3D printed microfluidics: advances in strategies, integration, and applications

The ability to construct multiplexed micro-systems for fluid regulation could substantially impact multiple fields, including chemistry, biology, biomedicine, tissue engineering, and soft robotics, among others. 3D printing is gaining traction as a compelling approach to fabricating microfluidic devices by providing unique capabilities, such as 1) rapid design iteration and prototyping, 2) the potential for automated manufacturing and alignment, 3) the incorporation of numerous classes of materials within a single platform, and 4) the integration of 3D microstructures with prefabricated devices, sensing arrays, and nonplanar substrates. However, to widely deploy 3D printed microfluidics at research and commercial scales, critical issues related to printing factors, device integration strategies, and incorporation of multiple functionalities require further development and optimization. In this review, we summarize important figures of merit of 3D printed microfluidics and inspect recent progress in the field, including ink properties, structural resolutions, and hierarchical levels of integration with functional platforms. Particularly, we highlight advances in microfluidic devices printed with thermosetting elastomers, printing methodologies with enhanced degrees of automation and resolution, and the direct printing of microfluidics on various 3D surfaces. The substantial progress in the performance and multifunctionality of 3D printed microfluidics suggests a rapidly approaching era in which these versatile devices could be untethered from microfabrication facilities and created on demand by users in arbitrary settings with minimal prior training.

A Bionic Testbed for Cardiac Ablation Tools

Bionic-engineered tissues have been proposed for testing the performance of cardiovascular medical devices and predicting clinical outcomes ex vivo. Progress has been made in the development of compliant electronics that are capable of monitoring treatment parameters and being coupled to engineered tissues; however, the scale of most engineered tissues is too small to accommodate the size of clinical-grade medical devices. Here, we show substantial progress toward bionic tissues for evaluating cardiac ablation tools by generating a centimeter-scale human cardiac disk and coupling it to a hydrogel-based soft-pressure sensor. The cardiac tissue with contiguous electromechanical function was made possible by our recently established method to 3D bioprint human pluripotent stem cells in an extracellular matrix-based bioink that allows for in situ cell expansion prior to cardiac differentiation. The pressure sensor described here utilized electrical impedance tomography to enable the real-time spatiotemporal mapping of pressure distribution. A cryoablation tip catheter was applied to the composite bionic tissues with varied pressure. We found a close correlation between the cell response to ablation and the applied pressure. Under some conditions, cardiomyocytes could survive in the ablated region with more rounded morphology compared to the unablated controls, and connectivity was disrupted. This is the first known functional characterization of living human cardiomyocytes following an ablation procedure that suggests several mechanisms by which arrhythmia might redevelop following an ablation. Thus, bionic-engineered testbeds of this type can be indicators of tissue health and function and provide unique insight into human cell responses to ablative interventions.

3D Printed Skin-Interfaced UV-Visible Hybrid Photodetectors

Photodetectors that are intimately interfaced with human skin and measure real-time optical irradiance are appealing in the medical profiling of photosensitive diseases. Developing compliant devices for this purpose requires the fabrication of photodetectors with ultraviolet (UV)-enhanced broadband photoresponse and high mechanical flexibility, to ensure precise irradiance measurements across the spectral band critical to dermatological health when directly applied onto curved skin surfaces. Here, a fully 3D printed flexible UV-visible photodetector array is reported that incorporates a hybrid organic-inorganic material system and is integrated with a custom-built portable console to continuously monitor broadband irradiance in-situ. The active materials are formulated by doping polymeric photoactive materials with zinc oxide nanoparticles in order to improve the UV photoresponse and trigger a photomultiplication (PM) effect. The ability of a stand-alone skin-interfaced light intensity monitoring system to detect natural irradiance within the wavelength range of 310-650 nm for nearly 24 h is demonstrated.

3D-printed flexible organic light-emitting diode displays

The ability to fully 3D-print active electronic and optoelectronic devices will enable unique device form factors via strategies untethered from conventional microfabrication facilities. Currently, the performance of 3D-printed optoelectronics can suffer from nonuniformities in the solution-deposited active layers and unstable polymer-metal junctions. Here, we demonstrate a multimodal printing methodology that results in fully 3D-printed flexible organic light-emitting diode displays. The electrodes, interconnects, insulation, and encapsulation are all extrusion-printed, while the active layers are spray-printed. Spray printing leads to improved layer uniformity via suppression of directional mass transport in the printed droplets. By exploiting the viscoelastic oxide surface of the printed cathode droplets, a mechanical reconfiguration process is achieved to increase the contact area of the polymer-metal junctions. The uniform cathode array is intimately interfaced with the top interconnects. This hybrid approach creates a fully 3D-printed flexible 8 × 8 display with all pixels turning on successfully.

In Science Journals

Highlights from the Science family of journals

Conduction Cooling and Plasmonic Heating Dramatically Increase Droplet Vitrification Volumes for Cell Cryopreservation

Droplet vitrification has emerged as a promising ice-free cryopreservation approach to provide a supply chain for off-the-shelf cell products in cell therapy and regenerative medicine applications. Translation of this approach requires the use of low concentration (i.e., low toxicity) permeable cryoprotectant agents (CPA) and high post cryopreservation viability (>90%), thereby demanding fast cooling and warming rates. Unfortunately, with traditional approaches using convective heat transfer, the droplet volumes that can be successfully vitrified and rewarmed are impractically small (i.e., 180 picoliter) for <2.5 m permeable CPA. Herein, a novel approach to achieve 90-95% viability in micro-liter size droplets with 2 m permeable CPA, is presented. Droplets with plasmonic gold nanorods (GNRs) are printed onto a cryogenic copper substrate for improved cooling rates via conduction, while plasmonic laser heating yields >400-fold improvement in warming rates over traditional convective approach. High viability cryopreservation is then demonstrated in a model cell line (human dermal fibroblasts) and an important regenerative medicine cell line (human umbilical cord blood stem cells). This approach opens a new paradigm for cryopreservation and rewarming of dramatically larger volume droplets at lower CPA concentration for cell therapy and other regenerative medicine applications.

