3D printed waveguides for optogenetics applications: design optimization and optical characterization

TL;DR

This work addresses delivering precise optogenetic stimulation to 3D tissue constructs by fabricating a hollow, 3D-printed waveguide via projection microstereolithography using the BMF acrylate resin. It combines detailed optical characterization (refractive index $n$ and extinction coefficient $\kappa$), FEM-based design optimization (favoring star-shaped openings and specific row/column counts and cylinder radius), and a functional proof-of-concept with ChR2-modified neural cells that exhibit light-evoked dopamine release. The study reports $n(470\,\mathrm{nm})\approx 1.52$, $n(590\,\mathrm{nm})\approx 1.51$, and $\kappa$ on the order of $10^{-6}$, enabling high transparency; it demonstrates that a 3D-printed waveguide can transmit sufficient light to trigger cellular responses, albeit with a measured stimulation efficiency of $\approx 2.8\%$ that informs future design iterations. Overall, the work validates a new route toward customizable, implantable optogenetic devices suitable for organoids and 3D tissue models, with clear pathways for reducing losses and enhancing stimulation efficacy in future iterations.

Abstract

Optogenetics has emerged as a powerful tool for disease modeling, enabling precise control of cellular activities through light stimulation and providing a valuable insights into disease mechanisms and therapeutic possibilities. Innovative materials and technologies such as micro-LEDs, optical fibers and micro/nano probes have been developed to allow precise spatial and temporal control of light delivery to target cells. Recent advances in 3D printing have further enhanced optogenetic applications by enabling the fabrication of implantable, customizable, and miniaturized light stimulation systems with high spatial resolution. In this study, we introduce a novel concept of a 3D printed light delivery system for brain organoid stimulation exploring the capabilities of projection microstereolithography (P$μ$SL). We characterized the optical properties of the high-resolution acrylate-based 3D print resin, i.e., refractive index and extinction coefficient, to evaluate if the light transmission efficiency might limit the performance of the optogenetic stimulation systems. Finite element method simulations were employed to optimize the 3D printed design. An optogenetic setup was developed for optimal light delivery, and initial tests with optogenetically modified cells showed light-induced dopamine release with a stimulation efficiency of 2.8\%, confirming the 3D printed waveguide functionality and guiding future optimization. Our results demonstrate that this light stimulation tool offers strong potential for advancing customizable optogenetic applications.

Figures (8)

• Figure 1: Schematic representation of organoid photostimulation systems. (Left) 2D photoexcitation approaches: (top) full implanted Micro-LED; (bottom) partially implanted cannula coupled to an optical fiber. (Right) Innovative 3D optogenetic stimulation system proposed in this work for 3D optogenetic cells and organoids stimulation.
• Figure 2: a-b Optical transmission characterization setup. a 3D printed disks with different thickness: $h_\mathrm{A}=0.5$ mm, $h_\mathrm{B}=1.0$ mm, $h_\mathrm{C}=1.2$ mm. b A schematic of the experimental setup. d-e Light divergence characterization setup. a A schematic view of the distribution and the expansion of the light beam. b A lateral view of the experimental setup.
• Figure 3: Experimental setup and FEM modeling. a Schematic sketch of the optogenetic system testing . b Definition of the power detector (light red arrows indicate the scattered rays absorbed and measured by the detector). c Design of the device, with a focus on the main geometrical parameters used for optimization. d Definition of the light source at the bottom surface of input intensity $I_0$ (the red arrows indicate the directionality of the emission).
• Figure 4: Optical characterization of BMF 3D print resin and comparison to other materials. a Ellipsometric determination of $n$ as a function of the excitation wavelength $\lambda$ for different incident angles $\phi$. b Spectrophotometric measurements of light transmission $T$ as a function of $\lambda$ (black solid curve) together with the corresponding calculated absorbance $A$ (red dashed curve). c-d Comparison of the BMF resin ($n$, $\kappa$) with the other materials exploited in optogenetics at 470 nm (c) and 590 nm (d) probing wavelength.
• Figure 5: Characterization of light transmission and divergence. a-b Detected power $P_{det}$ emitted by the (a) LED$_{470}$ (blue curve) and (b) LED$_{590}$ (orange curve) as a function of the LED-photodetector distance ($d$), with no resin in between. The performed curve fitting (black solid curve) according to Equation \ref{['eq:P_det']} gives a divergence coefficient ($\beta$) of (a) $22$ m$^{-1}$ and (b) $59$ m$^{-1}$. The dashed lines indicate $P_{det}=P_0/2$ (horizontal) and the corresponding $d$ (vertical). c Comparison between the experimental (solid curves) and nominal (dashed curves) intensity profiles for the LED$_{470}$ (blue) and LED$_{590}$ (orange). Both profiles are Gaussian kashiwao2025modeling. d-e Intensity attenuation for (d) LED$_{470}$ and (e) LED$_{590}$ as a function of the disk thickness $h_{cyl}$. In d, an absorption coefficient $\alpha=250$ m$^{-1}$ has been extracted based on linear regression. The horizontal dashed line indicates the threshold intensity for ChR2 photostimulation. Solid lines indicates the intensity reduction within the BMF disk, for different LED-to-disk distance $d$.