An on-demand bench-top fabrication process for fluidic chips based on cross-diffusion through photopolymerization

Abstract

In this paper, we propose a novel approach to fabricate fluidic chips. The method utilizes molecular cross-diffusion, induced by photopolymerization under ultraviolet (UV) irradiation in a channel pattern, to form the channel structures. During channel structure formation, the photopolymer layer still contains many uncured molecules. Subsequently, a top substrate is attached to the channel structure under adequate pressure, and the entire chip is homogenously irradiated by UV light. Immediately thereafter, a sufficiently sealed fluidic chip is formed. Using this fabrication process, the channel pattern of a chip can be designed quickly by a computer as binary images, and practical chips can be produced on demand at a benchtop, instead of awaiting production in specialized factories.

I. INTRODUCTION

At present, fluidic chips are used for many objectives in the research fields of biology and medicine,^1–5 in which the fluidic devices have highly integrated sequential functions, earning the title “lab-on-a-chip.” Previous studies have also aimed to develop each chip element to realize its functions, including as microelectrodes for electrochemical reactions,⁶ a check valve integrated with a micropump,⁷ reactors,^8,9 micromixers,¹⁰ and microcontainers for cell cultures.^11,12 More recently, nano-laser chips have been proposed, which are suitable for use under a microcoherent light source in fluidic chips for optical analysis or photo-induced reactions.¹³ Consequently, various advantageous uses of fluidic chips, such as in extremely compact PCR systems,¹⁴ in a water purification system,⁸ and as sensing chips,^15–17 have been reported.

However, the fabrication or preparation of the microfluidic chips themselves still faces difficulties in rapid fabrication or rapid prototyping. Presently, rapid progress is being made in biological and medical studies using fluidic chips. Therefore, the chips are often individually designed to satisfy each specific condition in the cutting-edge methods. As a result, an on-demand and bench-top fabrication process is required to quickly manufacture a wide variety of products in small quantities. In addition, such an on-demand fabrication process would benefit onto a sudden and unexpected incident such as the pandemic of a new pathogenic virus. Some rapid-prototyping processes or bench-top fabrication processes have been reported.^18–23 For example, an effective approach using a dry film resist (DFR) has been proposed to realize low-cost rapid prototyping.²⁴ As a result, low-cost and high-throughput fabrication was demonstrated although the fabrication process needs the repetitive lamination of DFR and immersion into the developer solution. Among these studies, an unavoidable problem exists: the need for master molds or photomasks for which the preparation takes time. The preparation of master molds can be replaced by direct fabrication of photo-curable materials.^20,22 However, a rinsing or washing out process must still be used in such cases to remove the unreacted or otherwise remaining portions. Photomask preparation can also be replaced by employing maskless irradiations using spatial light modulators.^25,26 Nevertheless, the rinsing process similarly remains necessary. Therefore, realizing an on-demand bench-top fabrication process for fluidic devices requires two considerations. First, the fabrication process must be maskless or moldless. Second, the process should not include rinsing or etching. In addition, a robust and facile final sealing process is preferable.

In this paper, we propose an approach that employs molecular cross-diffusion through photopolymerization to prepare the fluidic chip channel structures based on the above background. The fundamental concept is illustrated in Fig. 1. The photopolymer films were irradiated by ultraviolet (UV) light with a single line pattern. The film surface changes during the irradiation time, according to the schematic illustrations in Figs. 1(a)–1(c). In the initial stage, the molecules are polymerized, and the polymerizable molecules (monomers) are reduced in the bright region. As a result, diffusion of polymerizable molecules from the dark regions occurs, which results in the convex surface structure visible in Fig. 1(a). As polymerization progresses, unreacted monomers and unreactive molecules are eliminated from the bright region [Fig. 1(b)] due to the increase in polymerized moieties. In the advanced stage of the process, molecular diffusion toward the dark region is enhanced, and a pronounced concave structure is formed, as seen in Fig. 1(c). In this study, we applied this concave structure formation to fluidic chip fabrication. In addition, the photopolymerization process was applied for sealing the chip after a top substrate was attached on the channel structure. This report presents the evaluation of this facile fabrication process.

FIG. 1.

(a)–(c) Schematic illustrations of molecular cross-diffusion under photopolymerization and (d) a patterned UV irradiation system.

