Introduction

The well-of-the-well (WOW) culture system1,2 is a platform for in-vitro mammalian embryo culture that has been shown to enhance the developmental competence of embryos and clinical pregnancy rates in humans compared to traditional embryo culture dishes3,4,5. In a WOW dish, each embryo is cultured in a confined space termed a “microwell” but has access to a larger “macrowell” holding a small volume of culture media that is shared among all embryos with overlay oil (Fig. 1a). In contrast, embryos cultured in a traditional in vitro fertilization (IVF) dish are not individually confined but share a droplet of culture media with overlay oil (Fig. 1b). Research shows that the shapes and dimensions of microwells in WOWs affect embryo culture outcomes; 6,7 however, rigorous, systematic research is lacking about which microwell shape provides the best culture quality. A major limitation in the ability to conduct this research has been the lack of cost-effective tools to rapidly iterate the fabrication of WOW dishes with arbitrary shapes and dimensions. Given the low success of current human IVF procedures, there is a clear need for new tools, such as WOW dishes of optimal design, that may improve clinical outcomes.

Fig. 1
figure 1

Options for embryo culture. (a) Embryos cultured in a WOW dish are isolated and inhabit a confined space but share a small volume of culture media and overlay oil. (b) Embryos cultured in an IVF dish are not isolated and share a droplet of media and oil. Note: not drawn to scale.

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When first introduced in 2000, WOW dishes were handmade by using heated darning needles to melt the base of a standard IVF dish1,3, resulting in microwells of semi-ellipsoid shape. The manual nature of the hand-melting process, however, can introduce heterogeneities in the dimensions of the fabricated microwells, leading to variability in culture outcomes and hindering systematic assessment. Moreover, the melting process can lead to the release of toxic materials that are potentially harmful to human embryos. Today, commercial WOW dishes are typically manufactured by polystyrene (PS) mold injection2 and are available with microwells having truncated cone, semi-ellipsoid and conical shapes. This fabrication process is optimal for high-volume manufacturing, but it is not compatible with the need to fine-tune design parameters to test various microwell shapes and dimensions for research studies. Commercial WOW dishes are also 10 times more costly than traditional IVF dishes, which lowers their desirability and accessibility.

Photolithography has been used to create poly(dimethylsiloxane) (PDMS)-based WOW dishes templated from SU-8 molds8,9; however, the SU-8 mold development process is complex and laborious. Moreover, since multi-layer (i.e., 3D) structures are hard to achieve in conventional photolithography, the range of possible microwell shapes is limited with this strategy. To date, only single-layer, cylindrical microwells have been reported8. Conversely, the broader cell culture community has developed several methods to fabricate cell culture dishes having microwells with a variety of 3D shapes, including conical microwell arrays10, omega-shaped microwell arrays11, and pyramidal microwell arrays12. These methods often use commercially available structures (e.g., computer chips, acrylic sheets) as the mold or leverage advanced manufacturing processes to create custom molds. Unfortunately, the use of commercially available materials for molding limits the options for design of the microwell shape, and the custom mold-development processes used to date are time-consuming and costly, which restrict rapid prototyping and production. For example, using a projection micro stereolithography (PmSL) 3D printing service, a petri dish-sized mold (30 × 30 × 6 mm) can cost $1000 to $10,000 and take ten hours to seven days to print, respectively, depending on the printing quality.

Here we present a rapid and low-cost method for WOW fabrication to facilitate systematic investigation of the optimal WOW design for human embryo culture. Our strategy involves use of digital light projection (DLP) 3D printing to fabricate a low-cost PDMS-based WOW insert with controllable microwell shapes and dimensions. When paired with a standard IVF dish, one can quickly and easily create a novel WOW dish to explore optimal culture conditions. In this work, we validate the physical quality of the WOW inserts fabricated with this strategy, demonstrate the utility of the PDMS WOW dishes for observation and grading of mouse embryo quality, and report the first use of novel-shaped microwells (hemispherical and pyramidal) in WOW dishes for embryo culture.

