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Lesson 13.8Micro & nanoChapter 13 · Micro- and nanofabrication

Two-photon polymerization

Hardens resin only at a laser focus to print 3D micro-parts

Two-photon polymerization is 3D printing at the scale of a living cell. A focused laser hardens liquid resin only at its focal point, so moving that point draws objects with details under a micron (0.001 mm). It prints micro-lenses onto optical fiber tips and scaffolds for growing cells.

Two-photon polymerization: a real photo
Photo: Spanholz, CC BY-SA 4.0, via Wikimedia Commons
Resin hardens only at the focus of the laser, so the focus can draw in 3DTwo-photon polymerization · side view, not to scaleFemtosecond lasernear-infrared pulsesScannermoves the focusObjective lens2liquid resinglass slide1part being written4focus movesin 3D3Only the focus hardenszoom on the focal spottoo dim:stays liquidhardenedvoxeltwo photons absorbed at once5Wash off the liquidfree-standingmicro-partLiquid resinHardened partMachine partsLaser light
two-photon polymerization · printer side view, focus zoom and finished partCircled numbers match the steps below; the right-hand panels zoom in on the focus and show the finished part.

How it works

  1. Place a drop of resin. A drop of liquid photoresin (a plastic that hardens under light) goes on a glass slide, or directly on a fiber tip or chip.

  2. Focus the laser. Femtosecond pulses of near-infrared light (just beyond red, invisible to us) pass through a microscope objective, a powerful lens. They converge to a spot inside the resin.

  3. Harden only the focus. Only at the focus is light intense enough for molecules to absorb two photons (particles of light) at once. That hardens a voxel: a 3D pixel around a micron or smaller.

  4. Write in 3D. Mirrors and stages move the focus along paths sliced from a CAD model, point by point and layer by layer. The part grows inside the liquid, often without support structures.

  5. Wash off the liquid. A solvent rinses away the unhardened resin, revealing the solid micro-part. Delicate parts are dried carefully so liquid surface tension does not bend them.

Key facts

Materials
Photoresins: acrylates, epoxies such as SU-8, hybrid organic-glass resins; some biocompatible or glass-forming resins
Typical parts
Micro-lenses and optics, cell scaffolds, microfluidic filters, lattice metamaterials, record-small sculptures
Part size
Features from about 0.1 µm; whole parts usually micrometers to a few millimeters
Precision
Sub-micron detail; research systems reach features around 100 nm or less
Surface finish
Smooth enough for optics; fine layer steps visible under an electron microscope
Batch size it suits
Prototypes and small series; also masters for molding copies
Tooling and machine cost
No tooling; printers often cost several hundred thousand dollars
Speed
Slow: a part a few millimeters in size can take hours

Strengths and limits

Strengths

  • True free-form 3D at sub-micron resolution
  • No mask or mold: straight from CAD to part
  • Prints onto fibers, chips and inside channels
  • Smooth, optical-quality surfaces are possible

Limits

  • Very slow for anything above a few millimeters
  • Small build volumes and expensive printers
  • Limited to light-curing resins
  • Shrinkage distorts parts unless the design compensates

Where it's used

  • PhotonicsMicro-lenses on fiber tips, waveguides, diffractive optics
  • Life sciencesScaffolds for cells and stem-cell research, micro-needles
  • Microfluidics3D filters and mixers printed inside channels
  • MEMS and sensorsMicro-springs, micro-grippers, tips for atomic force microscopes
  • Materials researchMechanical metamaterials (lattices with unusual properties), nano-lattices
  • Art and outreachMicro-sculptures and record-small objects

How to recognize it

  • You almost never will: the parts are smaller than a grain of sand and live in labs, prototype optics and research devices.
  • Under a microscope: perfectly shaped 3D objects with fine stair-step layer lines, like a tiny 3D print.
  • Overhangs, lattices and closed hollow shapes that no mold could release.
  • A lens or structure sitting right on the cut end of an optical fiber.

Compare with

Stereolithography8.2also hardens resin with light, but layer by layer in a vat; much bigger and faster, with details of tens of microns.Femtosecond laser machining13.6the same kind of laser, but it removes material instead of building it.Volumetric printing8.10also cures resin inside the liquid, but a whole object at once from many angles; fast, but far coarser.

Watch it

g Fun Facts online · A short, simple explanation of the principle

scientificblogging · A race car smaller than a grain of sand being printed

Nanoscribe · Printed micro-parts and electron-microscope close-ups

UpNano · A commercial printer and how its parts are built

See real photos

Remember

  • The resin hardens only where two photons arrive at once: at the laser's focus.
  • Moving that focus draws true 3D shapes with sub-micron detail.
  • It is slow and small-scale: ideal for micro-optics, scaffolds and prototypes.

More in Micro- and nanofabrication

All 100 processes
13.1Czochralski crystal growth (silicon wafers)Pulls a giant single crystal of silicon slowly out of a meltOpen the lesson13.2PhotolithographyProjects a pattern with light onto a light-sensitive coating, then etches itOpen the lesson13.3Molecular beam epitaxy (MBE)Grows a crystal one atomic layer at a time from beams in ultra-high vacuumOpen the lesson13.4Atomic layer deposition (ALD)Coats a surface one atomic layer per cycle with alternating gasesOpen the lesson13.5Focused ion beam (FIB)Mills or adds material with a beam of ions under a microscopeOpen the lesson13.6Femtosecond laser machiningRemoves material with ultrashort laser pulses, without meltingOpen the lesson13.7LIGADraws deep slots with X-rays or UV, fills them with nickel, then molds copiesOpen the lesson13.9ElectrospinningPulls a polymer jet into nanofibers with high voltageOpen the lesson

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