Polymeric 3D Printed Functional Microcantilevers for Biosensing Applications

3D-Printed MEMS in Italy

MEMS devices are more and more commonly used as sensors, actuators, and microfluidic devices in different fields like electronics, opto-electronics, and biomedical engineering. Traditional fabrication technologies cannot meet the growing demand for device miniaturisation and fabrication time reduction, especially when customised devices are required. That is why additive manufacturing technologies are increasingly applied to MEMS. In this review, attention is focused on the Italian scenario in regard to 3D-printed MEMS, studying the techniques and materials used for their fabrication. To this aim, research has been conducted as follows: first, the commonly applied 3D-printing technologies for MEMS manufacturing have been illustrated, then some examples of 3D-printed MEMS have been reported. After that, the typical materials for these technologies have been presented, and finally, some examples of their application in MEMS fabrication have been described. In conclusion, the application of 3D-printing techniques, instead of traditional processes, is a growing trend in Italy, where some exciting and promising results have already been obtained, due to these new selected technologies and the new materials involved.

Three-Dimensional Printing of Triboelectric Nanogenerators by Digital Light Processing Technique for Mechanical Energy Harvesting

Triboelectric nanogenerators (TENGs) represent intriguing technology to harvest human mechanical movements for powering wearable and portable electronics. Differently, compared to conventional fabrication approaches, additive manufacturing can allow the fabrication of TENGs with good dimensional resolution, high reproducibility, and quick production processes and, in particular, the obtainment of complex and customized structures. Among 3D printing technologies, digital light processing (DLP) is well-known for being the most flexible to produce functional devices by controlling both the geometry and the different ingredients of printable resins. On the other hand, DLP was not exploited for TENG fabrication, and consequently, the knowledge of the performance of 3D printable materials as charge accumulators upon friction is limited. Here, the application of the DLP technique to the 3D printing of triboelectric nanogenerators is studied. First, several printable materials have been tested as triboelectric layers to define a triboelectric series of DLP 3D printable materials. Then, TENG devices with increased geometrical complexity were printed, showcasing the ability to harvest energy from human movement. The method presented in this work illustrates how the DLP may represent a valuable and flexible solution to fabricate triboelectric nanogenerators, also providing a triboelectric classification of the most common photocurable resins.

Chemistry in light-induced 3D printing

In the last few years, 3D printing has evolved from its original niche applications, such as rapid prototyping and hobbyists, towards many applications in industry, research and everyday life. This involved an evolution in terms of equipment, software and, most of all, in materials. Among the different available 3D printing technologies, the light activated ones need particular attention from a chemical point of view, since those are based on photocurable formulations and in situ rapid solidification via photopolymerization. In this article, the chemical aspects beyond the preparation of a formulation for light-induced 3D printing are analyzed and explained, aiming at giving more tools for the development of new photocurable materials that can be used for the fabrication of innovative 3D printable devices.

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Advances in Digital Light Processing of Hydrogels

Hydrogels, three-dimensional (3D) networks of hydrophilic polymers formed in water, are a significant type of soft matter used in fundamental and applied sciences. Hydrogels are of particular interest for biomedical applications, owing to their soft elasticity and good biocompatibility. However, the high water content and soft nature of hydrogels often make it difficult to process them into desirable solid forms. The development of 3D printing (3DP) technologies has provided opportunities for the manufacturing of hydrogels, by adopting a freeform fabrication method. Owing to its high printing speed and resolution, vat photopolymerization 3DP has recently attracted considerable interest for hydrogel fabrication, with digital light processing (DLP) becoming a widespread representative technique. Whilst acknowledging that other types of vat photopolymerization 3DP have also been applied for this purpose, we here only focus on DLP and its derivatives. In this review, we first comprehensively outline the most recent advances in both materials and fabrication, including the adaptation of novel hydrogel systems and advances in processing (e.g., volumetric printing and multimaterial integration). Secondly, we summarize the applications of hydrogel DLP, including regenerative medicine, functional microdevices, and soft robotics. To the best of our knowledge, this is the first time that either of these specific review focuses has been adopted in the literature. More importantly, we discuss the major challenges associated with hydrogel DLP and provide our perspectives on future trends. To summarize, this review aims to aid and inspire other researchers investigatng DLP, photocurable hydrogels, and the research fields related to them.

