Fused Filament Fabrication Technology Research Articles

Development of functional gradient multi-material composites using Poly Lactic Acid and walnut shell reinforced Poly Lactic Acid filaments by fused filament fabrication technology

In construction and architectural industries, multi-material additive manufacturing is gaining more interest because of obtaining the combined material properties in a single additive process by multiple materials. The present work concentrates on developing functionally gradient multi-material composite using Poly Lactic Acid (PLA), and novel extruded walnut shell particle reinforced Poly Lactic Acid (WPLA) by Fused Filament Fabrication technology. This work followed single and multi-gradient material deposition approaches, and their mechanical, thermal, and tribological properties were compared with standard 3D printed PLA and walnut shell-reinforced PLA composite. Among the 3D printed functional graded multi-materials, the composites with single-gradient PW configuration exhibits better compressive and D shore hardness value of 63.68 MPa and 76.2 compared with the neat PLA composite. The results show that considering the tensile and flexural samples the single gradient PLA samples shows better values when compared with all other multi gradient prepared composites. The highest heat deflection temperature of 56 °C was observed on the WPLA composite and for gradient materials PW composite which is PLA (50%) and walnut shell reinforced PLA (50%) sample exhibits the maximum heat deflection temperature of 54.4 °C. Furthermore, considering the tribological and thermal properties, the multi-gradient composite of WPW configured with PLA of 40% as inner core exhibits the lowest thermal conductivity of 0.087 W/mK and a specific wear rate of 1.631 × 10−6 mm3/Nm when compared with neat PLA composite. Therefore, among the novel functional graded multi-material, the sample PW with PLA (50%) and walnut shell reinforced PLA (50%) composite will be more suitable for semi-structural applications, and the composite with WPW designation (PLA of 40% as inner core) will suit the high wear-resistant and constructional building panel material for thermal proofing applications.

Preparation of 3D Models of Cultural Heritage Objects to Be Recognised by Touch by the Blind—Case Studies

Providing access to and the protection of cultural goods—intangible and tangible heritage—is carried out primarily by institutions such as museums, galleries or local cultural centres where temporary exhibitions are shown. The international community also attempts to protect architectural objects or entire urban layouts, raising their status by inscribing them on the UNESCO World Heritage List. Contemporary museums, however, are not properly prepared to make museum exhibits available to the blind and visually impaired, which is confirmed by both the literature studies on the subject and the occasional solutions that are put in place. The development of various computer graphics technologies allows for the digitisation of cultural heritage objects by 3D scanning. Such a record, after processing, can be used to create virtual museums accessible via computer networks, as well as to make copies of objects by 3D printing. This article presents an example of the use of scanning, modelling and 3D printing to prepare prototypes of copies of museum objects from the Silk Road area, dedicated to blind people and to be recognised by touch. The surface of an object has information about it written in Braille before the copy-making process is initiated. The results of the pilot studies carried out on a group of people with simulated visual impairment and on a person who is blind from birth indicate that 3D models printed on 3D replicators with the fused filament fabrication technology are useful for sharing cultural heritage objects. The models are light—thanks to which they can be freely manipulated, as well as having the appropriate smoothness—which enables the recognition of decorative details present on them, as well as reading texts in Braille. Integrating a copy of an exhibit with a description about it in Braille into one 3D object is an innovative solution that should contribute to a better access to cultural goods for the blind.

Design and manufacturing of graded density components by material extrusion technologies

Innovative design methods and manufacturing technologies, such as lattice structures optimization and additive manufacturing, allow for the production of functional and extremely complex components. Recent literature shows limits in geometric modeling and data exchange, highlighting some improvements in the design of variable density lattice structures mainly for powder bed fusion technologies. Similar improvements are not available for material extrusion (MEX) technologies which show technological and numerical limits related to the computer numerical control programming language (G-code) generated by computer aided manufacturing (CAM) software. This work aims at overcoming the limits in fabricating graded density shell-based lattice structures for MEX technology by using the infill patterns available in the CAM software and editing the G-code based on a density map defined by volumetric models. Combining two usually separated phases, i.e., the geometric modeling and the CAM processing, several advantages are obtained, considering at the same time some of the technological constraints.The proposed approach is tested on a cubic sample and on a bracket fabricated by a fused filament fabrication technology. The results show that the method allows for the reduction of design efforts, amount of data exchanged, and processing time, obtaining an effective G-code and consistent components.

