The motivation behind the project lies in a quiet revolution sweeping through structural engineering. Fiber-reinforced thermoplastic composites are increasingly favored over traditional thermoset materials in aerospace, automotive and wind energy applications because they offer exceptional specific strength, corrosion resistance, fatigue tolerance and damage resistance. The aerospace sector in particular has embraced these materials, reporting weight savings of 20 to 30 percent alongside an approximate 25 percent reduction in total production costs. Unlike thermosets, which cure irreversibly, thermoplastic matrices can be melted and re-consolidated, which makes them inherently weldable. That weldability allows large structures to be joined without rivets or adhesives, and it has already transformed flagship aircraft components.

Resistance welding is widely regarded as one of the most efficient and promising techniques for joining these materials. The principle is elegantly simple. An electric current is passed through a resistive heating element placed at the joint interface, and Joule heating, governed by the relationship Q = I²Rt, raises the temperature until the thermoplastic matrix melts. Polymer chains then diffuse across the interface, and the joint consolidates under pressure as the assembly cools. Crucially, even though melting temporarily reduces molecular weight, thermoplastics retain their original mechanical properties once cooled, provided the joint achieves low void content and strong interlaminar bonding. The technology has a distinguished industrial pedigree: the Fokker 50’s main landing gear doors were among the earliest aerospace applications, and the J-nose component on the wing leading edges of the Airbus A340-500/600 and A380 was re-engineered from multiple aluminum pieces into a single polyphenylene sulfide composite weldment, eliminating thousands of rivets and shedding nearly 20 percent of the component’s weight.

Yet resistance welding is notoriously difficult to execute well. Current leakage is a serious risk, especially when welding composites reinforced with conductive carbon fibers. Temperature distribution along the joint interface is rarely uniform, and overheating at the edges can thermally degrade the polymer while the center remains insufficiently melted, producing voids and partially fused zones that weaken the bond. Weld quality is acutely sensitive to pressure and timing, and inadequate parameters can cause joint deconsolidation, excessive resin squeeze-out or fiber displacement. The process also demands access to both sides of the workpiece and highly specific tooling for each component geometry. Conventional alternatives fare no better: mechanical fastening introduces stress concentrations, galvanic corrosion, delamination from drilling and weight penalties, while adhesive bonding requires long curing times, extensive surface preparation and joints whose quality degrades under environmental exposure.

The OpenWelT platform tackles each of these challenges through a combination of clever design and low-cost automation. The machine’s main structure is built from modular aluminum V-slot profiles, providing lightness and portability while the welding base itself is made of wood, chosen deliberately because it acts as both a structural support and an electrical insulator, preventing current from escaping through the machine frame. During operation, external ceramic insulator plates, which are not permanent parts of the equipment, are positioned at the joint interface to serve as thermal barriers, concentrating energy within the weld zone and shielding the wooden base from localized heat. This dual insulation strategy directly addresses the leakage and thermal-control problems that plague conventional setups.

Perhaps the most distinctive innovation is the machine’s autonomous pressure management. Instead of relying on manual pneumatic presses or expensive hydraulic systems, OpenWelT employs an electric linear actuator driven by an Arduino Uno microcontroller that processes signals from a 100-kilogram load cell in real time through an HX711 24-bit amplifier. This closed feedback loop applies and maintains the perpendicular consolidation force with precision and repeatability, eliminating dependence on external pressure systems. The power subsystem pairs a Huawei R4850G2 rectifier with a Juntek DPM8624 programmable buck-boost converter, delivering a regulated welding output of up to 60 volts and 24 amps with fine decimal adjustment, which helps prevent both edge overheating and incomplete melting at the weld center. Galvanic separation between the high-power welding circuit and the low-voltage control electronics ensures that back-electromotive forces and ground loops cannot corrupt the sensitive measurement chain.

The entire apparatus is orchestrated by a custom Python-based graphical interface that acquires voltage, current, power, temperature via a Type-K thermocouple and pressure at one hertz, logging every variable with timestamps and exporting standardized PDF reports alongside raw data files. The software automatically validates each incoming data frame, imposes upper safety limits of 24 amps and 1.5 megapascals, and generates voltage-versus-time curves without requiring expensive laboratory data acquisition boards. Because the firmware and interface are fully open, researchers can freely modify the control algorithms, implement alternative feedback loops or adapt the platform for entirely different experimental setups, something impossible with proprietary commercial stations.