II. EXPERIMENTAL

An authorized photopolymer material (NOA 81, Norland Products, Inc.) was used in this study.^18,27–29 Curing occurs under UV irradiation based on a thiol-ene reaction.³⁰ The photopolymer material was coated through a spin-coating method onto pre-cleaned glass substrates. The rotation speed in each experiment was adjusted to obtain the intended layer thicknesses.

The prepared photopolymer films were subsequently irradiated by UV light with the desired pattern for the fluidic chips, as shown in Fig. 1(d). The LED used in this study emits UV light with a wavelength of 365 nm. The UV light was spatially modulated by a digital mirror device (DMD, DLP6500, Texas Instruments, Inc.) and the modulation pattern was irradiated on the film via an imaging lens system. The power density for the irradiation pattern was adjusted for each experiment. The surfaces of the fabricated structures were then analyzed and measured using a laser microscope (VK-X210, Keyence Co.).

III. RESULTS AND DISCUSSION

A. Fundamental study of channel structure formation

We have studied the fundamental characteristics of the surface volume modulation on the photopolymer films under photopolymerization. First, the outline of the temporal changes under UV irradiation, illustrated in Fig. 2, was investigated. In this investigation, five similar films with a thickness of 25 μm were prepared on glass substrates. A single line pattern with a width of 500 μm was then irradiated at a power density of 11.1 mW/cm² for 5, 15, 30, 60, and 120 s, respectively, on five films. After the patterned irradiation, each sample was exposed immediately to homogenous UV light to complete the photoreaction over the entire film. In Fig. 2, the laser microscope images clearly display the surface volume changes, and their dependency on the irradiation duration, as expected from Fig. 1. In the initial stage, a convex structure was formed in the central part of the bright region.²⁹ As the irradiation-time increased, the convex structure shifted toward the dark region. In the advanced stage, a deep valley (channel structure) was formed due to the formation of two walls.

FIG. 2.

Surface volume changes with respect to the UV irradiation time.

Such surface modulation under photopolymerization has been previously recognized and well-investigated.^31–35 Molecular migration in photopolymer layers can be represented using the diffusion theory, similarly to molecular diffusions in other cases, such as in metals or crystals. The flux $(J)$ of a targeted substance with an inhomogeneous concentration is described by the well-known partial differential equation:

$$J=-D{\displaystyle \frac{\mathrm{\partial }c}{\mathrm{\partial }x},}$$ (1)

where D is the diffusion coefficient, c is the concentration, and x is the focused direction based on Fick's law. This equation can be expanded for the time domain,

$${\displaystyle \frac{\mathrm{\partial }c}{\mathrm{\partial }t}=D{\displaystyle \frac{{\mathrm{\partial }}^{2}c}{\mathrm{\partial }{x}^2}=-\mathrm{div}\mathit{J},}}$$ (2)

where $\mathit{J}$ is the flux vector. Baten’kin and Mensov studied and reported the molecular diffusion under photopolymerization in more detail.³⁶ When monomer $(\mathrm{M})$, polymer $(\mathrm{P})$, and non-reactive components $(\mathrm{N})$ such as photo-initiators and plasticizers are considered in a photopolymer layer, the concentrations of these three components can be described as ${\rho }_{\mathrm{M}}m,{\rho }_{\mathrm{P}}p,\mathrm{and}{\rho }_{\mathrm{N}}n$, respectively, where $m,p,\mathrm{and}n$ are the respective volume fractions, and ${\rho }_{\mathrm{M}},{\rho }_{\mathrm{P}},\mathrm{and}{\rho }_{\mathrm{N}}$ are the densities of $\mathrm{M},\mathrm{P}\mathrm{and}\mathrm{N}$, respectively. Therefore, Eq. (2) can be rewritten for each component as

$${\displaystyle \frac{\mathrm{\partial }({\rho }_{\mathrm{M}}m)}{\mathrm{\partial }t}=-\mathrm{div}{\mathit{J}}_{\mathrm{M}}-\mathit{R},}$$ (3)

$${\displaystyle \frac{\mathrm{\partial }({\rho }_{\mathrm{P}}p)}{\mathrm{\partial }t}=-\mathrm{div}{\mathit{J}}_{\mathrm{P}}+\mathit{R},}$$ (4)

$${\displaystyle \frac{\mathrm{\partial }({\rho }_{\mathrm{N}}n)}{\mathrm{\partial }t}=-\mathrm{div}{\mathit{J}}_{\mathrm{N}},}$$ (5)

where $\mathit{R}$ indicates the polymerization rate. In the unit volume (i.e., the considered area including the bright region and sufficient dark regions), the sum of all the volume fractions must be constant, as

$$m+p+n=1.$$ (6)