Experimental study on the effect of selected sterilization methods on mechanical properties of polylactide FFF specimens

PurposeBiodegradable polymers are widely used in personalized medical devices or scaffolds for tissue engineering. The manufacturing process should be finished with sterilization procedure. However, it is not clear how the different sterilization methods have an impact on the mechanical strength of the three-dimensional (3D)-printed parts, such as bone models or personalized mechanical devices. This paper aims to present the results of mechanical testing of polylactide-based bone models before and after sterilization.Design/methodology/approachPolylactide specimens prepared in fused filament fabrication technology were sterilized with different sterilization methods: ultraviolet (UV) and ethylene oxide. Mechanical properties were determined by testing tensile strength, Young’s modulus and toughness.FindingsThe tensile strength of material after sterilization was significantly higher after ethylene oxide sterilization compared to the UV sterilization, but in both sterilization methods, the specimens characterized lower tensile strength and Young’s modulus when compared to the control. In comparison of toughness results, there was no statistically significant differences. The findings are particularly significant in the perspective of using individual implants, bone grafts and dental guides.Originality/valueAlthough fused filament fabrication (FFF) 3D printing devices equipped with UV light sterilization options are available, experimental results of the effect of selected sterilization methods on the mechanical strength of additively manufactured parts have not been described. This paper completes the present state of the art on the problem of sterilization of FFF parts from biodegradable materials.

Metal FFF sintering shrinkage rate measurements by X-ray computed tomography

ABSTRACT Fused filament fabrication (FFF), also known under the brand name fused deposition modelling, has recently emerged as a more straightforward and economic alternative to established 3D printing or additive manufacturing technologies such as powder bed fusion for metal part manufacturing. In this work, we report on possible approaches to the measurements of FFF sintering shrinkage rates from X-ray computed tomography (XCT) data, with particular focus on those that use greyscale data, and are thus independent of any errors introduced by segmentation. Metallic parts were produced using FFF technology, which relies on an isotropic scaling factor applied to the STL model of the part to be produced to compensate for the shrinkage happening during the sintering of the printed (green) part. We report that for the stainless-steel parts produced, the shrinkage rates were not isotropic and the one along the build direction was the largest, the other two being similar to one another. Overall, the shrinkage rate measured by the different methodologies all fell between 15 % and 16 %. XCT data also clearly showed the typical defects encountered with this technology, such as voids resulting from insufficient overlap or caused by print head blockages, are not mitigated by the sintering process. Autocorrelation was introduced as a way of characterising the 3D structure of the sample before and after sintering, and thus the sintering shrinkage rates.

Influence of Geometric Parameters on Buckling Behavior of 3D Printed Anisogrid Structures

The present work aims at studying the buckling behavior of lattice structures realized by additive manufacturing technology. To this purpose, carbon fiber reinforced thermoplastic filaments have been used to realize anisogrid structure at different geometric parameters by means of Fused Filament Fabrication technology. Eight configurations were realized varying the rib width and the rib thickness of the structures, and keeping constant the cell height value. Anisogrid structures were tested under compressive load in order to investigate the effect of geometric parameters on strength and specific strength exhibited by the structures. It has been shown that mechanical performances of lattice structures are highly affected by the geometric parameters of the anisogrids.

Effects of fused filament fabrication parameters on the manufacturing of 316L stainless-steel components: geometric and mechanical properties

PurposeThe extrusion-based additive manufacturing method followed by debinding and sintering steps can produce metal parts efficiently at a relatively low cost and material wastage. In this study, 316L stainless-steel metal filled filaments were used to print metal parts using the extrusion-based fused filament fabrication (FFF) approach. The purpose of this study is to assess the effects of common FFF printing parameters on the geometric and mechanical performance of FFF manufactured 316L stainless-steel components.Design/methodology/approachThe microstructural characteristics of the metal filled filament, three-dimensional (3D) printed green parts and final sintered parts were analysed. In addition, the dimensional accuracy of the green parts was evaluated, as well as the hardness, tensile properties, relative density, part shrinkage and the porosity of the sintered samples. Moreover, surface quality in terms of surface roughness after sintering was assessed. Predictive models based on artificial neural networks (ANNs) were used for characterizing dimensional accuracy, shrinkage, surface roughness and density. Additionally, the response surface method based on ANNs was applied to represent the behaviour of these parameters and to identify the optimum 3D printing conditions.FindingsThe effects of the FFF process parameters such as build orientation and nozzle diameter were significant. The pore distribution was strongly linked to the build orientation and printing strategy. Furthermore, porosity decreased with increased nozzle diameter, which increased mechanical performance. In contrast, lower nozzle diameters achieved lower roughness values and average deviations. Thus, it should be noted that the modification of process parameters to achieve greater geometrical accuracy weakened mechanical performance.Originality/valueNear-dense 316L austenitic stainless-steel components using FFF technology were successfully manufactured. This study provides print guidelines and further information regarding the impact of FFF process parameters on the mechanical, microstructural and geometric performance of 3D printed 316L components.