3D printed self-supporting elastomeric structures for multifunctional microfluidics

Microfluidic devices fabricated via soft lithography have demonstrated compelling applications such as lab-on-a-chip diagnostics, DNA microarrays, and cell-based assays. These technologies could be further developed by directly integrating microfluidics with electronic sensors and curvilinear substrates as well as improved automation for higher throughput. Current additive manufacturing methods, such as stereolithography and multi-jet printing, tend to contaminate substrates with uncured resins or supporting materials during printing. Here, we present a printing methodology based on precisely extruding viscoelastic inks into self-supporting microchannels and chambers without requiring sacrificial materials. We demonstrate that, in the submillimeter regime, the yield strength of the as-extruded silicone ink is sufficient to prevent creep within a certain angular range. Printing toolpaths are specifically designed to realize leakage-free connections between channels and chambers, T-shaped intersections, and overlapping channels. The self-supporting microfluidic structures enable the automatable fabrication of multifunctional devices, including multimaterial mixers, microfluidic-integrated sensors, automation components, and 3D microfluidics.

3D printed patient-specific aortic root models with internal sensors for minimally invasive applications

Minimally invasive surgeries have numerous advantages, yet complications may arise from limited knowledge about the anatomical site targeted for the delivery of therapy. Transcatheter aortic valve replacement (TAVR) is a minimally invasive procedure for treating aortic stenosis. Here, we demonstrate multimaterial three-dimensional printing of patient-specific soft aortic root models with internally integrated electronic sensor arrays that can augment testing for TAVR preprocedural planning. We evaluated the efficacies of the models by comparing their geometric fidelities with postoperative data from patients, as well as their in vitro hemodynamic performances in cases with and without leaflet calcifications. Furthermore, we demonstrated that internal sensor arrays can facilitate the optimization of bioprosthetic valve selections and in vitro placements via mapping of the pressures applied on the critical regions of the aortic anatomies. These models may pave exciting avenues for mitigating the risks of postoperative complications and facilitating the development of next-generation medical devices.

3D printed deformable sensors

The ability to directly print compliant biomedical devices on live human organs could benefit patient monitoring and wound treatment, which requires the 3D printer to adapt to the various deformations of the biological surface. We developed an in situ 3D printing system that estimates the motion and deformation of the target surface to adapt the toolpath in real time. With this printing system, a hydrogel-based sensor was printed on a porcine lung under respiration-induced deformation. The sensor was compliant to the tissue surface and provided continuous spatial mapping of deformation via electrical impedance tomography. This adaptive 3D printing approach may enhance robot-assisted medical treatments with additive manufacturing capabilities, enabling autonomous and direct printing of wearable electronics and biological materials on and inside the human body.

In Situ Expansion, Differentiation, and Electromechanical Coupling of Human Cardiac Muscle in a 3D Bioprinted, Chambered Organoid

RATIONALE

One goal of cardiac tissue engineering is the generation of a living, human pump in vitro that could replace animal models and eventually serve as an in vivo therapeutic. Models that replicate the geometrically complex structure of the heart, harboring chambers and large vessels with soft biomaterials, can be achieved using 3-dimensional bioprinting. Yet, inclusion of contiguous, living muscle to support pump function has not been achieved. This is largely due to the challenge of attaining high densities of cardiomyocytes-a notoriously nonproliferative cell type. An alternative strategy is to print with human induced pluripotent stem cells, which can proliferate to high densities and fill tissue spaces, and subsequently differentiate them into cardiomyocytes in situ.

OBJECTIVE

To develop a bioink capable of promoting human induced pluripotent stem cell proliferation and cardiomyocyte differentiation to 3-dimensionally print electromechanically functional, chambered organoids composed of contiguous cardiac muscle.

METHODS AND RESULTS

We optimized a photo-crosslinkable formulation of native ECM (extracellular matrix) proteins and used this bioink to 3-dimensionally print human induced pluripotent stem cell-laden structures with 2 chambers and a vessel inlet and outlet. After human induced pluripotent stem cells proliferated to a sufficient density, we differentiated the cells within the structure and demonstrated function of the resultant human chambered muscle pump. Human chambered muscle pumps demonstrated macroscale beating and continuous action potential propagation with responsiveness to drugs and pacing. The connected chambers allowed for perfusion and enabled replication of pressure/volume relationships fundamental to the study of heart function and remodeling with health and disease.

CONCLUSIONS

This advance represents a critical step toward generating macroscale tissues, akin to aggregate-based organoids, but with the critical advantage of harboring geometric structures essential to the pump function of cardiac muscle. Looking forward, human chambered organoids of this type might also serve as a test bed for cardiac medical devices and eventually lead to therapeutic tissue grafting.

3D Printed Neural Regeneration Devices

Neural regeneration devices interface with the nervous system and can provide flexibility in material choice, implantation without the need for additional surgeries, and the ability to serve as guides augmented with physical, biological (e.g., cellular), and biochemical functionalities. Given the complexity and challenges associated with neural regeneration, a 3D printing approach to the design and manufacturing of neural devices could provide next-generation opportunities for advanced neural regeneration via the production of anatomically accurate geometries, spatial distributions of cellular components, and incorporation of therapeutic biomolecules. A 3D printing-based approach offers compatibility with 3D scanning, computer modeling, choice of input material, and increasing control over hierarchical integration. Therefore, a 3D printed implantable platform could ultimately be used to prepare novel biomimetic scaffolds and model complex tissue architectures for clinical implants in order to treat neurological diseases and injuries. Further, the flexibility and specificity offered by 3D printed in vitro platforms have the potential to be a significant foundational breakthrough with broad research implications in cell signaling and drug screening for personalized healthcare. This progress report examines recent advances in 3D printing strategies for neural regeneration as well as insight into how these approaches can be improved in future studies.

Corrigendum to "3D printed bionic nanodevices" [Nano Today 11 (2016) 330-350]

[This corrects the article PMC5016035.].