Methods

WOW dish fabrication

Our WOW dish comprises two components: a PDMS WOW insert (containing both microwell and macrowell structures) and an IVF dish. The overall fabrication process (Fig. 2) involves creation of the WOW insert from a 3D-printed negative mold and attachment of the WOW insert to the IVF dish. The negative mold can be reused to fabricate numerous WOW inserts at low cost: the initial mold printing cost was roughly $60, and the materials cost for each molded insert was just $0.77. Details of the fabrication process and design we tested are described below.

Fig. 2
figure 2

Process to fabricate a WOW dish. (a) A negative mold comprising the desired microwell and macrowell shapes and dimensions is designed and printed using 3D-printing; (b) a WOW inserted is fabricated by casting PDMS over the mold; (c) the final WOW dish is assembled by adhering the insert to the bottom of an IVF dish with PDMS.

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Negative mold design and fabrication

We designed a 28-mm-diameter negative mold (Fig. 2a) in SolidWorks (Dassault Systèmes) to template WOW inserts comprising four cylindrical macrowells (diameter: 6 mm, depth: 3 mm), each capable of holding up to 81 µl of culture media. Each macrowell contains nine identical microwells in one of four different designs, each 200-µm deep: large truncated cones (top diameter: 400 µm, base diameter: 200 µm); small truncated cones (top diameter: 300 µm, base diameter: 100 µm); hemispheres (diameter: 400 µm) and square pyramids (side length: 400 µm). The microwell shapes and dimensions included in the design were selected to demonstrate the versatility of the fabrication technique: the specific shapes we chose correspond to a combination of existing (i.e., truncated conical) and novel (i.e., pyramidal and hemispherical) shapes for WOW culture. Note that pyramidal designs have been shown to enhance cell culture in other contexts12. The negative mold was 3D-printed (Core 530, B9Creations) using high-detail resin (B9R-8-HD-SLATE, B9Creations) with an axial resolution of 20 µm and a lateral resolution of 15 µm. A complete CAD drawing of the negative mold is included in Fig. S1. To prevent adhesion of PDMS to the mold in subsequent steps, the mold was cleaned by air plasma treatment in a plasma cleaner (PDC-32G, Harrick Plasma) for 30 s. It was then placed inside a desiccator along with a dish (08–732-105, Fisher Scientific) of 100 µl of trichloro(1H, 1H, 2H, 2H-perfluorooctyl)silane (PFOCTS, Sigma Aldrich) and subjected to high vacuum overnight (at least 12 h). Finally, the mold was transported to an oven and baked at 75 °C for an hour.

WOW insert fabrication

Figure 2b shows the workflow of WOW insert fabrication. PDMS (Sylgard 184, DOW) was prepared using a standard recipe of 10:1, base:curing agent, degassed for 30 min, and then poured directly on the negative mold until the mold was full. A second, 20-min degassing step was subsequently performed to remove any trapped air bubbles. Then, we covered the PDMS-filled mold with a glass slide (71860-01, Electron Microscopy Sciences) to form a flat bottom for the WOW insert and cured it at 75 °C in an oven for at least three hours. After that, we removed the glass slide and retrieved the cured WOW insert from the mold.

WOW dish assembly

Figure 2c shows the workflow of WOW dish assembly. The WOW insert was glued to an IVF dish (150,255, NUNC, Thermo Fisher) by casting a layer of uncured PDMS to the flat base of the dish and curing the assembly for at least 48 h at room temperature. The use of the IVF dish was necessary to provide a stable bottom support for the WOW dish and to leverage the vented lid that comes with the IVF dish to facilitate air exchange during culturing. The assembled WOW dishes were placed in an ultrasonic cleaner with distilled water for 20 min, dried with a nitrogen blow gun and sterilized under UV light for an hour before use.

