Manufacturable Single-Slot Flow Insert Update

Introduction

A recent collaborative publication from RIT developed a modular µSiM to address the lack of physiological fluid flow in conventional open-well devices (Figure 1). The magnetic latching assembly of this design allows for quick configuration into a fluidic setup. The flow module is a microfabricated PDMS insert that contains a channel over the membrane culture surface as well as access ports for tubing. The insert matches the “cauliflower” geometry of the µSiM’s open well to ensure alignment.

Figure 1. Schematic of modular fluidic m-µSiM (Mansouri, et al.).

Implementation in the Human Tendon-on-a-Chip (hToC)

Previous efforts by Raquel to utilize the modular fluidic m-µSiM with the hToC resulted in leakage issues. These issues were due to thickness differences between the hToC bottom component and the µSiM bottom component as well as tolerance discrepancies with top components. Despite efforts to adjust the housing dimensions to accommodate the hToC thickness, variable top component thicknesses created difficulties when latching the upper and lower housings and reliably sealing the insert to the membrane surface.

To overcome these sealing issues, the McGrath lab adopted a UV bonding process depicted in Figure 2.

Figure 2. Current solution for bonding PDMS insert to the membrane.

To validate this new bonding method, I cultured HUVECs in the hToC device and subjected the cells to 10 dynes/cm⁻² of shear stress for 24 hours using a peristaltic pump circuit. The devices did not exhibit leakage throughout the experiment and as shown in Figure 3 HUVECs cultured in fluidic conditions aligned in the direction of flow.

Figure 3. HUVEC shear alignment in hToC with cultured on NPN.

Manufacturing Design Process

Over the past few months, we have been working with ALine to develop a manufacturable version of the flow insert. By doing so, we will facilitate high-throughput flow experiments and increase the efficiency of the transition from static to fluidic mode. We began this process by defining the following design criteria:

1. Easy assembly (peel and stick with pressure-sensitive adhesive)
2. Maintain a similar shear stress profile at the culture surface as current flow module
3. Allow for sufficient gas exchange (ideally made from PDMS)
4. Include reservoirs to prevent evaporation in flow channel during static culture

Figure 4. Design criteria for manufacturable flow insert.

After multiple rounds of design changes and consultations with the team at ALine, we ultimately landed on a simple insert design that closely matches the original geometry. The rationale for these design changes is summarized in Figure 5.

Figure 5. Manufacturable flow insert design progression.

The main challenges when adapting our design for manufacturability were taking into account the complexity of the design and the feasible capabilities at ALine. The lid design aimed to provide additional media to prevent evaporation during the initial 24 hours of static culture before the addition of flow. Moreover, we aimed to eliminate concerns about the flow needles contacting and potentially damaging the membrane. However, these designs were deemed too complex as they would require PDMS thicknesses not attainable with high-throughput manufacturing methods.

After removing the lid portion, we needed to make additional changes to the geometry in order to maximize the bonding area of the PSA to the membrane. As shown in Figure 6, the channel length was shortened from 5 mm to 4.6 mm to increase the minimum distance between the edge of the flow channel and the membrane to 0.5 mm.

Figure 6. Bottom view of flow insert (dimensions shown in mm).

While our priority was to maintain the original flow channel geometry, this change does not significantly impact the uniform shear stress profile at the membrane (to be validated by a COMSOL simulation).

In addition to the channel geometry revision, the flow channel will be manufactured in a three-layer laminate: PSA + PET + PSA (Figure 7). This reduces the number of steps in the manufacturing process and eliminates the need to mold the channel. The 3 mm-thick PDMS portion will be molded and assembled to the laminate channel.

Figure 7. Final manufactured flow insert design.