Biomarkers and Detection Platforms for Human Health and Performance Monitoring: A Review

Human health and performance monitoring (HHPM) is imperative to provide information necessary for protecting, sustaining, evaluating, and improving personnel in various occupational sectors, such as industry, academy, sports, recreation, and military. While various commercially wearable sensors are on the market with their capability of "quantitative assessments" on human health, physical, and psychological states, their sensing is mostly based on physical traits, and thus lacks precision in HHPM. Minimally or noninvasive biomarkers detectable from the human body, such as body fluid (e.g., sweat, tear, urine, and interstitial fluid), exhaled breath, and skin surface, can provide abundant additional information to the HHPM. Detecting these biomarkers with novel or existing sensor technologies is emerging as critical human monitoring research. This review provides a broad perspective on the state of the art biosensor technologies for HHPM, including the list of biomarkers and their physiochemical/physical characteristics, fundamental sensing principles, and high-performance sensing transducers. Further, this paper expands to the additional scope on the key technical challenges in applying the current HHPM system to the real field.

Light manipulation for fabrication of hydrogels and their biological applications

The development of biocompatible materials with desired functions is essential for tissue engineering and biomedical applications. Hydrogels prepared from these materials represent an important class of soft matter for mimicking extracellular environments. In particular, dynamic hydrogels with responsiveness to environments are quite appealing because they can match the dynamics of biological processes. Among the external stimuli that can trigger responsive hydrogels, light is considered as a clean stimulus with high spatiotemporal resolution, complete bioorthogonality, and fine tunability regarding its wavelength and intensity. Therefore, photoresponsiveness has been broadly encoded in hydrogels for biological applications. Moreover, light can be used to initiate gelation during the fabrication of biocompatible hydrogels. Here, we present a critical review of light manipulation tools for the fabrication of hydrogels and for the regulation of physicochemical properties and functions of photoresponsive hydrogels. The materials, photo-initiated chemical reactions, and new prospects for light-induced gelation are introduced in the former part, while mechanisms to render hydrogels photoresponsive and their biological applications are discussed in the latter part. Subsequently, the challenges and potential research directions in this area are discussed, followed by a brief conclusion. STATEMENT OF SIGNIFICANCE: Hydrogels play a vital role in the field of biomaterials owing to their water retention ability and biocompatibility. However, static hydrogels cannot meet the dynamic requirements of the biomedical field. As a stimulus with high spatiotemporal resolution, light is an ideal tool for both the fabrication and operation of hydrogels. In this review, light-induced hydrogelation and photoresponsive hydrogels are discussed in detail, and new prospects and emerging biological applications are described. To inspire more research studies in this promising area, the challenges and possible solutions are also presented.

Reaching silicon-based NEMS performances with 3D printed nanomechanical resonators

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Young’s modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors.

NEMS devices, nano-electro-mechanical systems, by virtue of their minute size, offer ultra-high sensitivity, though at the expense of manufacturing complexity. Here, Stassi et al succeed in manufacturing high quality factor NEMS devices using high resolution 3D printing.

Review of 3D-Printed functionalized devices for chemical and biochemical analysis

Recent developments in three-dimensional printing (3DP) have attracted the attention of analytical scientists interested in fabricating 3D devices having promising geometric functions to achieve desirable analytical performance. To break through the barrier of limited availability of 3DP materials and to extend the chemical reactivity and functionalities of devices manufactured using conventional 3DP, new approaches are being developed for the functionalization of 3D-printed devices for chemical and biochemical analysis. This Review discusses recent advances in the chemical functionalization schemes used in the main 3DP technologies, including (i) post-printing modification and surface immobilization of reactive substances on printed materials, (ii) pre-printing incorporation of reactive substances into raw printing materials, and (iii) combinations of both strategies, and their effects on the selectivity and/or sensitivity of related analytical methods. In addition, the state of the art of 3D-printed functionalized analytical devices for enzymatic derivatization and sensing, electrochemical sensing, and sample pretreatment applications are also reviewed, highlighting the importance of introducing new functional and functionalized materials to facilitate future 3DP-enabled manufacturing of multifunctional analytical devices.