Mold and WOW insert characterization

To verify the printing and fabrication quality of the negative mold and resulting WOW inserts, we used micro-computed tomography (µCT) to visualize the large-scale structures (i.e., macrowells) and optical coherence tomography (OCT) to visualize small-scale structures (i.e., microwells).

µCT imaging

The macrowell templates (i.e., cylinders) in the negative mold and the macrowells of the WOW insert were imaged by µCT (µCT 40, Scanco Medical AG) at an energy of 70 kVp and resolution of 10 µm. The resulting volumetric data were loaded in ImageJ for visualization and measurement. Standard deviation (STD) projections along the z-axis were generated to characterize the diameters of the extensive wells. The heights of the macrowell templates and corresponding depths of the macrowells were measured along the two orthogonal planes. Each measurement was repeated three times to enable averaging.

OCT imaging

Despite the high resolution of µCT, the poor contrast of resin and PDMS in air limits the visualization of microstructures. Thus, we used optical coherence tomography (OCT, TEL221PSC1, Thorlabs) to characterize the dimensions of the microstructures. OCT images covering a 2 × 2-mm scanning area (1000 × 1000 points) were acquired for each of the four macrowell regions on the negative mold and WOW insert. Both the negative mold and the WOW insert were carefully adjusted to be flat to the OCT scanner head prior to imaging. The volumetric data were loaded in ImageJ for visualization and measurement. STD projections along the z-axis were generated to characterize the dimensions of lateral (i.e., x–y plane) structures. Specifically, we measured the following structure dimensions of all nine microwells in each macrowell: the average top and base diameters of truncated conical shapes, the average diameters of hemispherical shapes, and the average side lengths of pyramidal shapes. Three cross-sectional images spanning the centers of the microwell structures were extracted to measure their profiles.

Mold reusability testing

To test the reusability of the negative mold, we fabricated 30 WOW inserts from the same mold. The 15th and 30th WOW inserts were collected for examination and imaged with OCT as described above. STD projections along the z-axis were generated to compare the structural consistency. The corresponding projection images of the same microwell structures in the two inserts were registered in MATLAB (MathWorks) for direct comparison.

Embryo culture in PDMS WOW dishes

To demonstrate the utility of the PDMS WOW dishes for embryo culture, we cultured single-cell mouse embryos and graded the blastocysts that resulted after four days. In this study, only the pyramidal, hemispherical, and large truncated conical microwells were used: the small truncated conical microwells were not used so that their smaller opening dimension would not be a confounding factor in comparing the culture outcomes, as has been noted previously13. Table 1 summarizes the number of embryos included in the study. Each row indicates the number of embryos that were cultured over the same experimental period. The variability in the row totals is due to exclusion of embryos lost during handling and transport prior to culture. During each experiment, the control group of embryos was grown in a standard IVF dish, without the use of a WOW insert. The other experimental groups of embryos were all placed in the same WOW dish, with no more than one embryo in a given microwell.

Table 1 Number of embryos collected for culture quality comparison in PDMS WOW dishes.
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Culture dish preparation

On Day 0, we filled the microwells of a WOW dish with culture media using micropipettes (7–72-2170, Cooper Surgical) under a stereomicroscope (SM-2BYY, AmScope) with an attached, 37 °C warm plate (TCS-100, AmScope). We then pipetted 80 µl culture media into each of the macrowells and carefully inspected the microwells for the appearance of bubbles. In general, we found that heating the microwells and using warm media eliminates the presence of bubbles. Thus, WOW dishes were kept in a 37 °C incubator before use. During media loading, WOW dishes were kept on the warm plate, and the media was maintained at 37 °C by a dry bath (14955218, Fisher Scientific). If observed, trapped bubbles appearing in microwells were removed by pushing culture media through a micropipette to flush them away with gentle pressure. About 3 ml culture oil was then pipetted into the WOW dishes to cover the media. Preparation of the IVF culture dishes was as follows: a droplet of 80 µl of culture media was loaded and covered by 3 ml of culture oil. Both WOW dishes and IVF culture dishes were placed in the incubator to equilibrate overnight.

