)ethanammonium (NEA)) and lead(II) halide (i.e., PbX42–, X = Cl, Br, and I) frameworks, and their corresponding thin films were fabricated and examined for structure–property relationship. Several new or redetermined crystal structures are reported, including those for (NEA)2PbI4, (NEA)2PbBr4, (NMA)2PbBr4, (PMA)2PbBr4, and (PEA)2PbI4. Non-centrosymmetric structures from among these 2D HOIPs were confirmed by piezoresponse force microscopyespecially noteworthy is the structure of (PMA)2PbBr4, which was previously reported as centrosymmetric. Examination of the impact of organic cation and inorganic layer choice on the exciton absorption/emission properties, among the set of compounds considered, reveals that perovskite layer distortion (i.e., Pb–I–Pb bond angle between adjacent PbI6octahedra) has a more global effect on the exciton properties than octahedral distortion (i.e., variation of I–Pb–I bond angles and discrepancy among Pb–I bond lengths within each PbI6octahedron). In addition to the characteristic sharp exciton emission for each perovskite, (PMA)2PbCl4, (PEA)2PbCl4, (NMA)2PbCl4, and (PMA)2PbBr4exhibit separate, broad “white” emission in the long wavelength range. Piezoelectric compounds identified from these 2D HOIPs may be considered for future piezoresponse-type energy or electronic applications.

Hybrid organic–inorganic compounds are receiving increasing attention for photoelectrochemical (PEC) devices due to their high electron transport efficiency and facile synthesis. Biosynthesis is a potentially low-cost and eco-friendly method to precipitate transition-metal-based semiconductor nanoparticles (NPs) in an organic matrix. In this work, we examine the structure and composition of bacterially precipitated (BAC) cadmium sulfide (CdS) NPs using electron microscopy, and we determine their PEC properties and the energy band structure by electrochemical measurements. In addition, by taking advantage of the organic matrix, which is residual from the biosynthesis process, we fabricate a prototype photocharged capacitor electrode by incorporating the bacterially precipitated CdS with a reduced graphene oxide (RGO) sheet. Our results show that the hydrophilic groups associated with the organic matrix make BAC CdS NPs a potentially useful component of PEC devices with applications for energy conversion and storage.

Interfaces and subsurface layers are critical for the performance of devices made of 2D materials and heterostructures. Facile, nondestructive, and quantitative ways to characterize the structure of atomically thin, layered materials are thus essential to ensure control of the resultant properties. Here, we show that contact-resonance atomic force microscopywhich is exquisitely sensitive to stiffness changes that arise from even a single atomic layer of a van der Waals-adhered materialis a powerful experimental tool to address this challenge. A combined density functional theory and continuum modeling approach is introduced that yields sub-surface-sensitive, nanomechanical fingerprints associated with specific, well-defined structure models of individual surface domains. Where such models are known, this information can be correlated with experimentally obtained contact-resonance frequency maps to reveal the (sub)surface structure of different domains on the sample.

We report a targeted prodrug delivery platform that can deliver a cytostatic nucleobase analog with high drug loading. We chose fluorouracil (5FU), a drug used to treat various cancers, whose active metabolite 5-fluorodeoxyuridine monophosphate (5-FdUMP) is the antineoplastic agent. We use terminal deoxynucleotidyl transferase (TdT) to polymerize 5-fluorodeoxyuridine triphosphate (5-FdUTP) onto the 3′-end of an aptamer. We find that (i) addition of hydrophobic, unnatural nucleotides at the 3′-end of the 5-FdU polynucleotide by TdT leads to their spontaneous self-assembly into nuclease resistant micelles, (ii) aptamers presented on the micelle corona retain specificity for their cognate receptor on tumor cells, and (iii) the micelles deliver 5FU to tumor cells and exhibit greater cytotoxicity than the free drug. The modular design of our platform, consisting of a targeting moiety, a polynucleotide drug, and a self-assembly domain, can be adapted to encompass a range of polymerizable therapeutic nucleotides and targeting units.

