3D-printed Sensors Research Articles

Three-dimensional, soft magnetic-cored solenoids via multi-material extrusion

ABSTRACT This study reports fully 3D-printed, three-dimensional, soft magnetic-cored solenoids that generate three times the largest magnetic fields previously reported from 3D-printed solenoids. The devices are fabricated on a customised, multi-material 3D printer that can extrude both filaments and pellets. Three different kinds of materials are employed to manufacture the reported soft magnetic-cored solenoids: pure PLA (dielectric portions), PLA doped with copper particles (electrically conductive structures), and nylon or PLA doped with metallic particles (soft magnetic cores). Via manufacturing optimisation, the reported devices are 33% smaller and can withstand about twice the current, generating three times more magnetic field. The 3D-printed solenoids generate Gauss-level magnetic fields while drawing tens-of-milliamps currents and can be readily used to implement fully 3D-printed induction sensors. The results of this work extend the state of the art in 3D-printed electronics, enabling the creation of more complex and capable solenoids for in-situ manufactured and in-space manufactured electromagnetic systems.

Displacement Estimation via 3D-Printed RFID Sensors for Structural Health Monitoring: Leveraging Machine Learning and Photoluminescence to Overcome Data Gaps.

Monitoring object displacement is critical for structural health monitoring (SHM). Radio frequency identification (RFID) sensors can be used for this purpose. Using more sensors enhances displacement estimation accuracy, especially when it is realized through the use of machine learning (ML) algorithms for predicting the direction of arrival of the associated signals. Our research shows that ML algorithms, in conjunction with adequate RFID passive sensor data, can precisely evaluate azimuth angles. However, increasing the number of sensors can lead to gaps in the data, which typical numerical methods such as interpolation and imputation may not fully resolve. To overcome this challenge, we propose enhancing the sensitivity of 3D-printed passive RFID sensor arrays using a novel photoluminescence-based RF signal enhancement technique. This can boost received RF signal levels by 2 dB to 8 dB, depending on the propagation mode (near-field or far-field). Hence, it effectively mitigates the issue of missing data without necessitating changes in transmit power levels or the number of sensors. This approach, which enables remote shaping of radiation patterns via light, can herald new prospects in the development of smart antennas for various applications apart from SHM, such as biomedicine and aerospace.

The first attempt of utilizing mineral-impregnated carbon fibre reinforcements as load sensors for structural health monitoring

A novel reinforcement of mineral-impregnated carbon fiber (MCF) composite has been introduced to replace traditional steel or fiber-reinforced polymer (FRP) reinforcements, primarily attributed to its high-temperature endurance, cost-effectiveness, and sustainable utilization. Previous studies have investigated the advantages of MCF in terms of its processing and exceptional mechanical performance over a wide temperature range, however, superior inherent electrical conductivity and related piezoresistivity of carbon fibers (CFs) embedded within the matrix have not yet been explored systematically. In this study, we explore the utilization of MCF as highly sensitive stress sensors for structural health monitoring (SHM), including the electrical resistance, piezoresistivity, and underlying mechanisms of MCF composites under cyclic flexural stresses. Additionally, the effect of two different electrode configurations is examined. The findings reveal that the MCF exhibits a significant stress-sensing capacity under cyclic flexural loading within an elastic regime. Specifically, MCF with outer electrodes shows superior electrical conductivity, albeit with less significant fractional changes of resistance (FCR) and stress-sensing sensitivity. Conversely, MCF with inner electrodes exhibits the opposite trend. These outcomes provide valuable insights into the intrinsic self-sensing capability of MCF and propel its potential applications as a multifunctional composite, particularly in fields such as 3D-printed smart sensors and SHM for concrete infrastructures.

Highly Efficient 3D-Printed Graphene Strain Sensors Using Fused Deposition Modeling with Filament Deposition Techniques

ABSTRACT Graphene is a two-dimensional (2D) material known for its exceptional strength and high electrical conductivity, making it an ideal substance for resistive strain sensors. Recently, fused deposition modeling (FDM) in three-dimensional (3D) printing has gained attractiveness as a promising process due to its ability to produce 3D structured strain sensors by layer-by-layer melting and depositing conductive polymer composites. To ensure reliable strain sensors, comprehending how sensor properties change based on strain direction is crucial. In this study, graphene-based sensors with different slicing angles were successfully fabricated using FDM, enabling systematic study of the effect of strain angles on the performance of graphene-based sensors. The alignment of graphene filaments relative to the direction of applied strain was found to impact the gauge factor (GF) and other important sensor parameters. Our results showed that the 45° pattern exhibited higher sensitivity and stability compared to the 180° pattern, while the GF was greater for the 180° pattern. Additionally, we demonstrated high reliability and linearity through 1000 bending tests. The findings of this study will contribute to the growing body of research on FDM-fabricated graphene-based strain sensors.