To validate the machine, the team welded three replicate lap-joint specimens of glass fiber-reinforced polyetherimide (GF/PEI) thermoplastic composite according to the ASTM D5868 standard, using 10 volts, 14 amps, 1 megapascal of pressure and a 100-second weld time. The heating element was a woven stainless steel mesh with 40-micrometer wires, cleaned ultrasonically before each run to guarantee reproducible contact resistance. The welded joints achieved a mean lap shear strength of 29.83 ± 1.2 megapascals, with an average power of 45.69 ± 1.1 watts, a power density of 0.14 ± 0.03 W/mm², and an average electrical resistivity of 0.23 ± 0.02 ohms. Optical microscopy of the joint cross-sections revealed excellent interfacial consolidation with only minor porosity, attributed to residual solvent in the matrix or slight thermal gradients, while dark-field imaging confirmed that reinforcing fibers were undamaged, well encapsulated by the polymer and free of delamination. The low coefficient of variation across replicates, roughly 4 percent, demonstrated stable and repeatable performance under fixed conditions.

The implications extend well beyond one material system. The authors envision the platform serving as a general-purpose testbed for any process combining controlled force with localized Joule heating, including heat sealing of polymer films, thermal bonding of textiles and membranes, and thermally activated adhesive curing, requiring only adapted sample fixturing. Because the continuously recorded voltage at constant current is a direct signature of the heating element’s resistance, the machine can also characterize the electrothermal behavior of metallic meshes, conductive coatings and nanocomposite heating elements on their own. For teaching laboratories and institutions without access to industrial welding stations, the complete documentation and low cost make it feasible to prepare standard-compliant specimens and expose students to a full sensor-to-actuator control chain, from load-cell acquisition to automated report generation.

The researchers are candid about current limitations. Validation was performed on a single material system and joint geometry at one optimized parameter set, and thermocouple monitoring is restricted to the 50 to 400 °C range. The serial protocol lacks checksum verification and automatic reconnection, and the software does not yet offer closed-loop temperature control or machine-learning integration. Ongoing work aims to map the wider parameter space of voltage, current, pressure and time. Nevertheless, with hardware licensed under CERN-OHL-S v2.0, software under the MIT License, documentation under CC BY 4.0, and all design files hosted on a public repository, OpenWelT represents a significant democratization of advanced composites manufacturing research, proving that a benchtop machine assembled from off-the-shelf components, printed PETG brackets and reclaimed materials can produce joints of genuine structural quality.

Subject of Research: An open-source, low-cost resistance welding machine for joining and studying thermoplastic composite materials.

Article Title: Open-source modular resistance welding equipment for thermoplastic composites

Article References: Reis, J. F., Barbosa Marques, L. F., Lucas Vieira, M. O., Gomes, D. N., Kotz, T. A., & de Oliveira Hein, L. R. (2026). Open-source modular resistance welding equipment for thermoplastic composites. HardwareX, 28, Article e00839. https://doi.org/10.1016/j.ohx.2026.e00839

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00839

Keywords: resistance welding, thermoplastic composites, open-source hardware, aerospace materials, GF/PEI, Joule heating, Arduino control, lap shear strength, ASTM D5868, composite joining, HardwareX, low-cost laboratory equipment

Microjoining, as the authors define it, covers the joining of thin sheets up to 0.5 millimeters thick and fine wires with diameters no greater than one millimeter. The scale is not merely a smaller version of conventional welding. At these dimensions, physics itself changes character. Surface and interfacial effects begin to dominate bulk behavior, capillary forces can overwhelm gravity, and the limited material volume means heat transfer and solidification kinetics behave in ways that defy macroscale intuition. The result is that micro-joints carry thermal stresses and microstructures fundamentally different from their large-scale counterparts, making microjoining a discipline that demands its own scientific foundation rather than a simple downscaling of established practice.

The range of materials involved is striking. Metals remain the workhorses of the field, with stainless steel, titanium and magnesium alloys long favored for their mechanical performance, biocompatibility and cost-effectiveness. Stainless steel has served in surgical implants since the 1920s, but modern demands have pushed far beyond. Shape-memory alloys, particularly nickel-titanium or NiTi, owe their indispensability to reversible martensitic phase transformations that produce the shape memory effect and superelasticity, properties exploited in orthodontic archwires, stents and guidewires. Laser microwelding between NiTi microtubes and platinum-iridium wires has even been used to tailor transformation temperatures for specific device needs. Biodegradable magnesium and zinc alloys, noble metals such as gold, silver and platinum that enhance X-ray visibility, and refractory metals like niobium that serve as biocompatible intermediates round out an expanding metallic palette.

Polymers and composites bring their own advantages, particularly deformability, corrosion resistance and machinability. Laser welding has successfully bonded three-dimensional polymer fabrics for orthopedic applications, achieving reliable fiber-to-fiber joints without surface damage. Researchers have also microjoined polymers to dissimilar materials, as in polyimide-titanium and PETG-stainless steel systems, and integrated metal-coated chips into the inner surfaces of polymer stents. Commercially, the FARAWAVE pulsed field ablation catheter relies on joining metal-ring electrodes to polymer wires, balancing flexible motion with X-ray mapping capability. The looming replacement of PFAS with environmentally sustainable polymers, the authors note, may create new joining challenges as unfamiliar materials enter the production pipeline.