Self-assembled monolayers (SAMs) have been widely used to engineer the electronic properties of substrate-supported graphene devices. However, little is known about how the surface chemistry of SAMs affects the interfacial mechanical properties of graphene supported on SAMs. Fluctuations and changes in these properties affect the stress transfer between substrate and the supported graphene and thus the performance of graphene-based devices. The changes in interfacial mechanical properties can be characterized by measuring the out-of-plane elastic properties. Combining contact resonance atomic force microcopy experiments with molecular dynamics simulations, we show that the head group chemistry of a SAM, which affects the interfacial interactions, can have a significant effect on the out-of-plane elastic modulus of the graphene–SAM heterostructure. Graphene supported on hydrophobic SAMs leads to heterostructures stiffer than those of graphene supported on hydrophilic SAMs, which is largely due to fewer water molecules present at the graphene–SAM interface. Our results provide an important, and often overlooked, insight into the mechanical properties of substrate-supported graphene electronics.

Ferroelectric surfaces can have very high surface charge densities that can be harnessed for manipulation of charged colloidal particles and soft matter in aqueous environments. Here, we report on the electrical double layer (EDL) formed by polarized ultrasmooth lead zirconium titanate (US-PZT) thin films in dilute electrolyte solutions. Using colloidal probe force microscopy (CPFM) measurements, we show that the ion distribution within the double layer can be changed by reversing the ferroelectric polarization state of US-PZT. The interaction force in dilute 1:1 electrolyte solution between the negatively charged probe and a positive surface charge (upward polarized) US-PZT thin film is attractive, while the interaction force is repulsive for a negative surface charge (downward polarized) film. We modeled these interactions with a constant-potential EDL model between dissimilar surfaces with the inclusion of a Stern layer. We report the surface potentials at the inner and outer-Helmholtz planes both for polarization states and for a range of ionic strength solutions. Effects of free-charge carriers, limitations of the analytical model, and effects of surface roughness are discussed.

An important goal and major challenge of material science and nanotechnology is building nanomotors for manipulating the motion of nanoparticles (NPs). Here, it is demonstrated that patterned, stimulus‐responsive polymer brush microstructures can be used as motor arrays to manipulate the movement of gold NP aggregates in response to external stimuli that induce a conformational change in the brushes as the driving force. The motion of NP aggregates in the out‐of‐plane direction is achieved with displacements ranging from nanometers to sub‐micrometers. These patterned polymer‐brush microstructures can find applications as efficient motor arrays and nanosensors, and benefit the design of more complex nanodevices.

In articular cartilage, chondrocytes are surrounded by a narrow region called the pericellular matrix (PCM), which is biochemically, structurally, and mechanically distinct from the bulk extracellular matrix (ECM). Although multiple techniques have been used to measure the mechanical properties of the PCM using isolated chondrons (the PCM with enclosed cells), few studies have measured the biomechanical properties of the PCM in situ. The objective of this study was to quantify the in situ mechanical properties of the PCM and ECM of human, porcine, and murine articular cartilage using atomic force microscopy (AFM). Microscale elastic moduli were quantitatively measured for a region of interest using stiffness mapping, or force-volume mapping, via AFM. This technique was first validated by means of elastomeric models (polyacrylamide or polydimethylsiloxane) of a soft inclusion surrounded by a stiff medium. The elastic properties of the PCM were evaluated for regions surrounding cell voids in the middle/deep zone of sectioned articular cartilage samples. ECM elastic properties were evaluated in regions visually devoid of PCM. Stiffness mapping successfully depicted the spatial arrangement of moduli in both model and cartilage surfaces. The modulus of the PCM was significantly lower than that of the ECM in human, porcine, and murine articular cartilage, with a ratio of PCM to ECM properties of ∼0.35 for all species. These findings are consistent with previous studies of mechanically isolated chondrons, and suggest that stiffness mapping via AFM can provide a means of determining microscale inhomogeneities in the mechanical properties of articular cartilage in situ.

