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Holography speeds up microscale tissue engineering

BioPhotonics
Jan 2012
Ashley N. Paddock, ashley.paddock@photonics.com

HANNOVER, Germany – A new laser holography technique can speed up the construction of finely detailed scaffolds on which human cells can grow, paving the way for more widespread clinical use of microscale medical devices to replace lost or damaged tissue.

Most tissue engineering applications have been confined to research labs, which has been a challenge for manufacturing frameworks on which to grow human cells for replacement tissue.


A multilayer tissue scaffold created with single-focus two-photon polymerization (2PP) is shown (a) beside one created with multifoci two-photon polymerization (b). The two scaffolds are structurally similar, but the one created with multifoci 2PP was completed in approximately one-fourth the time. Cells from the inner lining of bovine blood vessels are shown (c) growing on the multifoci-created scaffold.


But a modified manufacturing technique called two-photon polymerization (2PP) can create finely detailed structures such as tissue scaffolds more quickly and efficiently, according to a research team from Laser Zentrum Hannover eV Institute and the Joint Department of Biomedical Engineering at the University of North Carolina at Chapel Hill and North Carolina State University in Raleigh.

The 2PP process occurs at the microscopic level. Current 2PP technology involves a laser pulse that lasts approximately one-quadrillionth of a second and sends an energy burst into unset resin, causing the molecules around the pulse to fuse together into two adjoining cone shapes. By focusing on multiple points in succession, 2PP can build up complex 3-D structures, one cone-shaped block at a time.


This scanning electron microscope image shows 16 microscopic Venus statue replicas that were produced simultaneously by a 16-beam two-photon polymerization system. The structures were produced in approximately 45 s. Images courtesy of Biomedical Optics Express.


Unfortunately, this process is too slow for biomedical applications, so the researchers used a computer-controlled hologram to split the 2PP laser into multiple beams, creating up to 16 different focus points that can work simultaneously.

The conventional fabrication time for a single-layer 1-mm square with 100-nm resolution was 2 h and 47 min, with a single-focus 2PP process. Using the hologram-enhanced 16-foci process, the team cut production time down to about 10 min.


An array of microneedles was created four needles at a time using a four-beam two-photon polymerization system.


The scientists first demonstrated the concept by creating 16 microscopic Venus statues. They also used the technique to manufacture cylindrical tissue scaffolds and an array of microneedles, which could be used to provide painless injections or to take blood samples. The work was published in Biomedical Optics Express (http://dx.doi.org/10.1364/BOE.2.003167).

The researchers plan to use the system to produce one large, complex 3-D structure. Once accomplished, producing such detailed features on such a large scale could prove useful for controlling cell attachment and alignment, which is important because cell orientation affects function in nerves, bones, muscle and blood vessels.


GLOSSARY
two-photon polymerization
An additive fabrication technique, referred to as TPP, used to make 3D microstructures with submicron feature sizes by using a near-infrared (NIR) emission that excites a photosensitive resin, triggering multiphoton absorption where light intensity is highest and a polymerization process that changes it from a liquid to a solid. When the volume of the focused laser beams, or voxels, are precisely overlapped, 3D microstructures are created and revealed by washing away unsolidified resin with an...
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