3D Bioprinted In Vitro Metastatic Models via Reconstruction of Tumor Microenvironments

The development of 3D in vitro models capable of recapitulating native tumor microenvironments could improve the translatability of potential anticancer drugs and treatments. Here, 3D bioprinting techniques are used to build tumor constructs via precise placement of living cells, functional biomaterials, and programmable release capsules. This enables the spatiotemporal control of signaling molecular gradients, thereby dynamically modulating cellular behaviors at a local level. Vascularized tumor models are created to mimic key steps of cancer dissemination (invasion, intravasation, and angiogenesis), based on guided migration of tumor cells and endothelial cells in the context of stromal cells and growth factors. The utility of the metastatic models for drug screening is demonstrated by evaluating the anticancer efficacy of immunotoxins. These 3D vascularized tumor tissues provide a proof-of-concept platform to i) fundamentally explore the molecular mechanisms of tumor progression and metastasis, and ii) preclinically identify therapeutic agents and screen anticancer drugs.

3D Printed Stem-Cell Derived Neural Progenitors Generate Spinal Cord Scaffolds

A bioengineered spinal cord is fabricated via extrusion-based multi-material 3D bioprinting, in which clusters of induced pluripotent stem cell (iPSC)-derived spinal neuronal progenitor cells (sNPCs) and oligodendrocyte progenitor cells (OPCs) are placed in precise positions within 3D printed biocompatible scaffolds during assembly. The location of a cluster of cells, of a single type or multiple types, is controlled using a point-dispensing printing method with a 200 μm center-to-center spacing within 150 μm wide channels. The bioprinted sNPCs differentiate and extend axons throughout microscale scaffold channels, and the activity of these neuronal networks is confirmed by physiological spontaneous calcium flux studies. Successful bioprinting of OPCs in combination with sNPCs demonstrates a multicellular neural tissue engineering approach, where the ability to direct the patterning and combination of transplanted neuronal and glial cells can be beneficial in rebuilding functional axonal connections across areas of central nervous system (CNS) tissue damage. This platform can be used to prepare novel biomimetic, hydrogel-based scaffolds modeling complex CNS tissue architecture in vitro and harnessed to develop new clinical approaches to treat neurological diseases, including spinal cord injury.

3D Printed Polymer Photodetectors

Extrusion-based 3D printing, an emerging technology, has been previously used in the comprehensive fabrication of light-emitting diodes using various functional inks, without cleanrooms or conventional microfabrication techniques. Here, polymer-based photodetectors exhibiting high performance are fully 3D printed and thoroughly characterized. A semiconducting polymer ink is printed and optimized for the active layer of the photodetector, achieving an external quantum efficiency of 25.3%, which is comparable to that of microfabricated counterparts and yet created solely via a one-pot custom built 3D-printing tool housed under ambient conditions. The devices are integrated into image sensing arrays with high sensitivity and wide field of view, by 3D printing interconnected photodetectors directly on flexible substrates and hemispherical surfaces. This approach is further extended to create integrated multifunctional devices consisting of optically coupled photodetectors and light-emitting diodes, demonstrating for the first time the multifunctional integration of multiple semiconducting device types which are fully 3D printed on a single platform. The 3D-printed optoelectronic devices are made without conventional microfabrication facilities, allowing for flexibility in the design and manufacturing of next-generation wearable and 3D-structured optoelectronics, and validating the potential of 3D printing to achieve high-performance integrated active electronic materials and devices.

3D Printed Organ Models for Surgical Applications

Medical errors are a major concern in clinical practice, suggesting the need for advanced surgical aids for preoperative planning and rehearsal. Conventionally, CT and MRI scans, as well as 3D visualization techniques, have been utilized as the primary tools for surgical planning. While effective, it would be useful if additional aids could be developed and utilized in particularly complex procedures involving unusual anatomical abnormalities that could benefit from tangible objects providing spatial sense, anatomical accuracy, and tactile feedback. Recent advancements in 3D printing technologies have facilitated the creation of patient-specific organ models with the purpose of providing an effective solution for preoperative planning, rehearsal, and spatiotemporal mapping. Here, we review the state-of-the-art in 3D printed, patient-specific organ models with an emphasis on 3D printing material systems, integrated functionalities, and their corresponding surgical applications and implications. Prior limitations, current progress, and future perspectives in this important area are also broadly discussed.

3D Printed Functional and Biological Materials on Moving Freeform Surfaces

Conventional 3D printing technologies typically rely on open-loop, calibrate-then-print operation procedures. An alternative approach is adaptive 3D printing, which is a closed-loop method that combines real-time feedback control and direct ink writing of functional materials in order to fabricate devices on moving freeform surfaces. Here, it is demonstrated that the changes of states in the 3D printing workspace in terms of the geometries and motions of target surfaces can be perceived by an integrated robotic system aided by computer vision. A hybrid fabrication procedure combining 3D printing of electrical connects with automatic pick-and-placing of surface-mounted electronic components yields functional electronic devices on a free-moving human hand. Using this same approach, cell-laden hydrogels are also printed on live mice, creating a model for future studies of wound-healing diseases. This adaptive 3D printing method may lead to new forms of smart manufacturing technologies for directly printed wearable devices on the body and for advanced medical treatments.

3D Printed Electrically-Driven Soft Actuators

Soft robotics is an emerging field enabled by advances in the development of soft materials with properties commensurate to their biological counterparts, for the purpose of reproducing locomotion and other distinctive capabilities of active biological organisms. The development of soft actuators is fundamental to the advancement of soft robots and bio-inspired machines. Among the different material systems incorporated in the fabrication of soft devices, ionic hydrogel-elastomer hybrids have recently attracted vast attention due to their favorable characteristics, including their analogy with human skin. Here, we demonstrate that this hybrid material system can be 3D printed as a soft dielectric elastomer actuator (DEA) with a unimorph configuration that is capable of generating high bending motion in response to an applied electrical stimulus. We characterized the device actuation performance via applied (i) ramp-up electrical input, (ii) cyclic electrical loading, and (iii) payload masses. A maximum vertical tip displacement of 9.78 ± 2.52 mm at 5.44 kV was achieved from the tested 3D printed DEAs. Furthermore, the nonlinear actuation behavior of the unimorph DEA was successfully modeled using analytical energetic formulation and a finite element method (FEM).

