Molly M. Stevens

Exploring and engineering the cell surface interface.

Cells are inherently sensitive to local mesoscale, microscale, and nanoscale patterns of chemistry and topography. We review current approaches to control cell behavior through the nanoscale engineering of materials surfaces. Far-reaching implications are emerging for applications including medical implants, cell supports, and materials that can be used as instructive three-dimensional environments for tissue regeneration.

Complexity in biomaterials for tissue engineering

The molecular and physical information coded within the extracellular milieu is informing the development of a new generation of biomaterials for tissue engineering. Several powerful extracellular influences have already found their way into cell-instructive scaffolds, while others remain largely unexplored. Yet for commercial success tissue engineering products must be not only efficacious but also cost-effective, introducing a potential dichotomy between the need for sophistication and ease of production. This is spurring interest in recreating extracellular influences in simplified forms, from the reduction of biopolymers into short functional domains, to the use of basic chemistries to manipulate cell fate. In the future these exciting developments are likely to help reconcile the clinical and commercial pressures on tissue engineering.

Biomaterials for bone tissue engineering

Materials that enhance bone regeneration have a wealth of potential clinical applications from the treatment of nonunion fractures to spinal fusion. The use of porous material scaffolds from bioceramic and polymer components to support bone cell and tissue growth is a longstanding area of interest. Current challenges include the engineering of materials that can match both the mechanical and biological context of real bone tissue matrix and support the vascularization of large tissue constructs. Scaffolds with new levels of biofunctionality that attempt to recreate nanoscale topographical and biofactor cues from the extracellular environment are emerging as interesting candidate biomimetic materials.

Colloidal nanoparticles as advanced biological sensors

Background Nanoparticle biosensors have the potential to enhance or supersede current analytical techniques, and their introduction could have a great impact in research and clinical practice. The unique optical properties of many nanomaterials make them ideal for biosensing. Colloidal fluorescent and plasmonic nanoparticles are particularly interesting, as they produce intense responses to incident light, and linking this response to the presence of a target analyte yields extremely sensitive detection in solution. Optimization of nanoparticle biosensors by considering the nanoparticle core, surface, and sensing properties, with an eye toward their application in research and as clinical tools. This translational process relies on the availability of high-quality nanoparticles with precisely engineered surfaces and biosensing mechanisms that allow detection with high sensitivity and specificity. Advances Much research has centered on fluorescent quantum dots (QDs) and plasmonic gold nanoparticles (AuNPs) and has expanded to include carbon dots, silicon dots, upconverting nanoparticles, alloyed plasmonic nanoparticles, and gold and silver nanoclusters, among a constantly growing repertoire. New tools for probing the nucleation and growth of nanoparticles, such as in situ synchrotron x-ray irradiation, liquid cell transmission electron microscopy, and computer simulation, are revealing ever more about fundamental processes—for example, the importance of particle coalescence and surface ligand conformation during particle growth. Precision engineering of particle surfaces is required to construct advanced nanoparticle biosensors, and this is being facilitated by a new generation of modular small-molecule and polymeric capping agents, the incorporation of new high-yield bio-orthogonal “click”-like functional groups, and advanced engineering of biomolecular sensing constructs (e.g., by recombinant protein engineering). Notable successes in the field include the precision synthesis of nanoparticles in microfluidic systems; ultrasensitive detection of cancer biomarkers in human serum with time-gated QD fluorescence; multiplexed intracellular sensing of mRNA using superquenching AuNPs; multiplexed detection of analytes with simple technologies such as smartphones; in vivo sensing of reactive oxygen species and methyl mercury; and the integration of nanoparticle biosensors with advanced DNA/RNA target amplification protocols. Outlook Many advanced applications are anticipated for nanoparticle biosensors, including as research analytical tools, intracellular sensors, and in vivo sensors for real-time detection and visualization of analytes and structures inside the body. There has been great success in synthesizing nanoparticles with excellent physical and chemical properties and in demonstrating their applications as biosensors. Looking toward high-impact applications, the main challenges are to develop techniques to synthesize reproducible high-quality nanoparticle biosensors on a mass scale and to maximize performance in physiological conditions. By drawing on many fields, including molecular biology, bioinformatics, computer modeling, bioengineering, and applied physics, we can compile the diverse toolset required to overcome these challenges and move toward high-impact applications of nanoparticle biosensors. Biological sensing using nanoparticles Colloidal fluorescent and plasmonic nanoparticles yield intense responses to incident light, making them useful as sensors or probes for sensitive detection in solution. Howes et al. review the potential uses of nanoparticle biosensors in research and diagnostics. A range of methods allow for the chemical modification of the particle surfaces so that they can be tuned for specific analytes and give optical signals for a range of biological conditions of interest. Signals can be detected in complex media or in vivo making the particles of interest for both laboratory research and in clinical settings. Science, this issue 10.1126/science.1247390 Colloidal nanoparticle biosensors have received intense scientific attention and offer promising applications in both research and medicine. We review the state of the art in nanoparticle development, surface chemistry, and biosensing mechanisms, discussing how a range of technologies are contributing toward commercial and clinical translation. Recent examples of success include the ultrasensitive detection of cancer biomarkers in human serum and in vivo sensing of methyl mercury. We identify five key materials challenges, including the development of robust mass-scale nanoparticle synthesis methods, and five broader challenges, including the use of simulations and bioinformatics-driven experimental approaches for predictive modeling of biosensor performance. The resultant generation of nanoparticle biosensors will form the basis of high-performance analytical assays, effective multiplexed intracellular sensors, and sophisticated in vivo probes.