Embryo thawing and culture

Cryopreserved one-cell murine embryos (B6C3F1 × B6D2F1, Embryotech Laboratories) were used in the study. On Day 1, a straw of 20 embryos was exposed to room temperature for two minutes and immersed in a 37 °C water bath for one minute. Then the straw was moved from the water bath and wiped dry with a tissue. We cut the straw at the plug and seal and pushed all of its contents into a droplet of 250 µl of embryo culture media (MR-106-D, EmbryoMax, Sigma) that had been pre-warmed in an IVF dish. After that, we immediately rinsed the embryos twice in two droplets of 250 µl of pre-warmed culture media in IVF dishes set aside for rinsing. The embryos were left in the second rinsing droplet and incubated for 10 min at 37 °C under 6% CO2 to rehydrate. Warmed embryos were then seeded in the Day 0, pre-equilibrated WOW dishes and IVF dishes and cultured for 96 h.

Embryo culture in PDMS dishes

Properties of a cell culture substrate, such as stiffness and hydrophobicity, are known to affect cell proliferation. To isolate the impacts of culture quality due to the PMDS substrate (i.e., material effects), we cultured single-cell mouse embryos on a layer of PDMS (i.e., PDMS dishes) or polystyrene (i.e., IVF dishes) and graded the blastocysts that resulted after 96 h for quality comparison. The protocols of PDMS dish fabrication, culture dish preparation, and embryo thawing and culture are described in the supplemental S1. The number of embryos used in the experiment is summarised in Table 2. Each row represents number of embryos uses in each replication. The variability of embryo numbers in different replication is due to the variations in number of embryos per straw and exclusion of embryos lost during handling and transport prior to culture.

Table 2 Number of embryos collected for culture quality comparison in PDMS dishes and IVF dishes.
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Embryo culture in commercial WOW dishes

To assess the performance of our PDMS WOW dishes to commercial WOW dishes (VitaVitro) made of polystyrene, we compared the embryo culture quality in the hemispherical microwells of PDMS WOW dishes (i.e., microwells of the best culture quality as described in results and discussion) and the semi-elliptical microwells of commercial WOW dishes. Each commercial WOW dish has 16 microwells in a macrowell. The microwell diameter measured 0.317 ± 0.001 mm (± 1 standard error of the mean, SEM) and the microwell depth measured 0.260 ± 0.001 mm (± 1 SEM) using OCT. The OCT images of the microwell are shown in supplemental Fig. S2. Comparing with the hemispherical microwells in our WOW dish, the semi-elliptical microwells in commercial WOW dish have a smaller diameter and larger depth. We cultured one-cell mouse embryos in the semi-elliptical microwells of commercial WOW dishes and graded the blastocysts that resulted after four days. During each experiment, a control group of embryos was grown in a standard IVF dish to ensure quality of the straw from the source. The protocols for commercial WOW dish preparation and embryo thawing and culture are described in the supplemental S2. The number of embryos used in the experiment is summarised in Table 3. Each row represents number of embryos uses in each replication. The variability of embryo numbers across replications is due to variations in the number of embryos per straw and exclusion of embryos lost during handling and transport prior to culture. Note that the macrowell size in the commercial WOW dishes is smaller than that in the PDMS WOW dishes. The commercial dishes used a of 50 µl of culture media as instructed by the manufacture, compared to 80 µl for which the PDMS WOW dishes were designed. Previous study has shown that embryo density in WOW dishes does not impact the blastocyst rate and quality14,15,16; therefore, the volume difference is unlikely impactful to culture quality in the study. The culture media in control groups was kept 80 µl to be consistent with the control group of PDMS WOW dishes.