3D Printed Organ Models with Physical Properties of Tissue and Integrated Sensors

The design and development of novel methodologies and customized materials to fabricate patient-specific 3D printed organ models with integrated sensing capabilities could yield advances in smart surgical aids for preoperative planning and rehearsal. Here, we demonstrate 3D printed prostate models with physical properties of tissue and integrated soft electronic sensors using custom-formulated polymeric inks. The models show high quantitative fidelity in static and dynamic mechanical properties, optical characteristics, and anatomical geometries to patient tissues and organs. The models offer tissue-mimicking tactile sensation and behavior and thus can be used for the prediction of organ physical behavior under deformation. The prediction results show good agreement with values obtained from simulations. The models also allow the application of surgical and diagnostic tools to their surface and inner channels. Finally, via the conformal integration of 3D printed soft electronic sensors, pressure applied to the models with surgical tools can be quantitatively measured.

The Role of Nanoparticle Design in Determining Analytical Performance of Lateral Flow Immunoassays

Rapid, simple, and cost-effective diagnostics are needed to improve healthcare at the point of care (POC). However, the most widely used POC diagnostic, the lateral flow immunoassay (LFA), is ∼1000-times less sensitive and has a smaller analytical range than laboratory tests, requiring a confirmatory test to establish truly negative results. Here, a rational and systematic strategy is used to design the LFA contrast label (i.e., gold nanoparticles) to improve the analytical sensitivity, analytical detection range, and antigen quantification of LFAs. Specifically, we discovered that the size (30, 60, or 100 nm) of the gold nanoparticles is a main contributor to the LFA analytical performance through both the degree of receptor interaction and the ultimate visual or thermal contrast signals. Using the optimal LFA design, we demonstrated the ability to improve the analytical sensitivity by 256-fold and expand the analytical detection range from 3 log10 to 6 log10 for diagnosing patients with inflammatory conditions by measuring C-reactive protein. This work demonstrates that, with appropriate design of the contrast label, a simple and commonly used diagnostic technology can compete with more expensive state-of-the-art laboratory tests.

3D Printed Stretchable Tactile Sensors

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human-machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.

3D Printed Bionic Nanodevices

The ability to three-dimensionally interweave biological and functional materials could enable the creation of bionic devices possessing unique and compelling geometries, properties, and functionalities. Indeed, interfacing high performance active devices with biology could impact a variety of fields, including regenerative bioelectronic medicines, smart prosthetics, medical robotics, and human-machine interfaces. Biology, from the molecular scale of DNA and proteins, to the macroscopic scale of tissues and organs, is three-dimensional, often soft and stretchable, and temperature sensitive. This renders most biological platforms incompatible with the fabrication and materials processing methods that have been developed and optimized for functional electronics, which are typically planar, rigid and brittle. A number of strategies have been developed to overcome these dichotomies. One particularly novel approach is the use of extrusion-based multi-material 3D printing, which is an additive manufacturing technology that offers a freeform fabrication strategy. This approach addresses the dichotomies presented above by (1) using 3D printing and imaging for customized, hierarchical, and interwoven device architectures; (2) employing nanotechnology as an enabling route for introducing high performance materials, with the potential for exhibiting properties not found in the bulk; and (3) 3D printing a range of soft and nanoscale materials to enable the integration of a diverse palette of high quality functional nanomaterials with biology. Further, 3D printing is a multi-scale platform, allowing for the incorporation of functional nanoscale inks, the printing of microscale features, and ultimately the creation of macroscale devices. This blending of 3D printing, novel nanomaterial properties, and 'living' platforms may enable next-generation bionic systems. In this review, we highlight this synergistic integration of the unique properties of nanomaterials with the versatility of extrusion-based 3D printing technologies to interweave nanomaterials and fabricate novel bionic devices.

Correction: 3D printed nervous system on a chip

Correction for '3D printed nervous system on a chip' by Blake N. Johnson et al., Lab Chip, 2016, 16, 1393-1400.

3D printed nervous system on a chip

Bioinspired organ-level in vitro platforms are emerging as effective technologies for fundamental research, drug discovery, and personalized healthcare. In particular, models for nervous system research are especially important, due to the complexity of neurological phenomena and challenges associated with developing targeted treatment of neurological disorders. Here we introduce an additive manufacturing-based approach in the form of a bioinspired, customizable 3D printed nervous system on a chip (3DNSC) for the study of viral infection in the nervous system. Micro-extrusion 3D printing strategies enabled the assembly of biomimetic scaffold components (microchannels and compartmented chambers) for the alignment of axonal networks and spatial organization of cellular components. Physiologically relevant studies of nervous system infection using the multiscale biomimetic device demonstrated the functionality of the in vitro platform. We found that Schwann cells participate in axon-to-cell viral spread but appear refractory to infection, exhibiting a multiplicity of infection (MOI) of 1.4 genomes per cell. These results suggest that 3D printing is a valuable approach for the prototyping of a customized model nervous system on a chip technology.

3D Printed Anatomical Nerve Regeneration Pathways

An imaging-coupled 3D printing methodology for the design, optimization, and fabrication of a customized nerve repair technology for complex injuries is presented. The custom scaffolds are deterministically fabricated via a microextrusion printing principle which enables the simultaneous incorporation of anatomical geometries, biomimetic physical cues, and spatially controlled biochemical gradients in a one-pot 3D manufacturing approach.

