1. Introduction

1.1. Optimal three-dimensional micro-fabrication

Three-dimensional microfabrication is an enabling technology that allows, for instance, the study of interactions at the cellular level [1, 2] by mimicking biological micro-architectures, or to manipulate light by building photonic microstructures [3–5].

The fabrication of these microstructures is best achieved without moving elements inside the build volume since they can distort the end product and decrease the axial printing resolution [6]. Hence, an optimal fabrication method consists in directly writing the solid three-dimensional microstructure deep into a liquid material. For a laser direct writing system, such a manufacturing method requires the ability to confine photopolymerization to a local voxel of material deep into a liquid photoresist.

This confinement cannot be achieved with a purely linear photopolymerization process. For instance, let us consider a laser beam tightly focused into a weakly absorbing photoresist, to neglect the attenuation of the beam. If the absorption is linear, as in single-photon absorption, one can demonstrate that the number of photo-activated molecules is constant in all planes transverse to the propagation direction of the beam [7]. As a consequence, if this linear absorption phenomenon is combined with a linear dose response of the photoresist, polymerization cannot be confined to a specific plane deep within a volume of photopolymer. Therefore, direct three-dimensional microfabrication into a material cannot be achieved with single-photon absorption and a linear photoresist [8].

1.2. Two-photon photopolymerization

Thus, the selective curing of a voxel of material requires inducing a non-linearity in the photopolymerization process, either in the photoresist dose response or in the laser beam absorption.

The latter solution is implemented in two-photon photopolymerization (TPP), which was first introduced in 1997 by Maruo et al. [9] and has become the technique of choice for producing three-dimensional microstructures [10]. In TPP, a photo-initiating molecule is activated upon simultaneous absorption of two photons [11, 12]. Owing to the extremely small two-photon absorption cross-section of photo-initiating molecules (10²–10³ · 10⁻⁵⁰cm⁴s photon⁻¹), which encompasses the limited lifetime (10⁻¹⁵–10⁻¹⁶ s) of their excited state after the first photon is absorbed [13], TPP is confined to voxels where there is both a high photon flux density and an intense photon flux [14]. Typically, in most TPP systems, such conditions for non-linear absorption are met by focusing femtosecond laser pulses with high numerical aperture (NA) objectives. Though TPP systems can achieve a linewidth resolution of 80 nm [15], the complexity and cost of femtosecond lasers prevent the integration of these printing systems within other manufacturing platforms and lessen their affordability.

1.3. Alternative non-linear photopolymerization processes

The implementation complexity of TPP have oriented research towards alternative non-linear photopolymerization processes. Scott et al. [16] investigated the lateral spatial confinement of polymerization using a two-color photo-initiation photo-inhibition scheme. The authors did not explore axial spatial confinement of polymerization but suggested the use of an inhibiting “bottle beam” [17] to achieve it. Recent progress in designing an efficient non-linear photo-inhibitor [18] could open up this possibility. Alternatively, several groups have recently demonstrated three-dimensional microfabrication deep into a photoresist through pointwise scanning of a highly-focused continuous-wave (CW) laser beam [8, 19–23]. The mechanisms behind this spatially confined photopolymerization process are still unclear: the authors respectively suggested a non-linear dose response of the photoresist [8], a possible photolysis of organic bonds [19] or an ultra-low single-photon absorption combined with photothermal effects [20,21,24]. Though simpler and more affordable than TPP, this direct writing technique still requires the generation of intensities as high as ∼10⁷W.cm⁻² to reach the polymerization threshold. As reported by several authors [8,19,25], such intensity levels can result in erratic microexplosions in the material, thus reducing the printing dynamic range and damaging the printed microstructure and its vicinity.

Another approach to induce a non-linear photopolymerization behavior consists in the combination of single-photon absorption and a non-linear dose response of the photoresist. Oxygen, as a strong radical scavenger [26], inhibits free-radical photopolymerization and has been used to create a so-called “dead-zone” of polymerization [27]. In these regions, the rate of radical photogeneration is lower than the rate of radical scavenging by oxygen and no polymerization takes place even after very long exposures. On the other hand, in regions exposed to a higher intensity, a similar dose will result in polymerization since the rate of radical photogeneration will overcome the scavenging threshold. Maruo et al. [22] took advantage of this non-linear dose response to build three-dimensional microstructures through single-photon polymerization. This was achieved by focusing a CW laser with a 1.0-NA microscope objective to confine photopolymerization to the focal volume without suffering from the cumulative dose effect in the surrounding “dead-zones”. With this method, intensities of 10⁻¹W.cm⁻², less harmful to materials, and off-the-shelf photo-initiators can be employed.