Undulating Seal Whiskers Evolved Optimal Wavelength-to-Diameter Ratio for Efficient Reduction in Vortex-Induced Vibrations.

Seals are well-known for their remarkable hydrodynamic trail-following capabilities made possible by undulating flow-sensing whiskers that enable the seals to detect fish swimming as far as 180m away. In this work, the form-function relationship in the undulating whiskers of two different phocid seal species, viz. harbor and gray seals, is studied. The geometry and material properties of excised harbor and grey seal whiskers are systematically characterized using blue light 3D scanning, optical and scanning electron microscopy, and nanoindentation. The effect of the undulating geometry on the whiskers' vibration in uniform water flow is studied using both experimental (piezoelectric MEMS and 3D-printed piezoresistive sensors developed in-house) and numerical (finite element method) techniques. The results indicate that the dimensionless ratio of undulation wavelength to mean whisker diameter (λ/Dm ) in phocid seals may have evolved to be in the optimal range of 4.4-4.6, enabling an order-of-magnitude reduction in vortex-induced vibrations (compared to a similarly-shaped circular cylinder) and, consequently, an enhanced flow sensing capability with minimal self-induced noise. The results highlight the importance of the dimensionless λ/Dm ratio in the biomimetic design of seal whisker-inspired vibration-resistant structures, such as marine risers and wake detection sensors for submarines.

3D Printed and Embedded Strain Sensors in Structural Composites for Loading Monitoring and Damage Diagnostics

The development of novel embedded sensors for structural health monitoring (SHM) is crucial to provide real-time assessments of composite structures, ensuring safety, and prolonging their service life. Early damage diagnostics through advanced sensors can lead to timely maintenance, reducing costs and preventing potential catastrophic failures. This paper presents the synthesis, 3D printing, and characterization of novel embedded strain sensors using multi-walled carbon nanotube (MWCNT) -enhanced nanocomposites in fiberglass reinforced composites for potential damage diagnostics and SHM applications. MWCNTs are dispersed within structural epoxy for the additive manufacturing of nanocomposites with piezoresistive sensing capability. The 3D-printed nanocomposite sensors are embedded in fiberglass-reinforced composite laminates. The piezoresistive sensing capabilities of the 3D-printed sensors within composites are characterized by applying different levels of maximum loads and load rates under three-point bending loads. Additionally, the long-term reliability of the developed strain sensors is evaluated up to 1000 cycles. The recorded piezoresistive sensing signals show high sensitivity for the externally applied bending loads with advanced gauge factor up to 100, resulting in potential load sensing capability for in-situ damage diagnostics and real-time SHM for structural composites.

Injectable, tough and adhesive zwitterionic hydrogels for 3D-printed wearable strain sensors

3D printing of high-strength hydrogel remains a great challenge owing to trade-off between strength and printability. Herein, a zwitterionic strategy was proposed through electrostatic reversible interaction to improve the rheology of shear-thinning based on an agarose-based network. Therefore, this mechanism of dual network (DN) ultimately imparts the hydrogel with remarkable strength (with a tensile strength of 0.79 MPa and a compressive strength of 14.18 MPa) and ductility (with a toughness of 766%), while simultaneously possessing multiple functionalities, such as high transparency, excellent biocompatibility, and adhesion. Furthermore, the developed agarose/poly([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium -co-N- hydroxyethylacrylamide) (A/D-H) DN hydrogel exhibited impressive strain sensitivity with a strain gauge factor GF of 4.33 even under large deformations. Various forms of flexible sensors based on this printed gel were successfully utilized in human motion sensing and remote heartbeat monitoring, thereby opening up new perspectives for the further expansion of injectable materials.