3D Printing of PDMS-Like Polymer Nanocomposites with Enhanced Thermal Conductivity: Boron Nitride Based Photocuring System

This study demonstrates the possibility of forming 3D structures with enhanced thermal conductivity (k) by vat printing a silicone-acrylate based nanocomposite. Polydimethylsiloxane (PDSM) represent a common silicone-based polymer used in several applications from electronics to microfluidics. Unfortunately, the k value of the polymer is low, so a composite is required to be formed in order to increase its thermal conductivity. Several types of fillers are available to reach this result. In this study, boron nitride (BN) nanoparticles were used to increase the thermal conductivity of a PDMS-like photocurable matrix. A digital light processing (DLP) system was employed to form complex structures. The viscosity of the formulation was firstly investigated; photorheology and attenuate total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) analyses were done to check the reactivity of the system that resulted as suitable for DLP printing. Mechanical and thermal analyses were performed on printed samples through dynamic mechanical thermal analysis (DMTA) and tensile tests, revealing a positive effect of the BN nanoparticles. Morphological characterization was performed by scanning electron microscopy (SEM). Finally, thermal analysis demonstrated that the thermal conductivity of the material was improved, maintaining the possibility of producing 3D printable formulations.

Functional 3D printing: Approaches and bioapplications

3D printing technology has become a mature manufacturing technique, widely used for its advantages over the traditional methods, such as the end-user customization and rapid prototyping, useful in different application fields, including the biomedical one. Indeed, it represents a helpful tool for the realization of biodevices (i.e. biosensors, microfluidic bioreactors, drug delivery systems and Lab-On-Chip). In this perspective, the development of 3D printable materials with intrinsic functionalities, through the so-called 4D printing, introduces novel opportunities for the fabrication of "smart" or stimuli-responsive devices. Indeed, functional 3D printable materials can modify their surfaces, structures, properties or even shape in response to specific stimuli (such as pressure, temperature or light radiation), adding to the printed object new interesting properties exploited after the fabrication process. In this context, by combining 3D printing technology with an accurate materials' design, functional 3D objects with built-in (bio)chemical functionalities, having biorecognition, biocatalytic and drug delivery capabilities are here reported.

Materials Testing for the Development of Biocompatible Devices through Vat-Polymerization 3D Printing

Light-based 3D printing techniques could be a valuable instrument in the development of customized and affordable biomedical devices, basically for high precision and high flexibility in terms of materials of these technologies. However, more studies related to the biocompatibility of the printed objects are required to expand the use of these techniques in the health sector. In this work, 3D printed polymeric parts are produced in lab conditions using a commercial Digital Light Processing (DLP) 3D printer and then successfully tested to fabricate components suitable for biological studies. For this purpose, different 3D printable formulations based on commercially available resins are compared. The biocompatibility of the 3D printed objects toward A549 cell line is investigated by adjusting the composition of the resins and optimizing post-printing protocols; those include washing in common solvents and UV post-curing treatments for removing unreacted and cytotoxic products. It is noteworthy that not only the selection of suitable materials but also the development of an adequate post-printing protocol is necessary for the development of biocompatible devices.