In this paper, we exploit the same non-linear photopolymerization phenomenon through a thin multimode optical fiber (∅70 μm, NA 0.64) with the aim of providing a compact and affordable alternative to TPP systems. First, we investigate the kinetics of single-photon photopolymerization and show that photopolymerization can be confined to a focal spot generated 50 μm deep into an off-the-shelf photoresist. Using a specific curing method, we then demonstrate three-dimensional microfabrication of solid and hollow microstructures by digitally focusing and scanning CW laser light through the multimode fiber.

2. Materials and methods

2.1. Calibration of the multimode fiber for the distal generation of focused laser spots

Three-dimensional microstructures are built through a multimode optical fiber (MMF) by adapting the endoscopic imaging setup of Loterie et al. [28] for micro-additive manufacturing.

Multimode fibers are scrambling media. Owing to the different propagation constants of the fiber’s modes, an image supplied at the input of a MMF will be distorted as it travels through it; which eventually results in a speckle pattern at the output. However, MMFs remain interesting tools for compact and low-cost imaging systems since the effect of modal scrambling on image transmission is deterministic and can be compensated by measuring the MMF’s propagation characteristics [29–31].

Hence, the generation of focused laser spots through a MMF requires a calibration step prior to each microfabrication experiment. Here, the fiber’s transmission matrix [32] is determined (see Fig. 1(a)) by measuring with off-axis holography the MMF’s output response (on the distal side in Fig. 1(a)) to a series of independent plane waves that are fed to the MMF’s input using wavefront shaping (on the proximal side in Fig. 1(a)).

 

Fig. 1 (a) Experimental setup for the calibration of the multimode fiber (MMF) prior to each microfabrication experiment. The MMF transmission matrix is determined by feeding a series of independent plane waves to its proximal side using wavefront shaping, the MMF’s response to these inputs is measured on the distal side with off-axis holography. (b) Experimental setup for single-photon micro-additive manufacturing through the calibrated MMF. Following the calibration, the MMF distal tip is dipped into a droplet of photoresist and three-dimensional microstructures are built by digitally focusing and scanning CW light through the MMF. (c) Close-up view of Fig. 1(b) for the definition of the build volume under the MMF distal tip and the position of the microtube printed through the system as discussed in section 3.2 (d) Experimental measurement of the uniformity of light focusing through the MMF over the build volume defined in Fig. 1(c).

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Briefly, as shown in Fig. 1(a), CW laser light at 488 nm (Coherent, Sapphire 488 SF) is coupled into two polarization-maintaining single mode fibers MOD and REF with a 90:10 ratio. The linearly-polarized MOD beam is collimated by lens L_C (f = 150 mm) and modulated by a spatial light modulator (SLM; Pluto VIS, Holoeye). Half of the beam is sent to a powermeter PWM (Model 2936-C, Newport) through a 50:50 beamsplitter BS to monitor the beam power. The modulated beam is fed to the multimode optical fiber (MMF; GOF85, NA 0.64, ∅70 μm, Schott), held in a syringe needle, through a 4-f system made of lens L₁ (f = 175 mm) and microscope objective OBJ (NA 0.8, 100×, Zeiss). The beam is further filtered by a pinhole F in the Fourier plane to retain only its modulated component. To better exploit the modes of the fiber, the beam is circularly polarized with a quarter-wave plate λ/4 before being coupled to the fiber, and then set back to a linear polarization at the fiber output [33]. The resulting speckle pattern at the MMF distal side is combined with a tilted reference plane wave REF and imaged onto a camera CAM. Using this off-axis holographic system, we thus extract the amplitude and phase response of the MMF to a series of independent plane waves (which are points in the Fourier domain) to determine the fiber’s transmission matrix. Further details on this measurement can be found in our previous works [28,34].

2.2. Experimental setup for single-photon microfabrication

Following the calibration of the MMF’s transmission matrix, focused CW laser spots can be generated ahead of the MMF distal facet using the proximal wavefront shaping system. We generate these focused laser spots on the distal side by calculating the associated wavefront to feed to the fiber on the proximal side. More precisely, this wavefront is determined by multiplying the wavefront desired on the distal side of the MMF with the inverted transmission matrix [34]. The obtained wavefront is then generated on the spatial light modulator and sent to the MMF proximal facet.