3D printed bioresorbable scaffolds for articular cartilage tissue engineering: a comparative study between neat polycaprolactone (PCL) and poly(lactide-b-ethylene glycol) (PLA-PEG) block copolymer

This work identifies and describes different material-scaffold geometry combinations for cartilage tissue engineering (CTE). Previously reported potentially interesting scaffold geometries were tuned and printed using bioresorbable polycaprolactone and poly(lactide-b-ethylene) block copolymer. Medical grades of both polymers were 3D printed with fused filament fabrication technology within an ISO 7 classified cleanroom. Resulting scaffolds were then optically, mechanically and biologically tested. Results indicated that a few material-scaffold geometry combinations present potential for excellent cell viability as well as for an enhance of the chondrogenic properties of the cells, hence suggesting their suitability for CTE applications.

Thermal characterization of short carbon fiber reinforced high temperature polymer material produced using the fused filament fabrication process

The objective of this research is to perform the thermal analysis on high-temperature polymer (polycarbonate) based composite material reinforced with short carbon fiber (SCF) produced by utilizing the fused filament fabrication (FFF) process. Different percentages of SCF (by vol.), i.e., pure (0 %), 3 %, 5 %, and 10 %, were taken to fabricate the additively manufactured part for thermal analysis. The entire part manufacturing process was described, starting with material mixture generation, composite filament fabrication with different SCF percentages, and finally, FFF printing. Micrography analysis was performed to observe the fiber behavior, including fiber-matrix adhesion, fiber orientation, and fiber distribution. The thermal behavior was investigated by determining the thermal degradation temperature, viscoelastic properties like loss modulus, storage modulus, and creep behavior at high temperature (100 °C) by varying the SCF percentage. Due to the anisotropic nature of the FFF-made parts, thermal conductivity was analyzed on 0° layup and 90° layup orientations in the X-Y plane to observe the behavior of thermal properties in different directions. The results of the study indicated that the addition of fibers has maintained or improved the thermal properties of PC. The incorporation of SCF has enhanced the degradation temperature and viscoelastic properties of the 3D printed composite parts. Thermal conductivity results also showed a significant improvement with the increase in SCF amount. In summary, PC-based composites with SCF incorporation were found to execute well with FFF technology and allow the fabrication of electronic products and heat exchangers with satisfactory thermal performance.

Hybrid Additive Manufacturing of Fused Filament Fabrication and Ultrasonic Consolidation.

Fused filament fabrication (FFF) additive manufacturing technology has the advantages of being low cost, having a simple operation, using wide types of molding materials, and producing less pollution during the printing process. However, the mechanical properties of the molded sample are unsatisfactory due to the limited bonding force between the filaments during the forming process, which limits its further development and application in the engineering field. Herein, the hybrid additive manufacturing technology for heterogeneous materials based on the ultrasonic-assisted enhanced fused filament fabrication technology was proposed. The mechanism of ultrasonic vibration on the strengthening of FFF samples was explored. The influence mechanisms of bonding time and ultrasonic strengthening times, ultrasonic strengthening and static load compression on the strengthening of mechanical properties of the sample were investigated. The effects of the thickness and printing angle of the FFF samples on the ultrasonic-enhanced mechanical properties were explored. The tensile strength of the one-time ultrasonic-strengthened sample is up to 43.43 MPa, which is 16.12% higher than that of the original. The maximum bending strength of the four-time ultrasonic-strengthened sample is 73.38 MPa, which is 78.98% higher than that of the original. Ultrasonic strengthening not only re-fused the pores inside the sample, but also improved the bond strength between the rasters. With the increase in the thickness of the sample, the increase rate of ultrasonic to the strength of the sample decreased significantly. The effects of ultrasound on the interlayer adhesion of samples with various printing angles were different. Based on the systematic research on the influence mechanism of ultrasonic process parameters and molding process parameters on the strengthening of FFF, a molding method for additively manufacturing heterogeneous material parts while strengthening the mechanical properties of FFF samples was proposed, and the influence mechanisms of the molding process on the mechanical properties and shape memory properties of the sample were explored, which can broaden the application of FFF technology in the engineering field.