3D Printed Programmable Release Capsules

The development of methods for achieving precise spatiotemporal control over chemical and biomolecular gradients could enable significant advances in areas such as synthetic tissue engineering, biotic-abiotic interfaces, and bionanotechnology. Living organisms guide tissue development through highly orchestrated gradients of biomolecules that direct cell growth, migration, and differentiation. While numerous methods have been developed to manipulate and implement biomolecular gradients, integrating gradients into multiplexed, three-dimensional (3D) matrices remains a critical challenge. Here we present a method to 3D print stimuli-responsive core/shell capsules for programmable release of multiplexed gradients within hydrogel matrices. These capsules are composed of an aqueous core, which can be formulated to maintain the activity of payload biomolecules, and a poly(lactic-co-glycolic) acid (PLGA, an FDA approved polymer) shell. Importantly, the shell can be loaded with plasmonic gold nanorods (AuNRs), which permits selective rupturing of the capsule when irradiated with a laser wavelength specifically determined by the lengths of the nanorods. This precise control over space, time, and selectivity allows for the ability to pattern 2D and 3D multiplexed arrays of enzyme-loaded capsules along with tunable laser-triggered rupture and release of active enzymes into a hydrogel ambient. The advantages of this 3D printing-based method include (1) highly monodisperse capsules, (2) efficient encapsulation of biomolecular payloads, (3) precise spatial patterning of capsule arrays, (4) "on the fly" programmable reconfiguration of gradients, and (5) versatility for incorporation in hierarchical architectures. Indeed, 3D printing of programmable release capsules may represent a powerful new tool to enable spatiotemporal control over biomolecular gradients.

3D printed quantum dot light-emitting diodes

Developing the ability to 3D print various classes of materials possessing distinct properties could enable the freeform generation of active electronics in unique functional, interwoven architectures. Achieving seamless integration of diverse materials with 3D printing is a significant challenge that requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. To date, 3D printing has been limited to specific plastics, passive conductors, and a few biological materials. Here, we show that diverse classes of materials can be 3D printed and fully integrated into device components with active properties. Specifically, we demonstrate the seamless interweaving of five different materials, including (1) emissive semiconducting inorganic nanoparticles, (2) an elastomeric matrix, (3) organic polymers as charge transport layers, (4) solid and liquid metal leads, and (5) a UV-adhesive transparent substrate layer. As a proof of concept for demonstrating the integrated functionality of these materials, we 3D printed quantum dot-based light-emitting diodes (QD-LEDs) that exhibit pure and tunable color emission properties. By further incorporating the 3D scanning of surface topologies, we demonstrate the ability to conformally print devices onto curvilinear surfaces, such as contact lenses. Finally, we show that novel architectures that are not easily accessed using standard microfabrication techniques can be constructed, by 3D printing a 2 × 2 × 2 cube of encapsulated LEDs, in which every component of the cube and electronics are 3D printed. Overall, these results suggest that 3D printing is more versatile than has been demonstrated to date and is capable of integrating many distinct classes of materials.

Biotemplated synthesis of PZT nanowires

Piezoelectric nanowires are an important class of smart materials for next-generation applications including energy harvesting, robotic actuation, and bioMEMS. Lead zirconate titanate (PZT), in particular, has attracted significant attention, owing to its superior electromechanical conversion performance. Yet, the ability to synthesize crystalline PZT nanowires with well-controlled properties remains a challenge. Applications of common nanosynthesis methods to PZT are hampered by issues such as slow kinetics, lack of suitable catalysts, and harsh reaction conditions. Here we report a versatile biomimetic method, in which biotemplates are used to define PZT nanostructures, allowing for rational control over composition and crystallinity. Specifically, stoichiometric PZT nanowires were synthesized using both polysaccharide (alginate) and bacteriophage templates. The wires possessed measured piezoelectric constants of up to 132 pm/V after poling, among the highest reported for PZT nanomaterials. Further, integrated devices can generate up to 0.820 μW/cm(2) of power. These results suggest that biotemplated piezoelectric nanowires are attractive candidates for stimuli-responsive nanosensors, adaptive nanoactuators, and nanoscale energy harvesters.

Tension-induced neurite growth in microfluidic channels

The generation of an effective method for stimulating neuronal growth in specific directions, along well-defined geometries, and in numerous cells could impact areas ranging from fundamental studies of neuronal evolution and morphogenesis, to applications in biomedical diagnostics and nerve regeneration. Applied mechanical stress can regulate neurite growth. Indeed, previous studies have shown that neuronal cells can develop and extend neurites with rapid growth rates under applied "towing" tensions imparted by micropipettes. Yet, such methods are complex and exhibit low throughputs, as the tension is applied serially to individual cells. Here we present a novel approach to inducing neurite growth in multiple cells in parallel, by using a miniaturized platform with numerous microchannels. Upon connection of a vacuum to these microchannels, tension can be applied on multiple cells simultaneously to induce the growth of neurites. A theoretical model was also developed to understand the effect of tension on the dynamics of neurite development.

Flexible piezoelectric PMN-PT nanowire-based nanocomposite and device

Piezoelectric nanocomposites represent a unique class of materials that synergize the advantageous features of polymers and piezoelectric nanostructures and have attracted extensive attention for the applications of energy harvesting and self-powered sensing recently. Currently, most of the piezoelectric nanocomposites were synthesized using piezoelectric nanostructures with relatively low piezoelectric constants, resulting in lower output currents and lower output voltages. Here, we report a synthesis of piezoelectric (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) nanowire-based nanocomposite with significantly improved performances for energy harvesting and self-powered sensing. With the high piezoelectric constant (d33) and the unique hierarchical structure of the PMN-PT nanowires, the PMN-PT nanowire-based nanocomposite demonstrated an output voltage up to 7.8 V and an output current up to 2.29 μA (current density of 4.58 μA/cm(2)); this output voltage is more than double that of other reported piezoelectric nanocomposites, and the output current is at least 6 times greater. The PMN-PT nanowire-based nanocomposite also showed a linear relationship of output voltage versus strain with a high sensitivity. The enhanced performance and the flexibility of the PMN-PT nanowire-based nanocomposite make it a promising building block for energy harvesting and self-powered sensing applications.