The single-photon direct-writing experiments are then carried out with the experimental setup described in Fig. 1(b). The MMF and proximal side of the calibration setup shown in Fig. 1(a) are jointly vertically displaced and the distal end of the MMF is dipped into a droplet of photoresist deposited on a plasma-cleaned glass slide. Three-dimensional microstructures are then fabricated in open-air conditions by digitally focusing and scanning point-by-point the CW laser light through the MMF using the wavefront shaping system at the proximal end.

The microstructures are written into a cubic volume of 30-μm edge (see the close-up view in Fig. 1(c)). This build volume is centered on the optical axis of the fiber and microstructures are fabricated with a bottom-up approach, starting from the glass slide, 50 μm below the MMF distal facet. The uniformity of light delivery over the build volume is of paramount importance to correctly print microstructures. Therefore, the ratio of energy focused in the phase-controlled spot over the total field energy was measured and we observe a maximal energy gap of 20% over the build volume (see Fig. 1(d)), which we assume to be reasonable to print simple structures within this volume. The uniformity of light delivery could be improved to further enlarge the build volume, for instance by compensating for the spot distortion as light is focused off the fiber’s axis [35] however this technical improvement is beyond the scope of our paper.

The photoresist in which microstructures are directly written is made of off-the-shelf chemical components: an organic polymer precursor trimethylolpropane triacrylate (TMPTA; >70%, Aldrich, USA), 1wt% of the Norrish type II photoinitiator camphorquinone (CQ; 97%, Aldrich, USA) and 0.5wt% of the synergist Ethyl 4-(dimethylamino)benzoate (EDAB; 97%, Aldrich, USA).

3. Results and discussion

3.1. Single-photon non-linear photopolymerization

The selective curing of a voxel of material for true three-dimensional microfabrication requires inducing a non-linearity in the photopolymerization process, either in the photoresist dose response or in the laser beam absorption.

The intensity attenuation of a light beam undergoing a N-photon absorption process in a material can be generally expressed as: $\frac{dI(z)}{dz}=-\alpha \cdot I{\left(z\right)}^N$, where I(z) is the intensity of the light beam propagating along the z-axis and α is the N-photon absorption coefficient of the material [12]. Similarly, the effective non-linearity or linearity of CW-induced photopolymerization can be determined by measuring the exponent N of the relationship between the exposure dose D, the threshold power for photopolymerization P_th and the exposure time Δt: D ∝ Δt · P_th^N [6, 8, 19, 25, 36]. Rewriting this relationship as P_th ∝ Δt^−1/N, we infer that the non-linearity exponent N can be measured by varying the photoresist exposure time Δt and determining the associated threshold power for polymerization P_th [19].

To calibrate the photoresist polymerization kinetics, we experimentally implement this method by focusing CW laser light at 488 nm into a droplet of photoresist prepared as described in section 2.2 and deposited onto a plasma-cleaned glass slide. The impact of oxygen on the photopolymerization threshold is measured by imaging the induced polymer spots with a DIC-microscope. Prior to the experiment, the photoresist is either bubbled with O₂ and the droplet is let to reach gas equilibrium onto the glass slide for a few minutes, or the photoresist is bubbled with N₂ and enclosed between a glass slide and a coverslip. Laser light is focused in an inverted configuration through a microscope objective matching the numerical aperture of the MMF and reflections at the air-glass interface are taken into account. The same measurement was then performed with the setup of Fig. 1(b) for the oxygen-bubbled configuration and exhibited consistent results. Another approach for time-resolved measurement of the photopolymerization threshold [36, 37] was also implemented but was not taken into account as the measured polymerization time was greatly overestimated compared to direct DIC imaging of the induced polymer spots (see Supplemental Information in Appendix A).