Fully FFF-Printed Capacitive Displacement Sensor Based on Graphene/PLA Composite and Thermoplastic Elastomer Filaments

In this paper, an organic capacitive displacement sensor was fully realized using Fused Filament Fabrication (FFF) technology. The fabricated sensor consists of two circular electrodes, the spacer, and the casing. The FFF-printed electrodes were made of a conductive graphene/Polylactic acid (PLA) composite filament. For sensor’s spacer a thermoplastic elastomer filament was used. Using these filaments gives us the possibility to design the extremely simple sensor with good sensitivity. The casing of the sensor and the applicator used for applying the displacement on the sensor were fabricated using a PLA. The sensor with three setup of spacer with different infill densities of the spacer (50%, 80%, and 100%) were realized and their characteristics were compared. The highest sensitivity was obtained for the sensor with infill density of spacer of 50%. The measurement results confirm the possibility of using FFF technology for the fabrication of low-cost and all organic and fully FFF-printed sensor with good characteristics.

Mechanical and biological behaviour of additive manufactured biomimetic biodegradable orthopaedic cortical screws

PurposeThe complications caused by metallic orthopaedic bone screws like stress-shielding effect, screw loosening, screw migration, higher density difference, painful reoperation and revision surgery for screw extraction can be overcome with the bioabsorbable bone screws. This study aims to use additive manufacturing (AM) technology to fabricate orthopaedic biodegradable cortical screws to reduce the bone-screw-related-complications.Design/methodology/approachThe fused filament fabrication technology (FFFT)-based AM technique is used to fabricate orthopaedic cortical screws. The influence of various process parameters like infill pattern, infill percentage, layer height, wall thickness and different biological solutions were observed on the compressive strength and degradation behaviour of cortical screws.FindingsThe porous lattice structures in cortical screws using the rapid prototyping technique were found to be better as porous screws can enhance bone growth and accelerate the osseointegration process with sufficient mechanical strength. The compressive strength and degradation rate of the screw is highly dependent on process parameters used during the fabrication of the screw. The compressive strength of screw is inversely proportional to the degradation rate of the cortical screw.Research limitations/implicationsThe present study is focused on cortical screws. Further different orthopaedic screws can be modified with the use of different rapid prototyping techniques.Originality/valueThe use of rapid prototyping techniques for patient-specific bone screw designs is scantly reported. This study uses FFFT-based AM technique to fabricate various infill patterns and porosity of cortical screws to enhance the design of orthopaedic cortical screws.

Mechanical Properties of Recycled Polymer Materials in Additive Manufacturing

The publication focuses on the testing of the tensile strength of samples produced by Fused Filament Fabrication additive manufacturing technology. Three materials rPLA, rPETG and rPET were tested. These materials are recycled PLA (Polylactic Acid), PETG (Polyethylene Terephthalate Glucol) and PET (Polyethylene Terephthalate). These materials were used to create samples for tensile testing according to ASTM D638. In addition to testing the materials, the methodology was also tested using sets of specimens printed with no outer perimeters, one outer perimeter and two outer perimeters. Tensile diagrams were created from the measured data. Statistical processing and comparison of the measured data is performed in the publication. In the discussion, a comparison of the mechanical properties of recycled materials for 3D printing versus virgin materials for 3D printing by Fused Filament Fabrication technology is made.

3D Printing for Cartilage Replacement: A Preliminary Study to Explore New Polymers.