When photopolymerization progresses under UV irradiation, the monomer concentration decreases at the polymerization rate, and increases in monomer diffusion originate from the spatial modulation of the fraction, as described in Eq. (3). Therefore, the dominant diffusion direction is from the dark region to the bright region. In contrast, the polymer concentration increases in the bright region according to the polymerization rate. While the non-reactive components diffuse from the bright region to the dark region due to the elimination of unreactive molecules during the polymerization reaction. (If the flowability in the layer is sufficiently sustained, the unreacted monomers and/or shorter polymerized units also diffuse from the bright region to the dark region.) This phenomenon of opposite-directional diffusion is termed cross-diffusion. The typical surface modulations resulting from cross-diffusion are (1) a convex structure in the initial stage; (2) a fractured structure in the mid stage; and (3) a concave structure in the advanced stage,^34,35 as observed in the bright region in Fig. 2. Therefore, the surface structuring can be realized through photo-driven molecular migration. We employed this cross-diffusion to form channel structures using patterned UV light irradiation, thus enabling on-demand and bench-top fabrication of micro fluidic devices. The channel structure formation on the basis of this concept is characterized in more detail below.

First, the dependence of the channel depth on irradiation time was investigated under various power densities of the patterned UV-irradiation, as plotted in Fig. 3(a). In this experiment, the photopolymer layer thickness and line pattern width were set to 17 μm and 500 μm, respectively. When the power density was relatively low (∼5.8 mW/cm²), the increase in the rate of the channel depth was relatively slow. In contrast, at relatively high power densities (>∼11.1 mW/cm²), the increase rate is relatively fast. This dependence can be explained clearly by the cross-diffusion mechanism. A relatively high power density of UV light induces rapid polymerization, as indicated by $\mathit{R}$ in Eqs. (3) and (4). Therefore, the diffusion progress for each component was accelerated according to the concentration changes.

FIG. 3.

Irradiation time dependencies of channel depths under (a) various UV irradiation power densities and (b) various layer thicknesses.

Furthermore, the dependencies of channel depth on irradiation time were investigated for various film thicknesses, as plotted in Fig. 3(b). In this investigation, the power density of the UV irradiation and the line pattern width were set to 11.1 mW/cm² and 500 μm, respectively. The channel depth increased with the increase in layer thickness. This dependence can be also explained explicitly. The channel is composed of two walls, formed by diffused molecules. Therefore, higher walls are obtained within thicker layers that include more molecules. In addition, sufficient fluidity is sustained for a longer time under the irradiation in the thicker layers, which results in a longer-term channel growth. The same phenomena arise in the case of relatively low power density due to the slowly progressed photopolymerization. As a result, a deeper channel was formed at a relatively low power density of 5.8 mW/cm² at 600 s, as shown in Fig. 3(a).

In the next stage, the channel profile changes depending on the UV irradiation time were investigated. Figures 4(a)–4(e) show the channel profiles for linewidths of 200, 400, 500, 750, and 1000 μm, respectively. Here, the channel width was defined as the distance between the top of two walls forming a channel structure. During this experiment, the photopolymer layer thickness and power density of the UV irradiation were set to 17 μm and 11.1 mW/cm², respectively. It is clear that the channel formation progressed based on the cross-diffusion. In the initial stage, narrow channels enclosed by low two walls were formed after the formation of lower convex structures. As the cross-diffusion progressed, the channel widths expanded and the walls were grown higher. However, when the UV irradiation linewidth was set to 200 μm, the growth in the channel depth was not sufficient compared with that of the other linewidths. This means that the finite surface tension of the photopolymer material does not allow the formation of very thin wall structures even under distinct cross-diffusion. To understand the channel characteristics with respect to the channel structure formation, the channel depths and widths under different irradiation times were measured from the profiles and plotted in Figs. 5(a) and 5(b), respectively. There is a clearly exhibited lower limit required for the linewidth to obtain the sufficient channel depth, equal to 400 μm, as shown in Fig. 5(a). Nevertheless, the channel widths reached an equivalent width to that of the UV irradiation linewidth at an irradiation time of 600 s for all conditions, as shown in Fig. 5(b). This time consumption data are an important factor in on-demand fabrication. As a result, the preferred channel width (>400 μm) and necessary irradiation time for a deeper channel (∼600 s) were estimated for the channel structure formation in the proposed fabrication process.