The calibration of the photopolymerization threshold clearly reveals a non-linear photopolymerization behavior for both the oxygen- and nitrogen-bubbled photoresists (see Fig. 2(a)) as the measured photopolymerization thresholds do not follow an isodose trend (see dashed lines in Fig. 2(a)), typical of a linear photopolymerization process. For the oxygen-bubbled photoresist, which we use for three-dimensional microfabrication, we find the non-linearity exponent to be N_O2 = 3.36±0.21 with a power-law fit on the data points with exposure time below 2 seconds. This value is similar to that obtained by Mueller et al. [36] with 7-Diethylamino-3-thenoylcoumarin (DETC) and Isopropylthioxanthone (ITX) photoinitiators. For exposure times longer than 2 s, the data points deviate from the power-law fit for both the oxygen- and nitrogen-bubbled photoresists, likely due to oxygen diffusion and photoinitiator depletion [36]. This deviation also evidences that bubbling the photoresist with nitrogen did not remove all the oxygen content, which might also account for the high non-linearity of the nitrogen-bubbled photoresist N_N2 = 2.41 ± 0.36. The non-linearities measured here would likely turn towards N = 1, that is to say a linear photoresist behavior, for sufficiently small exposure times, where diffusion effects are too slow to quench photopolymerization.

 

Fig. 2 (a) Polymerization threshold power of a single polymer spot versus the spot exposure time, the dashed lines are isodose lines, the parametric space for efficient printing of three-dimensional microstructures is also depicted in gray. (b) Absorption spectrum of camphorquinone (CQ) in C₆H₁₂ (l = 0.4 cm) (c) Simulation of the beam propagation and absorption within the photoresist. The beam is focused 50 μm below the MMF.

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Previous works reported on a similar non-linear photopolymerization behavior induced by CW laser light [8, 19–21, 24]. The authors speculated that the non-linearity stemmed from an ultra-low single-photon absorption [8, 20] as well as the photolysis of organic bonds [19], or single-photon photopolymerization inhibited by oxygen radical scavenging [22]. As to the non-linearity evidenced in our experiments, the intensity level we used and the absorbance of the photoresist unambiguously rule out a non-linear absorption phenomenon, the photolysis of organic bonds and an ultra-low single-photon absorption phenomenon. Indeed, the laser intensities used in our experiments ∼ 10¹–10²W.cm⁻² (see Fig. 2(a)), are too low by several orders of magnitude to induce multiphoton absorption or avalanche ionization ∼ 10¹²W.cm⁻² [14,38], or to induce the photolysis of organic bonds ∼ 10⁷W.cm⁻² [19]. In addition, as the name indicates, ultra-low single-photon absorption was performed with weak absorption conditions, for instance the absorbance is A = 0.0072 in the work of Do et al. [20], whereas our experiments are performed with a significant single-photon absorption (see Fig. 2(b)), which accounts for a 2.1% decrease of intensity after a 50-μm propagation within our photoresist. Moreover, photo-thermal effects at the focus can be ruled out as Tong et al. [24] showed that an absorbed light density of ∼ 10¹⁵W.m⁻³ is required to bring a photoresist to a temperature of 150°C, whereas the light density absorbed at focus is more than six orders of magnitude lower in our experiments (see Fig. 2(c)).

Hence, we speculate that the non-linear photopolymerization behavior we observe is a combination of single-photon absorption and a non-linearity of the photoresist, such as the one induced by oxygen radical scavenging. This hypothesis is supported by the lower non-linearity N_N2 = 2.41 ± 0.36 measured with a nitrogen-bubbled photoresist (see Fig. 2(a)) and the low intensity level ∼ 10⁻¹−10⁰W.cm⁻², close to ours, used by Maruo et al. in their seminal work on single-photon photopolymerization inhibited by oxygen radical scavenging [22].

The propagation and absorption of the laser beam from the distal facet of the MMF to a focal spot 50μm deep into the photoresist are simulated (see Fig. 2(c)) and indicate more than a 100-fold contrast between the focus and regions 5μm off-focus. The combination of this high contrast with the non-linear photopolymerization threshold depicted in Fig. 2(a) suggests that off-focal photopolymerization can potentially be removed by illuminating the photoresist so that only the focal volume reaches the photopolymerization threshold. This is discussed in the next section.

3.2. Curing method for optimal single-photon three-dimensional fabrication

The challenge of three-dimensional microfabrication using single-photon photopolymerization is to avoid the cumulative off-focal polymerization that may eventually solidifiy unwanted volumes. To prevent this cumulative polymerization and ensure the curing of a specific voxel element, the non-linear photopolymerization process determined in section 3.1 must be combined with a high contrast focus of light and an optimal curing method.