The use of additive manufacturing technologies for biomedical applications must begin with the knowledge of the material to be used, by envisaging a very specific application rather than a more general aim. In this work, the preliminary study was focused on considering the cartilaginous tissue. This biological tissue exhibits different characteristics, such as thickness and mechanical properties, depending on its specific function in the body. Due to the lack of vascularization, cartilage is a supporting connective tissue with limited capacity for recovery and regeneration. For this reason, any approach, whether to repair/regenerate or as a total replacement, needs to fulfill the adequate mechanical and chemical properties of the surrounding native cartilage to be successful. This work aims to explore the possibility of using new polymers for cartilage total replacement approaches with polymeric materials processed with the specific 3D printing technique of fused filament fabrication (FFF). The materials studied were Nylon® 12 (PA12), already described for this purpose, and LAY-FOMM® 60 (FOMM). FOMM has not been described in the literature for biomedical purposes. Therefore, the chemical, thermal, swelling capacity, and mechanical properties of the filaments were thoroughly characterized to better understand the structure–properties–application relationships of this new polymer. In addition, as the FFF technology is temperature based, the properties were also evaluated in the printed specimens. Due to the envisaged application, the specimens were also characterized in the wet state. When comparing the obtained results with the properties of native cartilage, it was possible to conclude that: (i) PA12 exhibits low swelling capacity, while FOMM, in its dry and wet forms, has a higher swelling capacity, closer to that of native cartilage; (ii) the mechanical properties of the polymeric materials, especially PA12, are higher than those of native cartilage; and (iii) from the mechanical properties evaluated by ultra-micro hardness tests, the values for FOMM indicate that this material could be a good alternative for cartilage replacement in older patients. This preliminary study, essentially devoted to expanding the frontiers of the current state of the art of new polymeric materials, provides valuable indications for future work targeting the envisaged applications.

Influence of contact geometry over the filament bond of polylactic acid blends

Fused Filament Fabrication is one of the most common Additive Manufacturing technologies among industrial and domestic users because of the satisfying quality of parts and the great variety of thermoplastic polymers available on the market. New possibilities were provided along with the equipment with multiple extrusion systems, referring to multi-colour and multi-material components. Besides part orientation during the manufacturing process, filament bond influences the mechanical properties of parts produced through Fused Filament Fabrication technology. This paper aims to investigate if the bond performance for polylactic acid-based polymers can be enhanced by designing new contact geometries, different from the current surface-to-surface approach. To comprehensively understand the influence of the designed bonding geometries over the tensile strength, all specimens were produced according to a factorial design of experiment method and contact geometries constrained with function related to the nozzle size.

Alumina Manufactured by Fused Filament Fabrication: A Comprehensive Study of Mechanical Properties and Porosity.

This article deals with a comprehensive study of the processing and mechanical properties of the ceramic material AlO on Fused Filament Fabrication technology (FFF). It describes the basic input analyses of the material, such as TGA, FTIR, and MVR. These analyses enabled the design and testing of process parameters for the 3D printing of parts. The article also presents the post-processes, including the technological parameters required to finalize parts made from this material, i.e., chemical debinding in acetone at elevated temperatures + thermal debinding and sintering. The microhardness was measured on the processed samples, depending on the density of the inner filling. The resulting hardness had an almost linear relationship between the percentage of filler (20–40–60–80–100%) and the resulting microhardness (1382–2428 HV10). Flexural strength was also measured on the test specimens with different degrees of internal filling (80–90–100%). However, inner filling do not affect the flexural strength (316.12–327.84–331.61 MPa). The relative density of the final parts was measured on a ZEISS METROTOM 1500 CT machine and reached 99.54%.

Adhesive bonding of 3D-printed short- and continuous-carbon-fiber composites: An experimental analysis of design methods to improve joint strength

This experimental work focuses on the effect of various joint-design factors on the adhesive bonding of mixed short- (Onyx) and continuous-carbon-fiber reinforced Nylon-6 parts, 3D-printed via Fused Filament Fabrication technology. A low-pressure plasma treatment has been preliminarily performed to maximize the adhesion at the composite's surface-adhesive interface. This approach has allowed the definition of a performance benchmark beyond which the intrinsic non-homogeneity of the composite determines joint failures due to delamination of substrates. Starting from this condition, adhesive-bonding parameters such as adherend overlap length (12.5, 25, or 50 mm) and geometry, adhesive type (epoxy or polyurethane), and substrate configuration are considered, investigating their influence on both the mechanical behavior of single-lap joints and modality of crack propagation across the bonded laminates. Supported by failure analysis, the experimental findings highlight that higher load levels (up to +251.4%) can be achieved by increasing the joint stiffness using higher values of overlap length thanks to lower peel stresses at the adherend ends where material discontinuities are mainly concentrated. Alternatively, joining of continuous-fiber-only adherends is considered to provide a further high-performing approach that involves the exclusion of the Onyx shell from the areas designed to be bonded. Moreover, heterogeneous joining between additively-manufactured composites and conventional carbon-fiber-reinforced-polymer materials is also investigated, providing pivotal insights about the applicability of 3D-printed composites also in pre-existing composite structures.