3D-printed flexible sensors for food monitoring

3D-printed flexible sensors for food monitoring

A strategy for human safety monitoring in high-temperature environments by 3D-printed heat-resistant TENG sensors

Flexible wearable devices based on triboelectric nanogenerators (TENGs) can be used to track many parameters around the human body, including signals from motions, gestures, and contacts, to perform a variety of tasks. However, it is still a challenge to achieve wide working temperature and multi-parameter body motion sensing with TENG sensors. To address that, this work reports a facile 3D printing-based strategy for preparing functionalized heat-resistant TENG sensors. In detail, a printable heat-resistant ink and multiple 3D printing-based assembly methods are proposed. The as-fabricated ink, which could maintain up to 200 ℃, then serves as multiple components in the TENG sensors through patterning. Functionalized TENG sensors and human–machine interfaces are printed and subsequently networked on a human body to sense comprehensive motion information. The results indicate six typical motions can be recognized with 94.85% accuracy based on neural network algorithms. A simple digital twin for motion capture and trace tracking in virtual space is furthermore implemented. Combining the proposed TENG sensors with virtual reality (VR) platforms opens up a new road for human safety monitoring in high-temperature environments.

Patterning (Electro)chemical Treatment-Free Electrodes with a 3D Printing Pen

A simple fabrication method to make electrochemical sensors is reported. The electrodes were fabricated with a commercial filament based on polylactic acid and carbon black (PLA/CB). They were engineered with a three-dimensional (3D) printing pen and poly(methyl methacrylate) template. The optimization parameters included the thickness and diameters of the electrodes. The electrode diameter was restricted by the 3D printing pen's nozzle dimension, and larger diameters generated small cracks on the electrode surface, compromising their analytical signal. The electrode thickness can increase the electrical resistance, affecting their electrochemical response. The fabrication showed reproducibility (RSD = 4%). The electrode surface was easily renewed by sanding the electrodes, making them reusable. Additionally, the proposed sensor provided comparable electrochemical responses over traditional glassy carbon electrodes. Moreover, no (electro)chemical surface treatment was required for sensing applications due to the compromise between the thickness and diameters of the electrodes, effectively translating the filaments' electrical properties to resulting materials. The electrodes' analytical performance was shown for organic and inorganic species, including paraquat, Pb2+, and caffeic acid. As proof of concept, the analytical applicability was demonstrated for total polyphenolic quantification in tea samples. Therefore, this work provides an alternative to fabricating miniaturized electrodes, bringing valuable insights into PLA/CB 3D-printed sensors and opening possibilities for designing electrode arrays. Moreover, the proposed electrodes are promising platforms for paper-based microfluidic systems.

3D-printed stretchable sensor based on double network PHI/PEDOT:PSS hydrogel annealed with cosolvent of H2O and DMSO

3D-printed stretchable sensors have promising applications in wearable devices, human–computer interaction, soft robotics, and many other fields. However, low conductivity and poor stretchability still hinder their extensive applications. Herein, we report a strategy of double networks and annealed PEDOT:PSS for the first time. The new material combines mechanical stretch performance and conductivity together, where 3D-printed complex stretchable poly(2-hydroxyethyl acrylate-isobornyl acrylate) (PHI) structure serves as the tough network and annealed highly conductive PEDOT:PSS serves as the functional network. It is adopted to build 3D-printed porous stereo structures with both stretchability and conductivity. It exhibits interconnected uniform networks, improved storage modulus (1.3 * 106 Pa), higher conductivity (The conductivity of PHI/PEDOT: PSS annealed is up to 3.12 * 10−1 S m−1, which is 441 times compared to PHI/PEDOT: PSS), excellent stability and durability at a large strain of 60%. Hence, it is expected that this new strategy will open up wide possibilities for designing stretchable sensors and smart devices.

ABS-PLA-Al composite for digital twinning of heritage buildings

In the past two decades, significant studies have been reported on the use of 3D-printed thermal sensors for online health monitoring of heritage buildings. But hitherto little has been reported on the 4D capability of such thermal sensors for the digital twinning of heritage buildings. In this study, 80% acrylonitrile butadiene styrene (ABS) and 20% polylactic acid (PLA) matrix (by wt %), was reinforced with 5% Al powder (≈45-50 µm) based on melt flow index (MFI). The substrate (as a passive sensor) of ABS-PLA-5 %Al was 3D printed with fused filament fabrication (FFF) for the construction of a micro-strip patch antenna (MPA) as an internet of things (IoT) based solution for online health monitoring and digital twinning of heritage buildings. The dielectric properties (dielectric constant (ɛr) and loss tangent (tanδ)) were explored by using a ring resonator test to ascertain the dimensions and design of the passive sensor. For 4D capabilities, the prepared sensor was exposed to two stimuli (temperature and moisture) for exploring its use as a digital twinning component.