Therefore, using the setup of Fig. 1(b), light is focused 50 μm below the MMF’s distal facet and the spot’s point spread function intensity distribution is measured (see Figs. 3(a)–3(b)), showing more than a 200-fold contrast between the peak focal intensity (in white in Figs. 3(a)–3(b)) and the direct surroundings (in green in Figs. 3(a)–3(b)).

 

Fig. 3 (a) Lateral PSF measured through the MMF in the focal plane (b) Axial PSF measured through the MMF (c) Fit of the data of Fig. 3(a) and experimental lateral overlapping of the voxels during 3D printing with the computed cumulative intensity (d) Fit of the data of Fig. 3(b) and experimental axial overlapping of the voxels during 3D printing with the computed cumulative intensity (e) SEM image of a non-optimal printing of a micro-hollow tube through the MMF via single-photon photopolymerization (0.1s exposure time per spot, 159±2 nW/spot). The axis of the microtube was printed orthogonally to the MMF optical axis, the microstructure fell aside during development revealing the tube’s cross-section. The arrows indicate the scanning direction for building the microstructure. (f) SEM image of a micro-hollow tube printed through the MMF via single-photon photopolymerization (0.06s exposure time per spot, 208±2 nW/spot).

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To determine the optimal curing method, using the setup of Fig. 1(b), we experimentally build test three-dimensional microstructures with various printing parameters. Our test microstructure is a hollow microtube (∅_in = 21.5μm, ∅_out = 31.5μm, length : 20μm) whose axis is perpendicular to the MMF’s optical axis (see Fig. 1(c) for the position of the microtube). Such a test microstructure allows us to study both the axial confinement of photopolymerization through imaging the void of the tube, and the ability of our technique to print suspended structures. These microstructures are built through pointwise scanning with a bottom-up approach, starting from the glass substrate, 50μm below the MMF’s distal facet. The lateral and axial sampling pitches are varied between $\frac{1}{3}$ and $\frac{3}{2}$ of the respective lateral and axial spot FWHM (see Figs. 3(c)–3(d)) fitted from the measurement of Figs. 3(a)–3(b). The optimal printing parameters are further investigated by varying the spot power from 70 nW to 800 nW, thus covering the range of polymerization threshold power determined in section 3.1 whereas the exposure time per scanning spot is fixed to 0.06s or 0.1s to build the microstructures as fast as technically possible (see section 3.3) and to avoid long-term power drifts.

We empirically determine that the optimal printing method consists in combining a large lateral and axial overlap of the scanning spots with an operating spot power below the polymerization threshold. More precisely, the respective optimal lateral and axial overlap of the spots are $~\frac{{\mathit{FWHM}}_{\mathit{lat}}}{2}$ and $~\frac{{\mathit{FWHM}}_{\mathit{ax}}}{2}$ (see the overcure parameters in Figs. 3(c)–3(d)) and the optimal experimental spot power is between ∼ 120nW and ∼ 250nW (see the gray area in Fig. 2(a)). With a looser lateral and axial sampling we observe a collapse of the microstructure, if printed at all. A higher spot power results in an overpolymerized microcylinder, conversely a lower spot power yields imperfectly printed microtubes (results not depicted here).

We hypothesize that the lateral and axial overlap of the sampled spots results in an overcuring of one voxel (see blue spots and curves in Figs. 3(c)–3(d)) by the next one (see orange spots and curves in Figs. 3(c)–3(d)), thus providing a cumulative dose (see yellow spots and curves in Figs. 3(c)–3(d)) that overcomes the photopolymerization threshold (see Fig. 2(a)). This hypothesis is further supported by the microtube depicted in Fig. 3(e), which is imperfectly printed. This microtube fell aside during development hence revealing the tube’s cross-section, the white and black arrows indicate the scanning directions adopted to print the curved structure layer-by-layer, from bottom to top in Fig. 3(e). Interestingly, the bottom of the microtube was incompletely sealed (see close-up view in Fig. 3(e)), and we speculate that this volume element did not reach the polymerization threshold as the contiguous spots located in this area were not consecutively overcured. Indeed, the pointwise scanning first took place in the direction of the black arrow. This lasted 5 seconds, during which the median free-radicals might have diffused or be scavenged by oxygen before the lateral overlapping scanning started in the direction of the white arrow. However, further time-resolved studies of the photopolymerization kinetics could help interpret the optimal printing parameters found in our work.