3D printed bionic ears

The ability to three-dimensionally interweave biological tissue with functional electronics could enable the creation of bionic organs possessing enhanced functionalities over their human counterparts. Conventional electronic devices are inherently two-dimensional, preventing seamless multidimensional integration with synthetic biology, as the processes and materials are very different. Here, we present a novel strategy for overcoming these difficulties via additive manufacturing of biological cells with structural and nanoparticle derived electronic elements. As a proof of concept, we generated a bionic ear via 3D printing of a cell-seeded hydrogel matrix in the anatomic geometry of a human ear, along with an intertwined conducting polymer consisting of infused silver nanoparticles. This allowed for in vitro culturing of cartilage tissue around an inductive coil antenna in the ear, which subsequently enables readout of inductively-coupled signals from cochlea-shaped electrodes. The printed ear exhibits enhanced auditory sensing for radio frequency reception, and complementary left and right ears can listen to stereo audio music. Overall, our approach suggests a means to intricately merge biologic and nanoelectronic functionalities via 3D printing.

Nanoscale flexoelectricity

Electromechanical effects are ubiquitous in biological and materials systems. Understanding the fundamentals of these coupling phenomena is critical to devising next-generation electromechanical transducers. Piezoelectricity has been studied in detail, in both the bulk and at mesoscopic scales. Recently, an increasing amount of attention has been paid to flexoelectricity: electrical polarization induced by a strain gradient. While piezoelectricity requires crystalline structures with no inversion symmetry, flexoelectricity does not carry this requirement, since the effect is caused by inhomogeneous strains. Flexoelectricity explains many interesting electromechanical behaviors in hard crystalline materials and underpins core mechanoelectric transduction phenomena in soft biomaterials. Most excitingly, flexoelectricity is a size-dependent effect which becomes more significant in nanoscale systems. With increasing interest in nanoscale and nano-bio hybrid materials, flexoelectricity will continue to gain prominence. This Review summarizes work in this area. First, methods to amplify or manipulate the flexoelectric effect to enhance material properties will be investigated, particularly at nanometer scales. Next, the nature and history of these effects in soft biomaterials will be explored. Finally, some theoretical interpretations for the effect will be presented. Overall, flexoelectricity represents an exciting phenomenon which is expected to become more considerable as materials continue to shrink.

Piezoelectric nanoribbons for monitoring cellular deformations

Methods for probing mechanical responses of mammalian cells to electrical excitations can improve our understanding of cellular physiology and function. The electrical response of neuronal cells to applied voltages has been studied in detail, but less is known about their mechanical response to electrical excitations. Studies using atomic force microscopes (AFMs) have shown that mammalian cells exhibit voltage-induced mechanical deflections at nanometre scales, but AFM measurements can be invasive and difficult to multiplex. Here we show that mechanical deformations of neuronal cells in response to electrical excitations can be measured using piezoelectric PbZr(x)Ti(1-x)O(3) (PZT) nanoribbons, and we find that cells deflect by 1 nm when 120 mV is applied to the cell membrane. The measured cellular forces agree with a theoretical model in which depolarization caused by an applied voltage induces a change in membrane tension, which results in the cell altering its radius so that the pressure remains constant across the membrane. We also transfer arrays of PZT nanoribbons onto a silicone elastomer and measure mechanical deformations on a cow lung that mimics respiration. The PZT nanoribbons offer a minimally invasive and scalable platform for electromechanical biosensing.

Biomimetic peptide nanosensors

The development of a miniaturized sensing platform tailored for sensitive and selective detection of a variety of biochemical analytes could offer transformative fundamental and technological opportunities. Due to their high surface-to-volume ratios, nanoscale materials are extremely sensitive sensors. Likewise, peptides represent robust substrates for selective recognition due to the potential for broad chemical diversity within their relatively compact size. Here we explore the possibilities of linking peptides to nanosensors for the selective detection of biochemical targets. Such systems raise a number of interesting fundamental challenges: What are the peptide sequences, and how can rational design be used to derive selective binders? What nanomaterials should be used, and what are some strategies for assembling hybrid nanosensors? What role does molecular modeling play in elucidating response mechanisms? What is the resulting performance of these sensors, in terms of sensitivity, selectivity, and response time? What are some potential applications? This Account will highlight our early attempts to address these research challenges. Specifically, we use natural peptide sequences or sequences identified from phage display as capture elements. The sensors are based on a variety of nanomaterials including nanowires, graphene, and carbon nanotubes. We couple peptides to the nanomaterial surfaces via traditional surface functionalization methods or self-assembly. Molecular modeling provides detailed insights into the hybrid nanostructure, as well as the sensor detection mechanisms. The peptide nanosensors can distinguish chemically camouflaged mixtures of vapors and detect chemical warfare agents with sensitivities as low as parts-per-billion levels. Finally, we anticipate future uses of this technology in biomedicine: for example, devices based on these sensors could detect disease from the molecular components in human breath. Overall, these results provide a novel platform for the development of highly sensitive and selective "nanoelectronic noses".

Graphene-based wireless bacteria detection on tooth enamel

Direct interfacing of nanosensors onto biomaterials could impact health quality monitoring and adaptive threat detection. Graphene is capable of highly sensitive analyte detection due to its nanoscale nature. Here we show that graphene can be printed onto water-soluble silk. This in turn permits intimate biotransfer of graphene nanosensors onto biomaterials, including tooth enamel. The result is a fully biointerfaced sensing platform, which can be tuned to detect target analytes. For example, via self-assembly of antimicrobial peptides onto graphene, we show bioselective detection of bacteria at single-cell levels. Incorporation of a resonant coil eliminates the need for onboard power and external connections. Combining these elements yields two-tiered interfacing of peptide-graphene nanosensors with biomaterials. In particular, we demonstrate integration onto a tooth for remote monitoring of respiration and bacteria detection in saliva. Overall, this strategy of interfacing graphene nanosensors with biomaterials represents a versatile approach for ubiquitous detection of biochemical targets.