Additive manufacturing of nylon composites with embedded multi-material piezoresistive strain sensors for structural health monitoring

A hybrid additive manufacturing process is proposed to incorporate piezoresistive sensors into a polyamide matrix for real-time structural health monitoring. 3D-printed samples were tested under tensile, flexural, and indentation testing using various sizes, numbers, and locations of the sensors, while the electrical resistance was recorded in real-time. Under uniaxial tensile loading, strength reached 36 MPa, and the resistance values linearly increased by up to 105% at ∼150% of strain. During flexural testing, the sensor had higher resistance when it was located above the supports rather than under the loading roller. The conductive paths were placed near the plate's top, middle and bottom sections to localise the damage during indentation testing. The energy absorbed during indentation testing reached 34–36 J. Resistance readings of sensors with 1 cm width showed better linearity and sensitivity than their wide (10 cm) counterparts. Three 2D sensors were interconnected to make a 3D sensor. The resistance values gradually increased after each strike during repeated indentation, but the overall strength remained similar. In summary, this work demonstrates a novel approach toward customisable 3D-printed piezoresistive sensors for detecting global or local damage.

SoJel –A 3D printed jellyfish-like robot using soft materials for underwater applications

Mass manufactured, biologically inspired soft robots are needed for safe robot-animal interactions. The key challenges are the design and manufacture of high-performance robots that meet stringent requirements in ocean environments. This work describes a 92% 3D-printed jellyfish-inspired soft robot, SoJel, which uses soft polymers for the bell and sensors. We show how slight variations in geometry affects the swimming performance of the robot, which demonstrates the need for advanced or custom-made 3D printers that avoid the difficulties in realizing controlled geometries by using traditional molding and casting techniques. The design reproduces important kinematic patterns of natural jellyfish, like the bell bending angle, actuation duty cycle, and bell kinematics. This work builds on previous generations of jellyfish-inspired robots that employed various types of actuators, but is largely polymer based, so it avoids metallic components that are susceptible to corrosion in the ocean. The realized cost of transport and vertical swimming height of robots powered by coiled nickel-titanium actuators are comparable to previous designs, however, the present robots have advantages in ease of design and manufacturing. A variety of soft robots and flexible structures can potentially be fabricated using this design principle. Flexible 3D-printed soft sensors enable us to determine the bending angles for motion estimation and control. Energy harvesting from water oscillation or free vibration is demonstrated using a twistron harvester and a piezoelectric composite integrated into the robot, which can be used for powering electronics, such as a LED for display and communication.

Lab-made 3D-printed electrochemical sensors for tetracycline determination

Antibiotics such as tetracycline (TC) are widely prescribed to treat humans or dairy animals. Therefore, it is important to establish affordable devices in laboratories with minimal infrastructure. 3D printing has proven to be a powerful and cost-effective tool that revolutionizes many applications in electrochemical sensing. In this work, we employ a conductive filament based on graphite (Gr) and polylactic acid (PLA) (40:60; w/w; synthesized in our lab) to manufacture 3D-printed electrodes. This electrode was used “as printed” and coupled to batch injection analysis with amperometric detection (BIA-AD) for TC sensing. Preliminary studies by cyclic voltammetry and differential pulse voltammetry revealed a mass transport governed by adsorption of the species and consequent fouling of the redox products on the 3D printed surface. Thus, a simple strategy (solution stirring and application of successive potentials, +0.95 V followed by +1.2 V) was associated with the BIA-AD system to solve this effect. The proposed electrode showed analytical performance comparable to costly conventional electrodes with linear response ranging from 0.5 to 50 μmol L−1 and a detection limit of 0.19 μmol L−1. Additionally, the developed method was applied to pharmaceutical, tap water, and milk samples, which required minimal sample preparation (simple dilution). Recovery values of 92–117% were obtained for tap water and milk samples, while the content found of TC in the capsule was close to the value reported by the manufacturer. These results indicate the feasibility of the method for routine analysis involving environmental, pharmaceutical, and food samples.