Nonetheless, using a shorter exposure time per spot (0.06 s), a sealed microtube can be correctly printed (see Fig. 3(f)). The printed microtube exhibits a smooth surface but deviates from the designed model, the microtube’s length is measured to be 20.0 ± 0.3μm, the outer diameter 33.5 ± 0.5μm and the inner diameter 17.6 ± 1.1μm versus respectively 20μm, 31.5μm and 21.5μm as defined in the model. The microtube’s walls are therefore too thick and further optimization of the printing parameters is required to produce accurate structures. Though not perfectly true to the model, this hollow microstructure demonstrates the possibility to remove the off-focal polymerization inherent to single-photon photopolymerization by taking advantage of the non-linearity of the photoresist.

Furthermore, the characterization of the MMF’s transmission matrix allows us to generate light spots through the MMF with small scanning steps (lateral: 300nm, axial: 1.5μm). The micro-hollow tube we printed (see Fig. 3(f)) appears smoother than the first attempt in printing microstructures with two-photon photopolymerization controlled by digital-phase conjugation through a MMF [35], which is likely due to the smaller scanning steps we employed.

3.3. Limitations of the current device

To determine the achievable complexity of the microstructures printed with our device, we investigate its lateral and axial printing resolution. The printing resolution is determined following Abbe’s criterion, that is to say we measure the minimal grating period achievable between two lines, both laterally and axially, which is often different from the minimal achievable linewidth [4]. Hence, using the setup of Fig. 1(b), we print a series of gratings with a decreasing period to determine the achievable lateral printing resolution (see Fig. 4(a)). As evidenced in Fig. 4(a), a grating with a lateral pitch of 1.05±0.06 μm can be resolved (see blue curve in Fig. 4(a)) whereas a grating period of 0.9 μm cannot be resolved (see red curve in Fig. 4(a)). The lateral printing resolution could likely be improved using the phenomenon observed in Fig. 3(e), that is to say by writing the adjacent lines with a long time period between each. In this way, the free-radicals generated when writing the first line would have time to diffuse or be inhibited before writing the second line, thus decreasing the cumulative dose of the interval between two lines.

 

Fig. 4 (a) Lateral printing resolution of our fiber-based single-photon micro-additive manufacturing device. Series of lines are printed with a decreasing pitch to determine the lateral printing resolution using Abbe’s criterion. The black dots are data points extracted from the DIC images and the smooth lines are cubic interpolant fits. (b) SEM top view of the smallest axial separation achieved between two solid lines. The model structure is the same as in Fig. 3(e) i.e. a micro-hollow tube of respectively 21.5μm and 31.5μm inner and outer diameter. (0.08s exposure time per spot, 197±2 nW/spot). The microtube fell aside during development. The slight overpolymerization of the structure results in a narrower hollow tube than designed. (c) SEM perspective view of an axially non-resolved hollow microtube. The model parameters are ∅_in = 7.5μm, ∅_out = 15μm, length = 10μm. (0.051s exposure time per spot, 149±1 nW/spot)

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Similarly, in order to determine the axial printing resolution, we measure the minimal axial separation between two polymer structures. As shown in Fig. 3(f), a hollow micro-tube of 21.5 μm inner diameter can be printed with our device, which means that two lines printed 21.5 μm axially apart can be resolved. Furthermore, by slightly overpolymerizing the same model structure, it is possible to generate axially adjacent lines separated by only 9.3±0.7 μm (see Fig. 4(b)). However, lines designed to be 7.5 μm axially apart cannot be resolved (see Fig. 4(c)), even though we targeted the lower end of the printing dynamic range (0.051s exposure per spot and 150nW per spot) to avoid overpolymerization. Our future work will be dedicated to the improvement of the axial printing resolution through specific curing strategy.

The printing speed of our system is currently limited by the refresh rate (20 Hz) of the spatial light modulator (SLM in Fig. 1(b)), which yields an one-hour printing time for the microtube depicted in Fig. 3(f). Digital micro-mirror device (DMD), capable of refresh rate over 20 kHz, could also be used as phase control system [39] at the cost of a lower efficiency of modulation than SLMs. Owing to the low power required by our writing method, the power budget of the phase control system is not of crucial importance, and the use of a DMD would dramatically increase the printing speed of our device. However, the scanning speed might ultimately be limited by the oxygen diffusion time [40].

