Most flexible 3D printed parts are dumb in the nicest possible way. A TPU gasket seals, a bumper absorbs a hit, and neither can tell you what just happened to it. A study reported this week points at a flexible part that gets stiffer, carries more load, and changes its electrical resistance as it stretches, so the part itself becomes the sensor.
What the study found
A team at VSB Technical University of Ostrava in the Czech Republic mixed TPU powder directly with micron sized carbon fiber powder before extruding it into 1.75 mm filament, instead of compounding pellets with a masterbatch. The work appeared in the Journal of Materials Research and Technology and was reported by Fabbaloo on September 3, 2026.
They tested five loadings, from plain TPU to 20 percent carbon fiber by weight, and printed all of them on a stock Prusa i3 MK3S with a 0.4 mm nozzle. The reported numbers:
- Tensile strength rose from 9.13 MPa (pure TPU) to 15.72 MPa at 20 percent carbon fiber.
- Young's modulus climbed from 118.78 MPa to 301.84 MPa, roughly 2.5 times stiffer.
- Shore A hardness moved only from about 87.5 to 93.5, so the material still behaves like a rubber, not a rigid plastic.
- Electrical conductivity jumped between 5 and 10 percent loading, reaching 12 S/m at 20 percent.
- In preliminary cyclic testing, the 20 percent blend showed a gauge factor of about 3.4 with repeatable resistance changes as it stretched.
That last point is the headline. Gauge factor is the basic figure of merit for a strain gauge, so a printed TPU part with a repeatable one can, in principle, report its own deformation without a separate sensor glued on.
I need a flexible 3D printed part that can sense when it bends. Is that actually possible?
Yes, in the lab: a 20 percent carbon fiber TPU blend printed on a standard desktop FDM printer changed its electrical resistance repeatably as it was stretched, with a gauge factor of about 3.4, but it is still research material, not a filament you can order today. Conductive TPU filaments exist today, but this study is notable for combining a real stiffness gain, retained elastomer behavior, and a repeatable strain response in one formulation. If your product depends on this now, the practical path is a printed flexible body plus an off the shelf flex sensor, with the geometry designed so the sensor sits where the bending happens. That is a design and prototyping job, which is what we do at Build and Ship.
The catch: rougher surfaces and gaps between lines
Nothing is free. Surface roughness went from 3.62 microns for pure TPU to 36.20 microns at 20 percent carbon, a ten fold increase, and microscopy showed larger gaps between extrusion lines at higher loadings. The researchers blame higher melt viscosity and poorer coalescence. For a self sensing part that matters twice, because the conductive network runs through those same lines: gaps in the print are gaps in the circuit. Treat this as a promising direction, not a finished material.
My TPU parts are too floppy but rigid filament cracks. What should I print them in?
Print in a harder TPU (95A or above) or a carbon fiber filled flexible filament, and add stiffness through geometry (ribs, thicker walls, more perimeters) before you reach for a completely different material. A 2.5 times jump in modulus while hardness barely moved means a part can still cushion and conform without sagging under its own load, which is the middle ground drone landing gear, gripper fingers, and strain reliefs usually need. When the rigid half of the assembly must survive heat or chemicals, that is where our PEEK and ULTEM printing service comes in.
What this means for makers
The powder first approach is the interesting bit for anyone who compounds their own filament. Fiber clumps that clog nozzles are the classic failure of home made composite filament, and starting from two similar sized powders is a cheap fix that seems to have worked, with nothing exotic on the printing side. To experiment with self sensing prints today, a commercial conductive TPU, a multimeter, and a bending fixture will get you far: measure flat, measure bent, repeat fifty times.
What this means for engineering buyers
Flexible printed components are getting more capable, and the properties are tunable at the filament stage. Instead of specifying "a rubber part," you can specify a target modulus, a hardness range, and whether it needs to conduct or insulate, all things a print shop can work toward with material choice and orientation. If you have a legacy flexible part with no drawings, our 3D scanning and reverse engineering service captures it, we rebuild it in CAD, and we pick a material that matches how the original behaved.
Filament makers: if you produce a conductive or carbon filled flexible filament and want it tested on real parts, send a spool to DC Additive Pros and we will write up what we find. Reach us at info@dcadditivepros.com.
Frequently asked questions
Can a 3D printed TPU part be electrically conductive?
Yes. Adding carbon fiber or carbon black to TPU creates a conductive network once the loading passes a threshold; in the new study, conductivity jumped between 5 and 10 percent carbon fiber and reached 12 S/m at 20 percent.
Does adding carbon fiber to TPU make it stiff like nylon?
No. In the study, Young's modulus rose about 2.5 times but Shore A hardness only moved from about 87.5 to 93.5, so the material stayed rubbery rather than becoming a rigid plastic.
Why do my carbon fiber TPU prints look rough and full of gaps?
Carbon filler raises melt viscosity and makes the extruded lines fuse poorly; the researchers measured surface roughness climbing from 3.62 to 36.20 microns at 20 percent loading. Slower speeds, higher temperatures, and a larger nozzle usually help.
Can I order a custom flexible 3D printed part in the USA with no minimum quantity?
Yes. DC Additive Pros in Rockville, Maryland prints one off and low volume flexible parts in TPU and other materials with no minimum order; email info@dcadditivepros.com with a file or a description.
Source: Fabbaloo, "Researchers Investigate TPU-CF Properties," September 3, 2026, reporting on a study in the Journal of Materials Research and Technology by researchers at VSB Technical University of Ostrava. All figures quoted are from that report.