FIG. 4.

(a)–(e) Channel profile changes with respect to the irradiation time for various line pattern widths of 200, 400, 500, 750, and 1000 μm, respectively.

FIG. 5.

Irradiation time dependencies of (a) channel depth and (b) channel width for various line pattern widths.

In these experiments, the most important finding is the range of channel depths achieved, which was approximately 0–70 μm, over the channel widths of several hundred micrometers. These values are substantially larger than those in previous reports. In the previous studies, typical channel (relief) depths range from several hundred nanometers to several micrometers,^31–37 which cannot be applied for forming channels with a sufficient flow rate in practical fluidic devices. The large surface modulations in this study are attributable to both the thicker layers and wider irradiated patterns in which a large amount of molecules are included, as described above. As a result, the available flow rate can be obtained by adequately adjusting both the channel depth and width. This means that relatively large structures in a range from several tens of micrometers to several hundred micrometers can be constructed by the molecular migration induced by the photochemical reaction. This finding is the conceptual origin of this study.

B. Demonstration of fluidic device construction

The actual fluidic device fabrication was evaluated systematically, using the experimental setup illustrated in Fig. 6(a). A glass substrate was placed on the prepared channel structure as a top substrate to seal the fluidic channel, and the top substrate was then compressed downward at a certain pressure. The pressure was determined using the force value indicated by a force gauge. Many identical channel structures were fabricated under the conditions of a 500 μm single line irradiation pattern, an 11.1 mW/cm² power density, and a 17 μm initial layer thickness. After the patterned irradiation, the top substrates were attached immediately onto each channel structure and compressed at various pressures. To complete the sealing process, each sample was homogeneously irradiated by a UV light while maintaining the various pressures. To investigate the effect of the sealing pressure on the cross area of the fluidic channel, the cross sections were observed using a scanning electron microscope (SEM, JSM-5510LV, JEOL Ltd.). The cross-sectional area was measured from the captured images through image processing techniques including binarization, error correction, and area calculation, as shown in Fig. 6(b). The resulting channel depths and cross-sectional areas in relation to the pressure applied to the top substrates are characterized in Fig. 6(c). The cross-sectional area decreased with increasing applied pressure. Therefore, it is possible to control the cross-sectional area directly based on the contact pressure applied to the top substrate on the channel structures formed by molecular diffusions. In the characteristic data, we noted a relatively flat region from 3 to 5 N/cm². This indicates a well-punctured state of the two walls surrounding the channel, resulting in a substantial sealing. Therefore, pressure control is applicable for fabricating fluidic chips with high sealing reliability and reproducibility, although the mechanical reliability for fluidic pressure must be investigated systematically for practical uses.

FIG. 6.

(a) Schematic illustration of pressure-controlled sealing, (b) SEM observation of the cross section of the resulting fluidic chip, and (c) depths and cross-sectional areas of the channels on the applied pressure.

In fact, we could produce various fluidic chips according to the above-mentioned sequence, as shown in Fig. 7. Each device pattern was irradiated by binary images prepared using a computer drawing software. Channel widths of 2.0, 1.0, and 0.5 mm were realized by simple modulation of the drawing software, as shown in Figs. 7(a)–7(c). Similarly, a three-branch retention area with a rectangular shape, and an entrapment structure for filtering impurities could be fabricated, as shown in Figs. 7(d) and 7(e). These results demonstrate the success of the proposed fabrication method clearly.

FIG. 7.

Typical examples of microfluidic chips with (a)–(c) various channel widths and (d) and (e) functional structures, fabricated through the proposed process.