Experimentally, we also observe that successful three-dimensional microfabrication requires to have more than ∼ 30% of the MMF’s output laser power focused within the phase-controlled spot. Otherwise, the laser power of the background speckle appears to cumulatively degrade the printing quality. Finally, all attempts to fabricate three-dimensional microstructures with a lower MMF’s numerical aperture (NA = 0.22, 0.39) did not succeed as no proper axial confinement of photopolymerization could be achieved (results not depicted here).

4. Conclusion

We demonstrate single-photon three-dimensional microfabrication through a multimode optical fiber. This is achieved without any motion of the multimode fiber, by digitally scanning a phase-controlled laser spot over the build volume. Single-photon off-focal photopolymerization is inhibited by taking advantage of the non-linearity of the photoresist, that we attribute to oxygen radical scavenging. Our experiments provide evidence that optimal writing of the microstructures is achieved by combining a large lateral and axial overlap of the scanning spots with an operating spot power below the photopolymerization threshold. Thus, it appears that a volume element is only polymerized if the cumulative exposure dose reaches the polymerization threshold within a writing time shorter than the termination time of free-radicals.

Furthermore, the 1.0-μm lateral and 21.5-μm axial printing resolution of our system could allow studying interactions at the cellular level by fabricating biologic scaffolds. Our proof-of-principle experiments with a compact multimode fiber (∅70μm, NA 0.64) and an inexpensive CW laser thus potentially open the way for in-vivo microfabrication through endoscopes.

Appendix A: Time-resolved measurement of the photopolymerization threshold

In order to obtain a better time resolution of the onset of photopolymerization as a function of the spot power than what was determined in Fig. 2(a), we implemented on our setup a measurement method based on the work of Engelhardt et al. [37] and Mueller et al. [36].

As shown in Fig. 5(a), the photopolymerization setup of Fig. 1(b) was adapted so that the laser spot generated into the photoresist, at the level of the glass substrate, is directly imaged onto a camera (CAM). According to previous studies [36, 37], the onset of polymerization locally modifies the refractive index of the polymer, thus leading to scattering of the laser used to induce photopolymerization [37] or of another light probe [36].

 

Fig. 5 (a) Experimental setup for time-resolved measurement of the photopolymerization threshold. Light is focused through the MMF into a droplet of photoresist, and the resulting focused spot is imaged on a camera as photopolymerization occurs. MOD: single-mode fiber, PWM: power-meter, BS: non-polarizing beam-splitter, SLM: spatial light modulator, L₁: lens, (f=175 mm), M:mirror, F: filtering diaphragm, λ/4: quarter wave-plate, OBJ: microscope objective (NA 0.8, 100x, Zeiss), MMF: multimode optical fiber (GOF85, NA 0.64, ∅70 μm, Schott), CAM:camera (b) Time-resolved measurement of the photopolymerization threshold for a spot power of 352 ± 3nW. The FWHM of the laser spot generated through the MMF is plotted over time (in blue, see left vertical axis), as well as the relative integrated power on the camera (in orange, see right vertical axis).

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Using the setup of Fig. 5(a), both the power integrated on the camera and the measured spot FWHM are monitored over time while photopolymerization occurs. However, a significant change of these signals that would evidence the onset of photopolymerization is not measured before time t ∼ 2s (see the blue curve in Fig. 5(b)), whereas DIC imaging of polymer spots generated with the same spot power showed that photopolymerization occurs within 0.8 ± 0.1s (see the gray area in Fig. 5(b)). The measurement setup of Fig. 5(a) therefore over-estimates the time necessary for the onset of photopolymerization. Mueller et al. [36] calculated that the relative scattering signal induced by the voxel onset is of the order of a few percents, to which our measurement setup might not be sensitive. Therefore, the parametric space for non-linear photopolymerization of Fig. 2(a) was determined by sampling the exposure times and spot powers, and imaging with a DIC microscope whether polymerization occured or not for a specific couple of parameters.

Funding

École Polytechnique Fédérale de Lausanne, School of Engineering (EPFL-STI) Advanced Additive Manufacturing grant; Swiss National Science Foundation (SNSF) project MuxWave (200021_160113\1).

Acknowledgments

Paul Delrot benefited from a grant from EPFL-STI for Advanced Additive Manufacturing research. This research was partially funded by a SNSF grant (project MuxWave). We acknowledge the kind help of Enrico Chinello (EPFL-LAPD) for preparing the glass substrates used for the calibration of the photopolymerization threshold.

Disclosures

The authors declare that there are no conflicts of interest related to this article.

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