Table 3 Number of embryos collected for culture quality comparison in PDMS dishes.
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Culture quality comparison

Embryo imaging

After 96 h of culture (Day 4), embryos were imaged with an inverted microscope for quality assessment. To prevent optical aberrations introduced by the base structure of the microwells from affecting the quality assessment, imaging of embryos cultured in both fabricated and commercial WOW dishes was performed in glass-bottom dishes (P35G-1.0-20-C, MatTek) that had been pre-equilibrated in the incubator the night before. For embryos in PDMS the WOW dish, each glass-bottom dish comprised three droplets of 30 µl of culture media covered with 3 ml of culture oil. Embryos from the same macrowell (i.e., cultured in same microwell shapes) were grouped together into the same droplet. For embryos in the commercial WOW dish, each glass-bottom dish comprised one droplet of 30 ul media covered with 3 ml culture oil, wherein embryos were grouped from commercial WOW dish. Then the dish was loaded into a stage-top incubator at 37 °C under 6% CO2 (ECU- HOC-IV, In Vivo Scientific) and imaged by integrated modulation contrast (IMC) microscopy (DMI8, Leica) using a 20 × objective (378–824-5, Mitutoyo). The PDMS dishes and IVF dishes were directly loaded to the stage-top incubator and imaged by the same microscope. Digital IMC images of the full field of view were saved. To avoid grading bias, any embryos seen in a collapsed state (due to spontaneous collapse or trauma from pipetting) were incubated for an additional 1.5 h to allow time for re-expansion prior to saving the final image.

Embryo grading

Each image was labelled with a random code known only to the experimenter. Randomly-labelled images were then sent to three embryologists for independent grading following the Gardner blastocyst grading system17, which involves assessing the degree of expansion, the quality of the inner cell mass (ICM) and the quality of the trophectoderm (TE) with a letter grade. The three-letter blastocyst morphology gradings were converted to a single-number score, the blastocyst quality score (BQS)18, which is the product of the degree of expansion (numbered one to six), and grades give for the ICM and TE (A = three, B = two and C = one). Embryos that did not become blastocysts were graded as zeros. The mean BQS across three embryologists was calculated for each embryo to represent its quality.

Blastocyst rate

We calculated the blastocyte rate of each replication for IVF dishes and for each microwell shape in PDMS WOW dishes. The mean blastocyst rate was then calculated across all replications for a given experimental condition. The mean blastocyst rate of the PDMS WOW dishes was obtained by averaging the blastocyst rates of all microwell groups. We confirmed the normal distribution of the blastocyst rate in all experimental groups by Kolmogorov–Smirnov normality test. Next, a paired Tukey one-way analysis of variance (ANOVA) was used to compare the blastocyst rates of each microwell shape, the PDMS WOW dish as a whole and the IVF dish. An embryo was considered not to have reached the blastocyst stage if at least two of the three embryologists graded it as a non-blastocyst. Such embryos were excluded from the subsequent BQS comparison.

Mean BQS analysis

We first confirmed the normal distribution of the mean BQS in all experimental groups by Kolmogorov–Smirnov normality test. With the confirmation of normal distribution, a paired Tukey one-way analysis of variance (ANOVA) was used to perform statistical analysis to compare culture outcomes in the study comparing PDMS WOW, flat PDMS, and commercial WOW dishes.

Ethical statements

This study is performed in accordance with relevant guidelines and regulations. All methods are reported in accordance with ARRIVE guidelines.

Results and discussion

Characterization of large-scale structures

Figure 3 shows µCT images of the 3D-printed mold and corresponding WOW insert. The values for the designed and measured dimensions are summarized in Table S1. The reported values represent the average of three repeated measurements at each location. Qualitatively, it is evident that the large-scale structures in the mold match the expected design. The large-scale lateral structures in the PDMS WOW inserts also nearly match the expected designs, although they are uniformly smaller than the corresponding mold dimensions. PDMS shrinkage, a known phenomenon during PDMS curing19, was observed in the axial structures (i.e., WOW insert thickness, H’, and macrowell depths, h1’–h4’). Notably, the shrinkage was limited (< 5.35%) and did not affect the use of WOW dishes.