The effects of strontium-substituted bioactive glasses on osteoblasts and osteoclasts in vitro.

Bioactive glasses (BG) which contain strontium have the potential to combine the known bone regenerative properties of BG with the anabolic and anti-catabolic effects of strontium cations. Here we created a BG series (SiO(2)-P(2)O(5)-Na(2)O-CaO) in which 0-100% of the calcium was substituted by strontium and tested their effects on osteoblasts and osteoclasts in vitro. We show that ions released from strontium-substituted BG enhance metabolic activity in osteoblasts. They also inhibit osteoclast activity by both reducing tartrate resistant acid phosphatase activity and inhibiting resorption of calcium phosphate films in a dose-dependent manner. Additionally, osteoblasts cultured in contact with BG show increased proliferation and alkaline phosphatase activity with increasing strontium substitution, while osteoclasts adopt typical resorption morphologies. These results suggest that similarly to the osteoporosis drug strontium ranelate, strontium-substituted BG may promote an anabolic effect on osteoblasts and an anti-catabolic effect on osteoclasts. These effects, when combined with the advantages of BG such as controlled ion release and delivery versatility, may make strontium-substituted BG an effective biomaterial choice for a range of bone regeneration therapies.

Enzyme-responsive nanoparticles for drug release and diagnostics ☆

Enzymes are key components of the bionanotechnology toolbox that possess exceptional biorecognition capabilities and outstanding catalytic properties. When combined with the unique physical properties of nanomaterials, the resulting enzyme-responsive nanoparticles can be designed to perform functions efficiently and with high specificity for the triggering stimulus. This powerful concept has been successfully applied to the fabrication of drug delivery schemes where the tissue of interest is targeted via release of cargo triggered by the biocatalytic action of an enzyme. Moreover, the chemical transformation of the carrier by the enzyme can also generate therapeutic molecules, therefore paving the way to design multimodal nanomedicines with synergistic effects. Dysregulation of enzymatic activity has been observed in a number of severe pathological conditions, and this observation is useful not only to program drug delivery in vivo but also to fabricate ultrasensitive sensors for diagnosing these diseases. In this review, several enzyme-responsive nanomaterials such as polymer-based nanoparticles, liposomes, gold nanoparticles and quantum dots are introduced, and the modulation of their physicochemical properties by enzymatic activity emphasized. When known, toxicological issues related to the utilization nanomaterials are highlighted. Key examples of enzyme-responsive nanomaterials for drug delivery and diagnostics are presented, classified by the type of effector biomolecule, including hydrolases such as proteases, lipases and glycosidases, and oxidoreductases.