Silk-based conformal, adhesive, edible food sensors

An array of passive metamaterial antennas fabricated on all protein-based silk substrates were conformally transferred and adhered to the surface of an apple. This process allows the opportunity for intimate contact of micro- and nanostructures that can probe, and accordingly monitor changes in, their surrounding environment. This provides in situ monitoring of food quality. It is to be noted that this type of sensor consists of all edible and biodegradable components, holding utility and potential relevance for healthcare and food/consumer products and markets.

Rapid, multiplexed microfluidic phage display

The development of a method for high-throughput, automated proteomic screening could impact areas ranging from fundamental molecular interactions to the discovery of novel disease markers and therapeutic targets. Surface display techniques allow for efficient handling of large molecular libraries in small volumes. In particular, phage display has emerged as a powerful technology for selecting peptides and proteins with enhanced, target-specific binding affinities. Yet, the process becomes cumbersome and time-consuming when multiple targets are involved. Here we demonstrate for the first time a microfluidic chip capable of identifying high affinity phage-displayed peptides for multiple targets in just a single round and without the need for bacterial infection. The chip is shown to be able to yield well-established control consensus sequences while simultaneously identifying new sequences for clinically important targets. Indeed, the confined parameters of the device allow not only for highly controlled assay conditions but also introduce a significant time-reduction to the phage display process. We anticipate that this easily-fabricated, disposable device has the potential to impact areas ranging from fundamental studies of protein, peptide, and molecular interactions, to applications such as fully automated proteomic screening.

Preferential binding of peptides to graphene edges and planes

Peptides identified from combinatorial peptide libraries have been shown to bind to a variety of abiotic surfaces. Biotic-abiotic interactions can be exploited to create hybrid materials with interesting electronic, optical, or catalytic properties. Here we show that peptides identified from a combinatorial phage display peptide library assemble preferentially to the edge or planar surface of graphene and can affect the electronic properties of graphene. Molecular dynamics simulations and experiments provide insight into the mechanism of peptide binding to the graphene edge.

Enhanced piezoelectricity and stretchability in energy harvesting devices fabricated from buckled PZT ribbons

The development of a method for integrating highly efficient energy conversion materials onto soft, biocompatible substrates could yield breakthroughs in implantable or wearable energy harvesting systems. Of particular interest are devices which can conform to irregular, curved surfaces, and operate in vital environments that may involve both flexing and stretching modes. Previous studies have shown significant advances in the integration of highly efficient piezoelectric nanocrystals on flexible and bendable substrates. Yet, such inorganic nanomaterials are mechanically incompatible with the extreme elasticity of elastomeric substrates. Here, we present a novel strategy for overcoming these limitations, by generating wavy piezoelectric ribbons on silicone rubber. Our results show that the amplitudes in the waves accommodate order-of-magnitude increases in maximum tensile strain without fracture. Further, local probing of the buckled ribbons reveals an enhancement in the piezoelectric effect of up to 70%, thus representing the highest reported piezoelectric response on a stretchable medium. These results allow for the integration of energy conversion devices which operate in stretching mode via reversible deformations in the wavy/buckled ribbons.

Wafer-scale nanopatterning and translation into high-performance piezoelectric nanowires

The development of a facile method for fabricating one-dimensional, precisely positioned nanostructures over large areas offers exciting opportunities in fundamental research and innovative applications. Large-scale nanofabrication methods have been restricted in accessibility due to their complexity and cost. Likewise, bottom-up synthesis of nanowires has been limited in methods to assemble these structures at precisely defined locations. Nanomaterials such as PbZr(x)Ti(1-x)O(3) (PZT) nanowires (NWs)--which may be useful for nonvolatile memory storage (FeRAM), nanoactuation, and nanoscale power generation--are difficult to synthesize without suffering from polycrystallinity or poor stoichiometric control. Here, we report a novel fabrication method which requires only low-resolution photolithography and electrochemical etching to generate ultrasmooth NWs over wafer scales. These nanostructures are subsequently used as patterning templates to generate PZT nanowires with the highest reported piezoelectric performance (d(eff) ∼ 145 pm/V). The combined large-scale nanopatterning with hierarchical assembly of functional nanomaterials could yield breakthroughs in areas ranging from nanodevice arrays to nanodevice powering.

Chemical functionalization of graphene enabled by phage displayed peptides

The development of a general approach for the nondestructive chemical and biological functionalization of graphene could expand opportunities for graphene in both fundamental studies and a variety of device platforms. Graphene is a delicate single-layer, two-dimensional network of carbon atoms whose properties can be affected by covalent modification. One method for functionalizing materials without fundamentally changing their inherent structure is using biorecognition moieties. In particular, oligopeptides are molecules containing a broad chemical diversity that can be achieved within a relatively compact size. Phage display is a dominant method for identifying peptides that possess enhanced selectivity toward a particular target. Here, we demonstrate a powerful yet benign approach for chemical functionalization of graphene via comprehensively screened phage displayed peptides. Our results show that graphene can be selectively recognized even in nanometer-defined strips. Further, modification of graphene with bifunctional peptides reveals both the ability to impart selective recognition of gold nanoparticles and the development of an ultrasensitive graphene-based TNT sensor. We anticipate that these results could open exciting opportunities in the use of graphene in fundamental biochemical recognition studies, as well as applications ranging from sensors to energy storage devices.