Fig. 3
figure 3

µCT inspection results of WOW the mold and insert (scale bar: 3.5 mm). (a) 3D rendering of the mold; (b) top-view STD z-projection of the mold; (c) cross-sectional images taken as shown in (b); (d) 3D rendering of the WOW insert; (e) STD z-projection of the insert; (f) cross-sectional images taken as shown in (e).

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Characterization of small-scale structures

Figure 4 shows OCT images of the microwell templates of the mold (a–h) and corresponding microwell structures of the insert (i–p). The measured dimensions are summarized in Table S2, where each measurement represents the average of single measurements taken at each of the nine microwells. As expected, the small-scale structures in the 3D-printed mold nearly matched the designs, as do the small-scale lateral structures in the PDMS WOW inserts. Similar to the macrowells, PDMS shrinkage was observed for the axial structures of microwells (i.e., microwell depths); however, the shrinkage was limited (< 7.84%) and did not affect the use of WOW dishes. Qualitatively, the images in Fig. 4 also show the strong similarity across microwell replicates, which suggests the repeatability and reliability of the embryo growth conditions across microwells.

Fig. 4
figure 4

OCT inspection of WOW mold and insert microstructures. (a–h) OCT images of the mold microwell structures, including STD z-projections and cross-sectional images (scale bar: 200 µm): (a,b) hemispheres; (c,d) large truncated cones; (e,f) pyramids; (g,h) small truncated cones. (i–p) OCT images of the insert microwell structures, including STD z-projections and cross-sectional images (scale bar: 200 µm): (i,j) hemispheres; (k,l) large truncated cones; (m,n) pyramids; (o,p) small truncated cones.

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Test of mold reusability

Figure 5 shows the consistency of inserts replicated with the same mold. STD z-projections of the OCT images of the 15th and 30th WOW insert replicates are shown as an interlaced pattern. The strong continuity of the image indicates that structures are well preserved even after many replicates, suggesting high reusability of the mold, which can lower fabrication and production costs.

Fig. 5
figure 5

STD z-projection overlays of OCT microwell images from the 15th (gray) and 30th (blue) WOW inserts (scale bar: 200 µm): (a) hemispherical microwells; (b) large truncated conical microwells; (c) pyramidal microwells; (d) small truncated conical microwell.

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Comparison of culture quality in PDMS WOW dishes

Of the 83 embryos cultured, 70 reached blastocysts, which is one measure of embryo quality. The average Day-5 blastocyst rate of embryos cultured in our WOW dish was 88.7%. In general, the blastocyst rate for each microwell shape was higher than that of control embryos grown in IVF dishes (78.0%) by nearly 10%; however, no significant differences were found in any paired groups, when evaluated using Tukey one-way ANOVA. The high blastocyst rate confirms the WOW dish culture conditions are suitable for embryo growth. Figure 6 presents a visual summary of these results. The hatched pattern of bars corresponds to dishes with flat bottoms, such as the truncated cone microwells and the standard IVF dish.

Fig. 6
figure 6

Blastocyst rates (± 1 SEM) of embryo cultured in WOW dishes with hemispherical (H), pyramidal (P), and truncated conical (TC) microwell shapes compared to those cultured in IVF dishes (I). The average blastocyst rate of all WOW microwell designs (WOW) is also calculated. Hatch pattern indicates a flat-bottomed culture vessel.