Enhanced RF response of 3D-printed wireless LC sensors using dielectrics with high permittivity

The development of wireless sensing technologies paves the way for advances in the fields of wearable devices, prosthetics and robotics. Wireless communication between sensors and readers plays an important role in recent Internet of Things technologies. Among many types of wireless sensing devices, wireless passive radio frequency devices including inductor-capacitor (LC) resonators have been spotlighted. However, passive LC sensors suffer from short-range wireless detection, and their fabrication requires several processes. Here, we design a 3D integrated wireless compact LC location sensor fabricated using the 3D printing method for multi-layered devices. The fabricated wireless sensing system shows an increased wireless readout distance of up to 10 cm. In addition, a dielectric material with high dielectric permittivity has been applied to enhance the quality factor of the sensors by 2.5 times with improved wireless detection.

Application of 3D Printing Technology in Sensor Development for Water Quality Monitoring.

The development of sensors for water quality monitoring is crucial to protect water quality, aquatic biota and human health. Traditional sensor manufacturing methods have significant drawbacks, such as low fabrication freedom, limited material choice and expensive manufacturing cost. As a possible alternative method, 3D printing technologies are increasingly popular in sensor development due to their high versatility, fast fabrication/modification, powerful processing of different materials and ease of incorporation with other sensor systems. Surprisingly, a systematic review examining the application of 3D printing technology in water monitoring sensors has not yet been conducted. Here, we summarized the development history, market share and advantages/disadvantages of typical 3D printing techniques. Specifically focused on the 3D-printed sensor for water quality monitoring, we then reviewed the applications of 3D printing in the development of sensors' supporting platform, cell, sensing electrode as well as all-3D-printed sensors. The fabrication materials and processing, and the sensor's performances regarding detected parameters, response time and detection limit/sensitivity, were also compared and analyzed. Finally, the current drawbacks of 3D-printed water sensors and potential directions for future study were discussed. This review will substantially promote the understanding of 3D printing technology used in water sensor development and benefit the protection of water resources.

On Comparison of 3D-Printed ABS and PVDF-Based Sensors for Body-Centric Utility

On Comparison of 3D-Printed ABS and PVDF-Based Sensors for Body-Centric Utility

Heat flux measurement using 3D-printed continuous wire polymer composite sensors

Fused filament fabrication (FFF) 3D printing was used to fabricate continuous wire polymer composite (CWPC) heat flux sensors; the integrated wires acted as resistive sensing elements. Sensor configurations consisting of polylactic acid (PLA) with copper wire (Cu), PLA with nickel (Ni) wire, and thermoplastic polyurethane (TPU) with Cu wire were investigated. For each composition, samples with different numbers of layers were 3D printed to investigate the effect of sensor thickness on performance. Element spacing, polymer conductivity, and wire temperature coefficient of resistances were quantified. Performance testing of the 3D-printed CWPC as a heat flux sensor showed promising results for all compositions and demonstrated their ability to be used as heat flux sensors for low temperature and low heat flux applications. Measurement errors were less than 17% for all configurations and less than 10% for 2-layer samples at the higher heat fluxes. A case study demonstrated the use of a 3D-printed flexible CWPC heat flux sensor to estimate heat loss from an insulated system with good accuracy. This sensor fabrication approach can potentially be employed in a wide range of applications because it allows for custom geometries and can use different types of polymers and sensing elements.

Modeling of Single-Process 3D-Printed Piezoelectric Sensors with Resistive Electrodes: The Low-Pass Filtering Effect

Three-dimensional printing by material extrusion enables the production of fully functional dynamic piezoelectric sensors in a single process. Because the complete product is finished without additional processes or assembly steps, single-process manufacturing opens up new possibilities in the field of smart dynamic structures. However, due to material limitations, the 3D-printed piezoelectric sensors contain electrodes with significantly higher electrical resistance than classical piezoelectric sensors. The continuous distribution of the capacitance of the piezoelectric layer and the resistance of the electrodes results in low-pass filtering of the collected charge. Consequently, the usable frequency range of 3D-printed piezoelectric sensors is limited not only by the structural properties but also by the electrical properties. This research introduces an analytical model for determining the usable frequency range of a 3D-printed piezoelectric sensor with resistive electrodes. The model was used to determine the low-pass cutoff frequency and thus the usable frequency range of the 3D-printed piezoelectric sensor. The low-pass electrical cutoff frequency of the 3D-printed piezoelectric sensor was also experimentally investigated and good agreement was found with the analytical model. Based on this research, it is possible to design the electrical and dynamic characteristics of 3D-printed piezoelectric sensors. This research opens new possibilities for the design of future intelligent dynamic systems 3D printed in a single process.