Electrical detection of pathogenic bacteria via immobilized antimicrobial peptides

The development of a robust and portable biosensor for the detection of pathogenic bacteria could impact areas ranging from water-quality monitoring to testing of pharmaceutical products for bacterial contamination. Of particular interest are detectors that combine the natural specificity of biological recognition with sensitive, label-free sensors providing electronic readout. Evolution has tailored antimicrobial peptides to exhibit broad-spectrum activity against pathogenic bacteria, while retaining a high degree of robustness. Here, we report selective and sensitive detection of infectious agents via electronic detection based on antimicrobial peptide-functionalized microcapacitive electrode arrays. The semiselective antimicrobial peptide magainin I--which occurs naturally on the skin of African clawed frogs--was immobilized on gold microelectrodes via a C-terminal cysteine residue. Significantly, exposing the sensor to various concentrations of pathogenic Escherichia coli revealed detection limits of approximately 1 bacterium/μL, a clinically useful detection range. The peptide-microcapacitive hybrid device was further able to demonstrate both Gram-selective detection as well as interbacterial strain differentiation, while maintaining recognition capabilities toward pathogenic strains of E. coli and Salmonella. Finally, we report a simulated "water-sampling" chip, consisting of a microfluidic flow cell integrated onto the hybrid sensor, which demonstrates real-time on-chip monitoring of the interaction of E. coli cells with the antimicrobial peptides. The combination of robust, evolutionarily tailored peptides with electronic read-out monitoring electrodes may open exciting avenues in both fundamental studies of the interactions of bacteria with antimicrobial peptides, as well as the practical use of these devices as portable pathogen detectors.

Recognition of patterned molecular ink with phage displayed peptides

The development of a reliable method for patterning and recognizing molecular inks could enable exciting avenues in fundamental research and novel applications. Phage display is a powerful method for identifying peptides that possess enhanced selectivity and binding affinity toward a variety of targets. Here, we demonstrate for the first time the immobilization and recognition of a small molecular ink with screened phage displayed peptides. Our approach is based on a unique mix of comprehensive phage displayed peptide screening processes, along with novel micropatterning techniques. These results, combined with the large variety of available inks and surface chemistries, could open up opportunities in cell biology, nanomaterials self-assembly, selective sensors, and even energy storage applications.

Piezoelectric ribbons printed onto rubber for flexible energy conversion

The development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials that function as sensors/actuators, bioMEMS devices, and energy converters. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. Here, we overcome these limitations by presenting a scalable and parallel process for transferring crystalline piezoelectric nanothick ribbons of lead zirconate titanate from host substrates onto flexible rubbers over macroscopic areas. Fundamental characterization of the ribbons by piezo-force microscopy indicates that their electromechanical energy conversion metrics are among the highest reported on a flexible medium. The excellent performance of the piezo-ribbon assemblies coupled with stretchable, biocompatible rubber may enable a host of exciting avenues in fundamental research and novel applications.

Peptide-nanowire hybrid materials for selective sensing of small molecules

The development of a miniaturized sensing platform for the selective detection of chemical odorants could stimulate exciting scientific and technological opportunities. Oligopeptides are robust substrates for the selective recognition of a variety of chemical and biological species. Likewise, semiconducting nanowires are extremely sensitive gas sensors. Here we explore the possibilities and chemistries of linking peptides to silicon nanowire sensors for the selective detection of small molecules. The silica surface of the nanowires is passivated with peptides using amide coupling chemistry. The peptide/nanowire sensors can be designed, through the peptide sequence, to exhibit orthogonal responses to acetic acid and ammonia vapors, and can detect traces of these gases from "chemically camouflaged" mixtures. Through both theory and experiment, we find that this sensing selectivity arises from both acid/base reactivity and from molecular structure. These results provide a model platform for what can be achieved in terms of selective and sensitive "electronic noses."

Si/a-Si core/shell nanowires as nonvolatile crossbar switches

Radial core/shell nanowires (NWs) represent an important class of nanoscale building blocks with substantial potential for exploring fundamental electronic properties and realizing novel device applications at the nanoscale. Here, we report the synthesis of crystalline silicon/amorphous silicon (Si/a-Si) core/shell NWs and studies of crossed Si/a-Si NW metal NW (Si/a-Si x M) devices and arrays. Room-temperature electrical measurements on single Si/a-Si x Ag NW devices exhibit bistable switching between high (off) and low (on) resistance states with well-defined switching threshold voltages, on/off ratios greater than 10(4), and current rectification in the on state. Temperature-dependent switching experiments suggest that rectification can be attributed to barriers to electric field-driven metal diffusion. Systematic studies of Si/a-Si x Ag NW devices show that (i) the bit size can be at least as small as 20 nm x 20 nm, (ii) the writing time is <100 ns, (iii) the retention time is >2 weeks, and (iv) devices can be switched >10(4) times without degradation in performance. In addition, studies of dense one-dimensional and two-dimensional Si/a-Si x Ag NW devices arrays fabricated on crystalline and plastic substrates show that elements within the arrays can be independently switched and read, and moreover that bends with radii of curvature as small as 0.3 cm cause little change in device characteristics. The Si/a-Si x Ag NW devices represent a highly scalable and promising nanodevice element for assembly and fabrication of dense nonvolatile memory and programmable nanoprocessors.

Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors

The development of a robust method for integrating high-performance semiconductors on flexible plastics could enable exciting avenues in fundamental research and novel applications. One area of vital relevance is chemical and biological sensing, which if implemented on biocompatible substrates, could yield breakthroughs in implantable or wearable monitoring systems. Semiconducting nanowires (and nanotubes) are particularly sensitive chemical sensors because of their high surface-to-volume ratios. Here, we present a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires are excellent field-effect transistors, and, as sensors, exhibit parts-per-billion sensitivity to NO2, a hazardous pollutant. We also use SiO2 surface chemistries to construct a 'nano-electronic nose' library, which can distinguish acetone and hexane vapours via distributed responses. The excellent sensing performance coupled with bendable plastic could open up opportunities in portable, wearable or even implantable sensors.

Nanotechnology: high-speed integrated nanowire circuits

Macroelectronic circuits made on substrates of glass or plastic could one day make computing devices ubiquitous owing to their light weight, flexibility and low cost. But these substrates deform at high temperatures so, until now, only semiconductors such as organics and amorphous silicon could be used, leading to poor performance. Here we present the use of low-temperature processes to integrate high-performance multi-nanowire transistors into logical inverters and fast ring oscillators on glass substrates. As well as potentially enabling powerful electronics to permeate all aspects of modern life, this advance could find application in devices such as low-cost radio-frequency tags and fully integrated high-refresh-rate displays.