Ceramics present the steepest hurdles. Bioinert oxides and nitrides, bioresorbable calcium phosphates and bioactive glasses are prized for bone and dental restoration, but their high melting points and poor deformability restrict both fusion and solid-state joining at the microscale. Soldering, brazing and adhesive bonding dominate instead. Implantable neural stimulators demonstrate the feasibility of micro-brazing platinum-iridium pins into ceramic substrates, and hermetically packaged neurostimulation devices have been built by co-firing ceramics with platinum tracks, pointing toward a ceramic-rich future for next-generation implantables.

Four families of joining methods have been adapted to the microscale. Fusion welding, the most widely used, employs laser beams, electron beams, plasma arcs or contact-based resistance heating to melt and re-solidify base materials. Laser microwelding leads the field thanks to its high energy density and precise controllability. Yet fusion welding of dissimilar metals reveals the discipline’s central difficulty: when NiTi wires are laser welded to stainless steel, rapid heating and cooling produce uneven mixing and non-equilibrium solidification, filling the fusion zone with brittle intermetallic compounds that cripple joint strength. Resistance microwelding offers an elegant workaround. Using miniature copper-chromium electrodes roughly one millimeter in diameter, researchers have squeezed molten liquid out of the interface entirely, achieving quasi-solid-state direct bonding between NiTi and stainless steel while preserving flexible deformability on one side and superelasticity on the other.

Solid-state welding sidesteps melting altogether, relying on interdiffusion under pressure through friction, ultrasonic vibration or diffusion bonding. Ultrasonic microwelding of thin titanium sheets produces interfacial regions less than five micrometers thick, while vapor foil actuator welding joins NiTi wires to brass sheets without significant alloy intermixing, suppressing intermetallic formation. Brazing and soldering with low-melting-point fillers enable complex configurations, exemplified by furnace micro-brazing of shape-memory alloy wires into honeycomb stent architectures that retain superelastic response, and laser micro-brazing of platinum rings to copper-coated stainless steel wires for interventional devices requiring both radiopacity and signal feedback. Adhesive bonding remains indispensable for assembling thermoplastic microfluidic substrates used in bioanalysis and diagnostics, though surface preparation through plasma cleaning proves critical to joint quality.

Evaluating micro-joints for biomedical service requires far more than simple strength measurements. Digital image correlation has revealed high strain localization around weld nuggets in resistance-welded shape-memory alloy sheets, identifying crack nucleation sites, while scanning electron microscopy enables in-situ testing of wire joints too small for optical strain mapping. Functional performance matters as much as mechanics: arc-welded NiTi wires show markedly increased residual strain during superelastic cycling compared with base material, attributed to grain coarsening and microstructural evolution in the weld zone. Thermomechanical fatigue testing under cyclic martensitic transformations shows welded joints suffer reduced fatigue life, though post-weld heat treatment can homogenize microstructure and restore durability. Corrosion and biocompatibility add further complexity, with studies showing that beneficial alloying element mixing during fusion welding can improve passive film formation, while toxic elements like copper, cobalt and nickel must be rigorously avoided.

The perspective outlines four frontiers for the coming decade. Smart devices such as MEMS sensor-equipped fractional flow reserve guidewires will demand increasingly sophisticated multi-material joining strategies combining soldering, adhesive bonding and fusion welding in a single assembly. Process optimization offers immediate gains, as demonstrated by electromagnetic-field-assisted laser microwelding of NiTi to stainless steel, where induced Lorentz forces drive the liquid fusion zone toward the steel side, limiting brittle iron-titanium phases and boosting tensile strength by 28 percent and ductility by 137 percent. Deeper fundamental understanding will come from in-situ X-ray imaging, smoothed particle hydrodynamics simulations and density functional theory calculations applied at microscale resolution. Finally, the authors call for standardized joining and testing protocols, whose current absence limits cross-study comparison and industrial adoption. With human lifespans lengthening and medical technology accelerating, the researchers argue that microjoining stands poised to become a defining enabler of the next generation of reliable, multifunctional biomedical devices.

Subject of Research: Microjoining technologies for the fabrication of biomedical devices

Article Title: Microjoining for biomedical device fabrication: a perspective

Article References: Zhang, K., Zhou, Y. N., & Peng, P. (2026). Microjoining for biomedical device fabrication: a perspective. Advanced Materials Joining, 1(1), Article 3. https://doi.org/10.1007/s44500-026-00006-z

Image Credits: AI Generated

DOI: 10.1007/s44500-026-00006-z

Keywords: microjoining, biomedical devices, laser microwelding, shape memory alloys, NiTi, stents, resistance welding, solid-state welding, brazing, biocompatibility, intermetallic compounds, implantable sensors