A zigzag stitch enables fabric to stretch until the thread is straight. University of Tartu researchers report in Advanced Materials that thread packing can encode fabric stretchability, leading the way to tailoring wearables at industrial scale.
As every body is unique, achieving a perfect dynamic fit of garments has to date relied on artisanal tailoring that cannot scale. Machine embroidery can place load-bearing thread in arbitrary patterns, but has been applied almost exclusively for visual appeal, such as logos and decorations. Embroidery machines are widely available in industry, hobby use, and as a service, yet their mechanical encoding potential remains largely unexplored, and available embroidery software cannot design for mechanics.
Mechanically active embroidery makes stretchable fabrics into a metamaterial that allows for unique stretchability patterns for each stitchout. "Embroidery is usually decorative - we normally don't want it to stretch. But what if we allowed it to?" said Leonid Zinatullin, the first author of the paper.
The researchers took inspiration from skin. Both textiles and skin act as fibrous metamaterials whose properties depend on how those fibers are packed. Wrinkles in skin are the most apparent form of packing; however, waves in collagen fibers allow for additional stretch. Regions where collagen is packed more densely stretch more. Textiles work in a similar way: looped threads can pack extra length to give fabrics direction-dependent stretchability.
To control stretchability, the research team embroidered short zigzag 'fibrous springs' of inelastic polyester thread on an elastic fabric to control stretchability. A straight seam doesn't stretch, and a zigzag seam stretches until it straightens. The zigzag amplitude defined how much extra thread was available for stretch before the 'fibrous spring' straightened.
The researchers tiled the 'fibrous springs' into a triangular mesh that is embroidered in a single pass. Where two springs meet, the machine loops new thread around the previously placed one, forming firm knots that cannot unravel. As a result, the whole fabric is covered in a fibrous spring mosaic with the stretch limit of each spring individually controlled. As a triangle cannot be stretched without stretching one of its sides, it was a perfect base unit for encoding stretchability.
To make the design practical, the authors used common drawing software, coupled with a custom Python library, to encode fabric mechanics. The three color channels of a raster image (red, green, and blue) provided a natural way to assign properties to the triangular mesh of fibrous springs, allowing designers to 'paint' mechanics using familiar visual workflows and convert artwork directly into stitch patterns.
The embroidered patterns gave researchers control over stretchability at a 7-mm resolution, sufficient to mimic the mechanics of skin. The mosaic surface can consist of thousands of such thread springs, each with an individual stretch limit. While each fibrous spring acted independently, neighbouring springs combined their stress limits to coordinate how the textile deformed. Embroidery can now locally set elastic and inelastic directions in a fabric, enabling garments that stretch with the body in some areas while restricting unwanted movement in others.
Directional stretchability is essential in skin biomechanics, where it maintains tension and guides body movement. Natural and synthetic leather are commonly used in wearables for aesthetics and moisture resistance, but tanning removes directional stretchability, and synthetic substitutes typically neglect it. As a result, leather in garments is, by default, treated as a uniform sheet, mechanically very different from skin. Encoded embroidery reproduces anisotropic stretch, providing an intrinsically compliant 'second skin' that follows the same mechanical properties as living tissue. "Although there are synthetic materials that look more skinlike than our embroidery, our solution is functionally much closer to natural skin", said Indrek Must, the last author of the work.
As a demonstrator, the researchers fabricated footwear from a single embroidered fabric piece, containing more than a thousand unit cells and almost twenty thousand stitches. Minimal sewing was needed to complete the footwear after embroidery. The demonstrator shoe showed a good fit to the foot, conforming to the heel without excessive slack and preventing toe torsion without restricting flexion. Encoded footwear could help reduce injury risk in activities that need high foot coordination, such as sports, and in occupations with heavy foot load, such as logistics.
The interconnected thread spring architecture functions as a physical neural network. Each node is a simple information processor, but together they act as more than the sum of their parts. Each node participates directly in the physical world, with fabric behavior emerging from external stimuli and a locally encoded program. The prototype shoe sensed foot-ground forces and adjusted gait immediately according to its stitched instructions. "We provide mass-customization at industrial speed using commercial instrumentation and materials, blending together software and hardware: the program code can be literally seen by eye and touched by finger", said Indrek Must. The visual appeal of embroidery is fully retained and even elevated, with the stitch pattern both distinctive and aesthetically pleasing, combining performance with desirability. "It is an example of how science can be beautiful - not only in visual appeal, but also in how a program code can hide in plain sight, masked as a natural cue in garments rather than an alienating add-on," added Must. In this way, the work also helps robotics feel more natural and socially accepted. Such physical groundedness could also provide a safety layer for robots and a direct interface for embodied artificial intelligence.
The footwear prototype promises a highly scalable solution to shoe fit. The same encoding principle could also extend to sportswear, orthopedic supports, and other garments, as well as engineered surfaces where graded stretch is needed.
By turning embroidery into a tool for programming mechanics, the researchers show how a simple software update can make textiles more comfortable, more adaptable, and smarter.