Chondrocytes in articular cartilage normally exhibit high expression of collagen II and aggrecan but rapidly dedifferentiate to a fibroblastic phenotype if passaged in culture. Previous studies have suggested that the loss of chondrocyte phenotype is associated with changes in the structure of the F-actin cytoskeleton, which also controls cell mechanical properties. In this study, we examined how dedifferentiation in monolayer influences the mechanical properties of chondrocytes isolated from different zones of articular cartilage. Atomic force microscopy was used to measure the mechanical properties of superficial and middle/deep zone chondrocytes as they underwent serial passaging and subsequent growth on fibronectin-coated, micropatterned self-assembled monolayers that restored a rounded cell shape in 2D culture. Chondrocytes exhibited significant increases in elastic and viscoelastic moduli with dedifferentiation in culture. These changes were only partially ameliorated by the restoration of a rounded shape on micropatterned surfaces. Furthermore, intrinsic zonal differences in cell mechanical properties were rapidly lost with passage. These findings indicate that cell mechanical properties may provide additional measures of phenotypic expression of chondrocytes as they undergo dedifferentiation and possibly redifferentiation in culture.

Biological surface science is a broad, interdisciplinary subfield of surface science, where properties and processes at biological and synthetic surfaces and interfaces are investigated, and where biofunctional surfaces are fabricated. The need to study and to understand biological surfaces and interfaces in liquid environments provides sizable challenges as well as fascinating opportunities. Here, we report on recent progress in biological surface science that was described within the program assembled by the Biomaterial Interface Division of the Science and Technology of Materials, Interfaces and Processes (www.avs.org) during their 55th International Symposium and Exhibition held in Boston, October 19−24, 2008. The selected examples show that the rapid progress in nanoscience and nanotechnology, hand-in-hand with theory and simulation, provides increasingly sophisticated methods and tools to unravel the mechanisms and details of complex processes at biological surfaces and in-depth understanding of biomolecular surface interactions.

Atomic force microscopy has rapidly become a valuable tool for quantifying the biophysical properties of single cells. The interpretation of atomic force microscopy-based indentation tests, however, is highly dependent on the use of an appropriate theoretical model of the testing configuration. In this study, a novel, thin-layer viscoelastic model for stress relaxation was developed to quantify the mechanical properties of chondrosarcoma cells in different configurations to examine the hypothesis that viscoelastic properties reflect the metastatic potential and invasiveness of the cell using three well-characterized human chondrosarcoma cell lines (JJ012, FS090, 105KC) that show increasing chondrocytic differentiation and decreasing malignancy, respectively. Single-cell stress relaxation tests were conducted at 2h and 2 days after plating to determine cell mechanical properties in either spherical or spread morphologies and analyzed using the new theoretical model. At both time points, JJ012 cells had the lowest moduli of the cell lines examined, whereas FS090 typically had the highest. At 2 days, all cells showed an increase in stiffness and a decrease in apparent viscosity compared to the 2-h time point. Fluorescent labeling showed that the F-actin structure in spread cells was significantly different between FS090 cells and JJ012/105KC cells. Taken together with results of previous studies, these findings indicate that cell transformation and tumorigenicity are associated with a decrease in cell modulus and apparent viscosity, suggesting that cell mechanical properties may provide insight into the metastatic potential and invasiveness of a cell.

We present a simple “top-down/bottom-up” strategy to fabricate nano- and micropatterned polymer brush arrays composed of pH- and salt-sensitive, weak polyelectrolyte copolymers poly(N-isopropylacrylamide-co-methacrylic acid, 3:1, poly(NIPAAM-co-MAA). In our approach, a silicon surface is first patterned with gold, using “lift-off” electron-beam lithography (“top-down”), and the resulting pattern is then amplified by surface-initiated photopolymerization by conventional, UV-light-induced free radical polymerization (“bottom-up”) from an immobilized 2,2‘-azobisisobutyronitrile (AIBN) type initiator. The use of pH- and ionic-strength-sensitive comonomers in the copolymer brush enables large, externally triggered conformational changes of the micro- and nanopatterned polymer brushes. We observed that the height of nanopatterned ionized polymer brushes increases with increasing feature size of the pattern. The design and fabrication of surfaces with conformationally switchable, patterned polymeric structures is important for sensing and actuation applications on the micro- and nanoscales.

