Multi-photon polymerization using upconversion nanoparticles for tunable feature-size printing

The recent development of light-based 3D printing technologies has marked a turning point in additive manufacturing. Through photopolymerization, liquid resins can be solidified into complex objects. Usually, the polymerization is triggered by exciting a photoinitiator with ultraviolet (UV) or blue light. In two-photon printing (TPP), the excitation is done through the non-linear absorption of two photons; it enables printing 100-nm voxels but requires expensive femtosecond lasers which strongly limits their broad dissemination. Upconversion nanoparticles (UCNPs) have recently been proposed as an alternative to TPP for photopolymerization but using continuous-wave lasers. UCNPs convert near-infrared (NIR) into visible/UV light to initiate the polymerization locally as in TPP. Here we provide a study of this multi-photon mechanism and demonstrate how the non-linearity impacts the printing process. In particular, we report on the possibility of fine-tuning the size of the printed voxel by adjusting the NIR excitation intensity. Using gelatin-based hydrogel, we are able to vary the transverse voxel size from 1.3 to 2.8 {\mu}m and the axial size from 7.7 to 59 {\mu}m by adjusting the NIR power without changing the degree of polymerization. This work opens up new opportunities for speeding up the fabrication while preserving the minimum feature size with cheap light sources

Light focusing and additive manufacturing through highly scattering media using upconversion nanoparticles

Light-based additive manufacturing holds great potential in the field of bioprinting due to its exceptional spatial resolution, enabling the reconstruction of intricate tissue structures. However, printing through biological tissues is severely limited due to the strong optical scattering within the tissues. The propagation of light is scrambled to form random speckle patterns, making it impossible to print features at the diffraction-limited size with conventional printing approaches. The poor tissue penetration depth of ultra-violet or blue light, which is commonly used to trigger photopolymerization, further limits the fabrication of high cell-density tissue constructs. Recently, several strategies based on wavefront shaping have been developed to manipulate the light and refocus it inside scattering media to a diffraction-limited spot. In this study, we present a high-resolution additive manufacturing technique using upconversion nanoparticles and a wavefront shaping method that does not require measurement from an invasive detector, i.e., it is a non-invasive technique. Upconversion nanoparticles convert near-infrared light to ultraviolet and visible light. The ultraviolet light serves as a light source for photopolymerization and the visible light as a guide star for digital light shaping. The incident light pattern is manipulated using the feedback information of the guide star to focus light through the tissue. In this way, we experimentally demonstrate that near-infrared light can be non-invasively focused through a strongly scattering medium. By exploiting the optical memory effect, we further demonstrate micro-meter resolution additive manufacturing through highly scattering media such as a 300-{\mu}m-thick chicken breast. This study provides a proof of concept of high-resolution additive manufacturing through turbid media with potential application in tissue engineering

Spontaneous Formation of CdSe Photoluminescent Nanotubes with Visible-Light Photocatalytic Performance

Two-dimensional (2D) colloidal CdSe nanocrystals (NCs) with precise atomic-scale thickness have attracted intensive attention in recent years due to their optical properties and quantum confinement effects originating from their particular band structure. Here, we report a solution-based and template-free protocol to synthesize CdSe nanotubes (NTs) having 3-6 walls, each of which has 3.5 molecular monolayers. Their crystal structure is zincblende, with Cd-terminated {100} planes at the top and bottom surfaces of each wall, which are passivated by short-chain Angular Attachment acetate ligands. After verifying the prominent role of the acetate ligand for NT synthesis, we elucidated the formation mechanism of these NTs. It starts by heterogeneous nucleation of 2D plateletlike nanoseeds from the amorphous Cd precursor matrix, followed by the growth via lateral and angular attachment of nanoplatelet building blocks into curved nanosheets, eventually resulting in NTs with sharp absorption and photoluminescence peak at around 460 nm. Moreover, the NTs show remarkable visible-light photocatalytic activity, as demonstrated by the reduction of the reddish Rhodamine B into its leuco form with a conversion rate of 92% in 1 min

Droplet-Based DNA Purification in a Magnetic Lab-on-a-Chip

Merge, Mix, Split and Transport, these are the main manipulation steps used for on-chip droplet-based DNA purification (see figure). The system is able to extract genomic material from dilute cell samples by using the actuation of magnetic microparticles within the droplets through a matrix of coils

Ultra-thick micro-optical components using the PRISM photosensitive flexopolymer

We present the photosensitive flexopolymer PRISM as a new promising material for the realization of thick optical components. The PRISM flexopolymer can be directly polymerized using conventional UV exposure and is simply developed in a water-based solution. A casting method is used to realize flexopolymer layers of a few millimetres thickness in a single application step. Optical components as thick as 2 mm have been fabricated using an exposure time of less than 1 min and a development time below 3 min. No baking process is required, making the process very fast and avoiding any temperature-induced stress problems. Due to its elastomeric nature, the material can be easily applied either on rigid or flexible supports. The good optical transmission of the PRISM flexopolymer in the 400–800 nm spectral range makes it a promising material for optical applications. Refractive index measurements are performed at different wavelengths in the UV–visible range and the flexopolymer refractive index dispersion behaviour is determined. Optical components such as right-angle prisms, penta-prisms and cylindrical lenses with thicknesses up to a few mm have been successfully fabricated

Insight into the Growth of Anisotropic CdSe Nanocrystals: Attachment of Intrinsically Different Building Blocks

An in-depth understanding of the growth mechanism of nanocrystals (NCs) is of great significance for shape control of colloidal semiconductor nanostructures. In this study, we elucidate the formation mechanism of anisotropic CdSe NCs by systematically investigating their growth in the presence of dioctylamine and carboxylate ligands with different chain lengths. A morphological transition from nanotubes and nanosheets to irregular nanorods and finally to nanodots was observed when the carbon number of the carboxylate ligands increased from 2 to 18. The traditional understanding of the growth of anisotropic CdSe nanostructures is mainly based on the monomer addition, which is difficult to explain the phenomena observed. We found that both short- and long-chain carboxylate ligands can lead to the anisotropic growth of CdSe NCs by attachment of early-formed building blocks, which is a nonclassical particle-mediated growth approach. The use of different carboxylate ligands plays a key role in the formation of different building blocks by affecting the local monomer supersaturation. We provide both experimental observations and first-principles simulations to support this hypothesis