CaCO3 Polymorphs Used as Additives in Filament Production for 3D Printing.

Nowadays, additive manufacturing—also called 3D printing—represents a well-established technology in the field of the processing of various types of materials manufacturing products used in many industrial sectors. The most common type of 3D printing uses the fused filament fabrication (FFF) method, in which materials based on thermoplastics or elastomers are processed into filaments. Much effort was dedicated to improving the properties and processing of such printed filaments, and various types of inorganic and organic additives have been found to play a beneficial role. One of them, calcium carbonate (CaCO3), is standardly used as filler for the processing of polymeric materials. However, it is well-known from its different applications that CaCO3 crystals may represent particles of different morphologies and shapes that may have a crucial impact on the final properties of the resulting products. For this reason, three different synthetic polymorphs of CaCO3 (aragonite, calcite, and vaterite) and commercially available calcite powders were applied as fillers for the fabrication of polymeric filaments. Analysis of obtained data from different testing techniques has shown significant influence of filament properties depending on the type of applied CaCO3 polymorph. Aragonite particles showed a beneficial impact on the mechanical properties of produced filaments. The obtained results may help to fabricate products with enhanced properties using 3D printing FFF technology.

Аддитивное производство изделий с пространственным армированием непрерывным волокном

One of the key challenges in additive manufacturing of plastic goods using the Fused Filament Fabrication (FFF) technology is to ensure their strength. The low strength of polymer materials and the distinct anisotropy of their mechanical properties limit the use of 3D printing as an alternative to the traditional small-scale production technologies. The most promising solution to the goal of increasing the strength of printed goods is the application of continuous fiber reinforcement. Several additive manufacturing devices and software products that allow preparing a control program for 3D printing with reinforcement are known, however, having all their advantages, they, like conventional printed products, have a wide spread in strength in various directions (in the plane of a layer and perpendicularly to it, in the direction of growing). In this paper, the authors propose using the continuous fiber reinforcement along the three-dimensional trajectories to smooth out the anisotropy of the products’ properties in the FFF technology and ensure wider possibilities for using them in the production of final goods. In the course of work, a 3D printer with the ability to print using five degrees of freedom and software for preparation of control programs were upgraded for the printing process with laying continuous fiber; printing modes with reinforcement were developed; samples were produced for standard static bending tests. The experiments show that reinforcement improves the printed specimen’s strength, and the proposed three-dimensional reinforcement technique ensures the lower flexing strength compared to standard flat reinforcement with uniaxial laying of fibers, though, the destruction of 3D reinforced specimens occurred without evident delamination.

Fused filament fabrication for one shot additive manufacturing of capacitive force sensors

Additive manufacturing (AM) technologies have been largely used for the sensor fabrication: fused filament fabrication (FFF) technology is very appealing for the fabrication of embedded piezoresistive sensors into 3D printed structures (such as strain gauges) leading to the fabrication of smart objects. 3D printed structures with embedded sensors can be exploited for the fabrication of biosensor, pushing AM technologies towards a new biomanufacturing paradigm.In the present paper, a new kind of sensor has been fabricated through FFF technology and characterized: capacitive sensor for force/pressure detection. The advantages in the exploitation of FFF for capacitive sensors are several: i) only one manufacturing process is involved, ii) no-assembly tasks are required and, iii) they can be easily integrated into any kind of structure.Two commercial materials (conductive and flexible thermoplastic) have been used to fabricate the sensors in a monolithic way: a robust printing strategy has been used for the electrodes fabrication and three different infill patterns have been studied for the dielectric separator layer. The sensors have been characterized in a range of force of [0, 400] N showing good sensitivity and repeatability: this work paves the way for a large exploitation of FFF in the field of capacitive sensors.