Summary:This paper demonstrates a new, reliable, and simple method for fabricating micropatterned nanoparticle arrays that can serve as templates for the surface‐initiated polymerization of polymer brushes. As a proof of concept, we micropatterned gold nanoparticles (Au‐NPs, ≈10 nm) onto glass, silicon, polystyrene, and gold surfaces by a simple three‐step process: (1) microcontact printing of soluble polymer, (2) incubation with a solution of Au‐NPs, and (3) lift‐off of the template in a mixture of ethanol and deionized water. 40 µm wide features were successfully fabricated without any significant defects or nonspecific adsorption on the background. To demonstrate the utility of these Au‐NP templates, we subsequently polymerized N‐isopropylacrylamide by surface‐initiated polymerization, using a surface‐bound initiator.

Colloidal probe microscopy was employed to study interactions between cellulose surfaces in aqueous solutions. Hydrodynamic forces must be accounted for in data analysis. Long-range interactions betweeen cellulose surfaces are governed by double-layer forces and, once surfaces contact, by osmotic repulsive forces and viscoelasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage. The conformation of adsorbed polyelectrolytes depends on the polymer concentration.

We studied the directed self-assembly of two types of complementary single-stranded DNA (ssDNA) strands [i.e., poly(dA) and poly(dT)] into more complex, organized, and percolating networks in dilute solutions and at surfaces. Understanding ssDNA self-assembly into 2D networks on surfaces is important for the use of such networks in the fabrication of well-defined nanotechnological devices, as, for instance, required in nanoelectronics or for biosensing. To control the formation of 2D networks on surfaces, it is important to know whether DNA assemblies are formed already in dilute solutions or only during the drying/immobilization process at the surface, where the concentration automatically increases. Fluorescence cross-correlation spectroscopy clearly shows the presence of larger DNA complexes in mixed poly(dA) and poly(dT) solutions already at very low DNA concentrations (<1 nM), that is, well below the overlap concentration. Here, we describe for the first time such supramolecular complexes in solution and how their structure depends on the ssDNA length and concentration and ionic strength. Hence, future attempts to control such networks should also focus on network precursors in solution and not only on their immobilization on surfaces.

Convergent advances in the field of soft matter, macromolecular chemistry, and engineering have led to the development of biomaterials that possess autonomous, adaptive, and self‐healing characteristics similar to living systems. These rationally designed biomaterials can surpass the capabilities of their parent material. Herein, the modification of hyaluronic acid (HA) to exhibit self‐healing properties is described, and its physical and biological function both in vitro and in vivo is studied. The in vitro findings showed that self‐healing HA designed to undergo self‐repair improves lubrication, enhances free radical scavenging, and attenuates enzymatic degradation compared to unmodified HA. Longitudinal imaging following intraarticular injection of self‐healing HA shows improved in vivo retention despite its low molecular weight. Concomitant with these functions, intraarticular injection of self‐healing HA mitigates anterior cruciate ligament injury‐mediated cartilage degeneration in rodents. This proof‐of‐concept study shows how incorporation of functional properties such as self‐healing can be used to surpass the existing capabilities of biolubricants. Modification of low molecular weight hyaluronic acid (HA) to exhibit self‐healing properties enhances its in vivo longevity and lubrication function. Concomitant with these functions, intraarticular injection of self‐healing HA mitigates joint injury‐mediated cartilage degeneration in rodents following anterior cruciate ligament transection.

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Our recent studies indicate that circulating PF4/heparin complexes activate complement and contribute to a subsequent immune response seen in heparin-induced thrombocytopenia (HIT) (Khandelwal, Blood 2016). We undertook studies to develop a sensitive assay for measuring circulating PF4/heparin. To do so, we utilize a quartz crystal microbalance (QCM) device, which is a piezoelectric acoustic resonator that detects decreases in the resonance frequency of the quartz crystal due to the increased mass caused by molecules binding to the electrode surface. We hypothesize that a novel label-free QCM biosensor functionalized with PF4/heparin capture antibodies can provide for robust quantitative antigen detection and physical characterization of antigen-antibody interactions.