C. Built-in functional surface

This promising approach has another advantage, regarding the in-building of functions for fluidic devices. Usually, functional elements must be introduced in the bottom part of the channel structures. If they were built into the top substrate, the sealing conditions would need to be modified to ensure proper sealing. However, in our method, sealing is performed by a photochemical reaction under the preferred pressure. Therefore, a functional structure prepared on the top substrate can be introduced easily and certainly into the fluidic device.³⁸

Figure 8 shows the result of the introduction of a plasmonic sensor exhibiting a sharp resonant spectrum.³⁹ A substrate with the sensing structure was attached to the channel structure and compressed by the proper pressure. After the subsequent homogeneous UV-irradiation, the sealing was complete, as shown in Fig. 6(a). To evaluate the built-in function, ethanol was injected in the right channel [Fig. 8(A)]. The plasmonic sensor can detect ambient refractive index changes through changes in the extinction spectrum. Therefore, ethanol injection can be confirmed by the change in coloration to green, as shown in Fig. 8(d). In this case, the left channel kept the initial red coloration because the sensing surface was still surrounded by air with no changes in the ambient refractive index, as shown in Fig. 8(b).

FIG. 8.

(A) Schematic illustration of the embedded plasmonic sensor and (B) a photograph of the fluidic chip undergoing coloration change.

This result demonstrates the high potential of this approach for benchtop fabrication of fluidic devices on demand, based on the confirmed quick design, robust sealing, and easy functionalization of the devices.

D. Consideration for practical application

The proposed fabrication process is outlined through partial photographs in Fig. 9. The photopolymer material was spin-coated on a glass substrate [Fig. 9(a)]. UV light was then irradiated for 120 s on the substrate in the pattern of the desired chip [Figs. 9(b) and 9(c)]. We can confirm that the channel structure was formed by only UV irradiation in Fig. 9(d). A top glass substrate was subsequently attached on top of the channel structure, after which the entire element was UV-irradiated homogeneously, resulting in robust sealing [Figs. 9(e) and 9(f)]. Notably, maintaining an adequate pressure during the homogeneous irradiation is required to obtain the desired cross-sectional area. After sealing, a solution can be introduced in the fluidic chips using the capillary forces, which confirms that the channel was sealed completely without breaks [Figs. 9(g) and 9(h)]. In addition, a typical pressure filling system using a pump can also be implemented in these fluidic devices by preparing the top substrates with tube connecters (Fig. S1 in the supplementary material). Recently, an effective approach for fluidic chip fabrication based on 3D printing was reported.⁴⁰ The printing of a microfluidic device was completed within 30 min and the availability for analytical uses was demonstrated. In our case, the sequential process took approximately 10 min and a functional structure can be introduced easily. Therefore, our approach could also be categorized as an effective approach, based on the on-demand bench-top process.

FIG. 9.

(a)–(h) Outline of the sequential fabrication process.

However, there are some concerns related to the fluidic device durability for use with organic solutions, as well as the bio-compatibility, because of the photo-curable material used. The former can be improved by additional treatment,^28,41 while the latter point was evaluated using HeLa cell adhesion.¹⁸ Unfortunately, the photopolymer area eliminated the living cells, but there were also living cells in the peripheral glass area. A glass substrate was used for the sealing of the channel structure in our fluidic chips. Therefore, it may be used as a fluidic device in some biological cases.

Another problem is that the shape of the channel cross-section is not preferred for the fluidic chips, as shown in Fig. 6(b). In general, the shape is preferred to be smooth such as an oval. Although a solution could be introduced in the fluidic chip smoothly as shown in Figs. 9(g) and 9(h), this problem should be solved in the future to realize the practical usage. The observed cross section in Fig. 6(b) suggests that the walls forming the channel structure collapsed toward the dark regions in which a soft phase is sustained sufficiently. This point may become the key point for the improvement.

IV. CONCLUSIONS

In this paper, an interesting approach to fabricating fluidic devices using cross-diffusion under photopolymerization was presented. Molecular diffusion in opposite directions generates a channel structure under a single line UV irradiation pattern. Sealing of the channels can be completed by attaching a top substrate to the channel structure and homogeneously irradiating the entire component with UV light due to the residual polymerizable molecules. Therefore, we can obtain the desired fluidic devices from binary computer-drawn images on demand and at the bench-top. In addition, a functional surface prepared on the top substrate can be introduced in the fluidic devices because of the easy and certain sealing process. In the future, such on-demand production will be used practically in research institutes and hospitals to supply the desired analysis chips and diagnostic test chips, respectively, without requiring an inventory storage of such devices. This concept therefore offers a small sustainable action for these institutions.

SUPPLEMENTARY MATERIAL

See the supplementary material for a pressure feed test of the fluidic chip.

DATA AVAILABILITY

The data that support the findings of this study are available within the article and its supplementary material.