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Overall, embryos grown in PDMS WOW dishes also show better blastocyst quality than those grown in IVF dishes when evaluated using the BQS score. The mean scores ± 1 SEM across all experiments for the different groups were as follows: 13.21 ± 1.45 (IVF dish), 28.63 ± 2.78 (hemispherical WOW), 17.35 ± 2.60 (pyramidal WOW) and 18.88 ± 3.01 (truncated conical WOW). Embryos cultured in hemispherical WOWs show the best blastocyst quality, and the trend was consistent with grades from all three embryologists despite general differences in the average BQS assigned by each embryologist (Fig. 7a). A Tukey one-way ANOVA was applied to analyse the BQS of each grader. In the BQS of the grader 1 (G1), statistically significant differences were found between the hemispherical and pyramidal dish groups, the hemispherical and truncated cone dish groups, and the hemispherical and IVF dish groups. In the BQS of the grader 2 (G2), statistically significant differences were found between the hemispherical and pyramidal dish groups, and the hemispherical and IVF dish groups. No significant differences were found in other paired groups. In the mean BQS comparison (Fig. 7b), statistically significant differences were found between the hemispherical and pyramidal dish groups, the hemispherical and truncated cone dish groups, and the hemispherical and IVF dish groups, while no significant differences were found in other paired groups, when evaluated using Tukey one-way ANOVA. Across all graders, hemispherical microwells yielded better BQS.

Fig. 7
figure 7

Culture quality comparison between PDMS WOW dishes and IVF dishes: (a) blastocyst quality score (BQS ± 1 SEM) of embryos cultured in hemispherical (H), pyramidal (P), truncated conical (TC) PDMS WOW and IVF dishes (I) for individual embryologist (Grader, G1, G2 and G3); (b) mean blastocyst quality score (BQS ± 1 SEM) of embryos cultured in PDMS WOW and IVF dishes. Hatch pattern indicates a flat-bottomed culture vessel.

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As this work is the first to use hemispherical microwells to culture embryos, one outcome of this fabrication process is that we have demonstrated a dish shape leading to better quality embryos. Previous efforts to demonstrate WOW culture of human embryos in truncated cone microwells5 yielded a live birth rate of 41.5%, an improvement over IVF dishes by 8.6%. Given our results showing that hemispherical wells outperform truncated cones IVF dishes in both blastocyst rate and BQS, we anticipate that this shape will also improve the live birth rate. In our experiment, all WOW-dish embryos were individually confined, while IVF-dish embryos were grown as a group. Notably, the WOW-dish embryos within a single microwell type were all connected by the same ~ 80-µL droplet formed in the macrowell, which is the definition of group culture2,5,20. Hence, this experiment is not a comparison of the effects of individual versus group culture. Indeed, group culture has already been shown to be superior to individual culture21, likely because it enables sharing of growth factors. We thus believe that the superior BQS results of the hemispherical microwell group are due to the shape alone, as the general size of the microwell was controlled by maintaining a constant opening and depth across all tested shapes. The superiority of the hemispherical shape is not surprising, as the 3D culture environment offered by the hemispherical microwell likely better mimics the in-vivo environment.

Comparison of culture quality in PDMS dishes

Of the 40 embryos cultured, 39 embryo reached blastocyst. The average Day-5 blastocyst rate of embryos cultured in PDMS dishes was 100% and in IVF dishes was 95%. The mean BQS scores ± 1 SEM for PDMS and IVF dishes are 15.33 ± 1.72 and 19.29 ± 0.94, respectively. As tested by Tukey one-way ANOVA, no statistically significant difference was found between the culture quality of PDMS and IVF dishes (Fig. 8), indicating that the PDMS substrate alone does not improve the culture quality. Therefore, the enhancement of culture quality in PDMS WOW dishes is not a material effect of using PDMS.

Fig. 8
figure 8

Culture quality comparison between PDMS dishes (PDMS) and IVF dishes (I): mean blastocyst quality score (BQS ± 1 SEM) of embryos cultured in PDMS and IVF dishes. Hatch pattern indicates a flat-bottomed culture vessel.