DLP 3D Printing Meets Lignocellulosic Biopolymers: Carboxymethyl Cellulose Inks for 3D Biocompatible Hydrogels

The development of new bio-based inks is a stringent request for the expansion of additive manufacturing towards the development of 3D-printed biocompatible hydrogels. Herein, methacrylated carboxymethyl cellulose (M-CMC) is investigated as a bio-based photocurable ink for digital light processing (DLP) 3D printing. CMC is chemically modified using methacrylic anhydride. Successful methacrylation is confirmed by ¹H NMR and FTIR spectroscopy. Aqueous formulations based on M-CMC/lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) photoinitiator and M-CMC/Dulbecco’s Modified Eagle Medium (DMEM)/LAP show high photoreactivity upon UV irradiation as confirmed by photorheology and FTIR. The same formulations can be easily 3D-printed through a DLP apparatus to produce 3D shaped hydrogels with excellent swelling ability and mechanical properties. Envisaging the application of the hydrogels in the biomedical field, cytotoxicity is also evaluated. The light-induced printing of cellulose-based hydrogels represents a significant step forward in the production of new DLP inks suitable for biomedical applications.

Control-Based 4D Printing: Adaptive 4D-Printed Systems

Building on the recent progress of four-dimensional (4D) printing to produce dynamic structures, this study aimed to bring this technology to the next level by introducing control-based 4D printing to develop adaptive 4D-printed systems with highly versatile multi-disciplinary applications, including medicine, in the form of assisted soft robots, smart textiles as wearable electronics and other industries such as agriculture and microfluidics. This study introduced and analysed adaptive 4D-printed systems with an advanced manufacturing approach for developing stimuli-responsive constructs that organically adapted to environmental dynamic situations and uncertainties as nature does. The adaptive 4D-printed systems incorporated synergic integration of three-dimensional (3D)-printed sensors into 4D-printing and control units, which could be assembled and programmed to transform their shapes based on the assigned tasks and environmental stimuli. This paper demonstrates the adaptivity of these systems via a combination of proprioceptive sensory feedback, modeling and controllers, as well as the challenges and future opportunities they present.

Micro/Nano fabricated cantilever based biosensor platform: A review and recent progress

Recent trends in biosensing research have motivated scientists and research professionals to investigate the development of miniaturized bioanalytical devices to make them portable, label-free and smaller in size. The performance of the cantilever-based devices which is one of the very important domains of sensitive field level detection has improved significantly with the development of new micro/nanofabrication technologies and surface functionalization techniques. The cantilevers have scaled down to Nano from micro-level and have become exceptionally sensitive and also have some anomalous associated properties due to the scale. In this review we have discussed about fundamental principles of cantilever operation, detection methods, and previous, present and future approaches of study through cantilever-based sensing platform. Other than that, we have also discussed the past major bio-sensing efforts through micro/nano cantilevers and about recent progress in the field.

A modular 3D printed lab-on-a-chip for early cancer detection

A functional polymeric 3D device is produced in a single step printing process using a stereolithography based 3D printer. The photocurable formulation is designed for introducing a controlled amount of carboxyl groups (-COOH), in order to perform a covalent immobilization of bioreceptors on the device. The effectiveness of the application is demonstrated by performing an immunoassay for the detection of protein biomarkers involved in angiogenesis, whose role is crucial in the onset of cancer and in the progressive metastatic behavior of tumors. The detection of angiogenesis biomarkers is necessary for an early diagnosis of the pathology, allowing the employment of a less invasive therapy for the patient. In particular, vascular endothelial growth factor and angiopoietin-2 biomarkers are detected with a limit of detection of 11 ng mL^-1 and 0.8 ng mL^-1, respectively. This study shows how 3D microfabrication techniques, material characterization, and device development could be combined to obtain an engineered polymeric chip with intrinsic tuned functionalities.

Tailored 3D printed micro-crystallization chip for versatile and high-efficiency droplet evaporative crystallization