Plasmonic nanosensors with inverse sensitivity by means of enzyme-guided crystal growth

Lowering the limit of detection is key to the design of sensors needed for food safety regulations, environmental policies and the diagnosis of severe diseases. However, because conventional transducers generate a signal that is directly proportional to the concentration of the target molecule, ultralow concentrations of the molecule result in variations in the physical properties of the sensor that are tiny, and therefore difficult to detect with confidence. Here we present a signal-generation mechanism that redefines the limit of detection of nanoparticle sensors by inducing a signal that is larger when the target molecule is less concentrated. The key step to achieve this inverse sensitivity is to use an enzyme that controls the rate of nucleation of silver nanocrystals on plasmonic transducers. We demonstrate the outstanding sensitivity and robustness of this approach by detecting the cancer biomarker prostate-specific antigen down to 10(-18) g ml(-1) (4 × 10(-20) M) in whole serum.

Biofunctionalization of Biomaterials for Accelerated in Situ Endothelialization: A Review

The higher patency rates of cardiovascular implants, including vascular bypass grafts, stents, and heart valves are related to their ability to inhibit thrombosis, intimal hyperplasia, and calcification. In native tissue, the endothelium plays a major role in inhibiting these processes. Various bioengineering research strategies thereby aspire to induce endothelialization of graft surfaces either prior to implantation or by accelerating in situ graft endothelialization. This article reviews potential bioresponsive molecular components that can be incorporated into (and/or released from) biomaterial surfaces to obtain accelerated in situ endothelialization of vascular grafts. These molecules could promote in situ endothelialization by the mobilization of endothelial progenitor cells (EPC) from the bone marrow, encouraging cell-specific adhesion (endothelial cells (EC) and/or EPC) to the graft and, once attached, by controlling the proliferation and differentiation of these cells. EC and EPC interactions with the extracellular matrix continue to be a principal source of inspiration for material biofunctionalization, and therefore, the latest developments in understanding these interactions will be discussed.

Technologies for global health

Institute for Global Health Innovation (L Conteh PhD, Prof A Darzi FRCS, P Howitt MA, K Kerr PhD, Prof S Matlin DSc, R Merrifi eld PhD, Prof G-Z Yang PhD), Centre for Environmental Policy (E Wilson MSc), Centre for Health Policy (D King MRCS, M Kulendran MRCS, Prof P C Smith BA), Department of Bioengineering (Prof A M J Bull PhD, Prof R A Malkin PhD, Prof M M Stevens PhD), Department of Civil and Environmental Engineering (M R Templeton PhD), Department of Infectious Diseases (G S Cooke PhD, N Ford PhD, S D Reid PhD), Department of Infectious Disease Epidemiology (S A J Gregson PhD), Department of Materials (Prof M M Stevens), Department of Medicine (A Blakemore PhD), Department of Primary Care & Public Health (Prof A Majeed MD), Department of Surgery and Cancer (H Ashrafi an MRCS, Prof C Vincent PhD), Faculty of Medicine (Prof R Atun FRCP), Global eHealth Unit (J Car PhD), Imperial College Business School (Prof R Atun FRCP, Prof J Barlow PhD), and Imperial Innovations (HA Penfold PhD), Imperial College London, London, UK Technologies for global health

Exploring and exploiting chemistry at the cell surface

Engineering the surface chemistry of a material so that it can interface with cells is an extraordinarily demanding task. The surface of a cell is composed of thousands of different lipids, proteins and carbohydrates, all intricately (and dynamically) arranged in three dimensions on multiple length scales. This complexity presents both a challenge and an opportunity to chemists working on bioactive interfaces. Here we discuss how some of these challenges can be met with interdisciplinary material synthesis. We also review the most popular classes of functional molecules grafted on engineered surfaces and explore some alternatives that may offer greater flexibility and specificity. Finally, we discuss the emerging field of dynamic surfaces capable of stimulating and responding to cellular activity in real time.