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Comparison of culture quality in commercial WOW dishes

Of the 37 embryos cultured, 30 embryo reached blastocyst. The average Day-5 blastocyst rate of embryos cultured in commercial WOW dishes was 79.0% and in IVF dishes (i.e., control) was 83.3%. The mean BQS scores ± 1 SEM for the IVF and commercial WOW dishes are 14.82 ± 1.01 and 18.42 ± 2.24 respectively. Tukey one-way ANOVA was performed on the mean BQS of the control groups of hemispherical PDMS WOW dishes and commercial WOW dishes to confirm that there is no quality difference of embryos used in these two experiments. The same analysis was performed between the embryos cultured in hemispherical PDMS WOW dishes and commercial WOW dishes (Fig. 9). A statistically significant difference was found between the mean BQS of hemispherical PDMS WOW dishes and commercial WOW dishes, indicating that our WOW dishes provide better culture quality than commercial WOW dishes with semi-elliptical shapes.

Fig. 9
figure 9

Culture quality comparison between hemispherical PDMS WOW dishes and commercial polystyrene WOW dishes: mean blastocyst quality score (BQS ± 1 SEM) of embryos cultured in hemispherical PDMS WOW dishes (H) and of commercial WOW dishes (cWOW).

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Conclusions

In this study, we proposed a new method for low-cost and rapid fabrication of WOW dishes with arbitrary microwell shapes and dimensions. The flexibility of the fabrication method enabled us to produce WOW dishes with multiple microwell designs in a single dish, including pyramidal and hemispherical designs that have never been used for embryo culture before. The dishes we present here used four different designs, but the method is versatile to produce any combination of microwell designs, which can facilitate highly parallelized experiments and testing to identify the optimal microwell shape and dimensions. In theory, one could conceive of having a larger variety of designs in a single dish or of varying the combination of designs that share a single macrowell. Although our WOW dishes show enhanced culture quality, the optimal WOW design is unknown. This experiment is to demonstrate that better culture quality can be achieved by varying WOW design. To that end, we proposed the fabrication method to offer a simple method for researchers to investigate and research the better WOW design.

The one-step PDMS molding method we introduce to fabricate WOW inserts is simple to perform and can be used to convert any standard IVF dish to a WOW dish. It is also convenient to make multiple WOW inserts, as may be necessary to perform large-scale studies that validate the statistical significance of the results. The low cost of each replicate contributes to lowering the cost to fabricate multiple dishes, as may be needed to enable large-scale studies. We found that pre-warming the WOW dishes and culture media to 37 °C before filling the wells is useful to reduce bubbles during handling. The PFOCTS coating applied to the mold to prevent adhesion of the PDMS can cause the PDMS not to cure. However, during the WOW dish assembly, we left the dish in room temperature for at least 48 h to let all PDMS cure. Therefore, this issue did not affect the integrity of WOW insert structures and embryo culture in this study. In the future, alternative coating materials, such as parylene-c, can be applied to eliminate the curing concern22.

Although this method is very versatile and can enable almost any arbitrary design, the limited axial resolution of the DLP 3D printer affects the accuracy of shapes that have round structures. This defect can be observed in Fig. 4a, where the hemisphere template appears slightly flattened on the top. The limited axial resolution also makes the roughness of the microwell edges hard to control. Such roughness can be observed in Fig. 4e as ridges on the pyramidal templates. These ridges are also seen in the microscope images of the mold and WOW insert shown in Figs. S3 and S4, respectively. The effects of microwell base curvature and edge roughness on embryo culture quality are unknown, but analysis of the embryo culture results reveals that the PDMS WOW dishes are robust for culture and that hemispherical microwells produce better quality blastocysts than IVF dishes and other microwell designs.

In conclusion, the proposed method for fabricating novel WOW dishes will make it possible to rigorously investigate the effects of microwell shapes and dimensions on embryo growth. Our results indicate that hemispherical microwells yield better culture outcomes than traditional IVF dishes, pyramidal microwells and truncated cone microwells. We also verify that the observed improvements in embryo quality are not due to the use of PDMS (i.e., material effects) alone. Our WOW dishes with hemispherical microwells also outperform commercial WOW dishes of semi-elliptical microwells. Altogether, these results suggest that translation of our technology to in vitro fertilization clinics may provide new tools to bolster IVF success rates.