Droplet evaporative crystallization on a micro-structured platform with limited interfacial area has potential applications in crystallization theory, bioengineering, and particle drug preparation. Here, an efficient and versatile approach is discussed for multiple drop-evaporative crystallization processes on a micro-crystallization chip fabricated via three-dimensional printing. A chip with limited interfacial area could be fabricated on a highly controlled crystallizer interface. During liquid injection, various drop locations and evaporative conditions can be used, which enables flexible and distinct crystallization processes. This reveals controlling mechanisms and identifies nucleation locations and growth paths. Various classic crystallization systems were introduced to evaluate the chip performance. Controlled nucleation and growth mechanisms at stable evaporative rates were revealed. From the final crystal morphologies, particle locations, and distributions, the effects of the initial concentration and droplet contact conditions at the triple-phase interface could be investigated with high adjustability. Moreover, the results can provide insights into the 'coffee ring' formation during evaporative crystallization, dendritic crystal growth, and hydrate crystallization mechanisms. In the limited microstructure, the capillary flow of a liquid drop can spontaneously drive the crystal distribution and morphology. Finally, incorrect liquid drop locations that led to unpredictable crystal formation and distributions were discussed to improve repeatability and efficiency. Applications include the manufacture of particle drugs and flow chemistry.

Thiol–yne chemistry for 3D printing: exploiting an off-stoichiometric route for selective functionalization of 3D objects

An alkyne monomer, bis(propargyl) fumarate, is synthesized and mixed to a thiol monomer to produce DLP-3D printable formulations. Using off-stoichiometric formulations it is possible to print functionalizable objects.

Three-Dimensional Printed Photoluminescent Polymeric Waveguides

In this work, we propose an innovative strategy for obtaining functional objects employing a light-activated three-dimensional (3D) printing process without affecting the materials' printability. In particular, a dye is a necessary ingredient in a formulation for a digital light processing 3D printing method to obtain precise and complex structures. Here, we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye toward different solvents in the printed structures, enabling the development of solvents' polarity sensors.

3D-Printing of Functional Biomedical Microdevices via Light- and Extrusion-Based Approaches

3D-printing is a powerful additive manufacturing tool, one that enables fabrication of biomedical devices and systems that would otherwise be challenging to create with more traditional methods such as machining or molding. Many different classes of 3D-printing technologies exist, most notably extrusion-based and light-based 3D-printers, which are popular in consumer markets, with advantages and limitations for each modality. The focus here is primarily on showcasing the ability of these 3D-printing platforms to create different types of functional biomedical microdevices-their advantages and limitations are covered with respect to other classes of 3D-printing, as well as the past, recent, and future efforts to advance the functional microdevice domain. In particular, the fabrication of micromachines/robotics, drug-delivery devices, biosensors, and microfluidics is addressed. The current challenges associated with 3D-printing of functional microdevices are also addressed, as well as future directions to improve both the printing techniques and the performance of the printed products.

A Review on Surface Stress-Based Miniaturized Piezoresistive SU-8 Polymeric Cantilever Sensors

In the last decade, microelectromechanical systems (MEMS) SU-8 polymeric cantilevers with piezoresistive readout combined with the advances in molecular recognition techniques have found versatile applications, especially in the field of chemical and biological sensing. Compared to conventional solid-state semiconductor-based piezoresistive cantilever sensors, SU-8 polymeric cantilevers have advantages in terms of better sensitivity along with reduced material and fabrication cost. In recent times, numerous researchers have investigated their potential as a sensing platform due to high performance-to-cost ratio of SU-8 polymer-based cantilever sensors. In this article, we critically review the design, fabrication, and performance aspects of surface stress-based piezoresistive SU-8 polymeric cantilever sensors. The evolution of surface stress-based piezoresistive cantilever sensors from solid-state semiconductor materials to polymers, especially SU-8 polymer, is discussed in detail. Theoretical principles of surface stress generation and their application in cantilever sensing technology are also devised. Variants of SU-8 polymeric cantilevers with different composition of materials in cantilever stacks are explained. Furthermore, the interdependence of the material selection, geometrical design parameters, and fabrication process of piezoresistive SU-8 polymeric cantilever sensors and their cumulative impact on the sensor response are also explained in detail. In addition to the design-, fabrication-, and performance-related factors, this article also describes various challenges in engineering SU-8 polymeric cantilevers as a universal sensing platform such as temperature and moisture vulnerability. This review article would serve as a guideline for researchers to understand specifics and functionality of surface stress-based piezoresistive SU-8 cantilever sensors.