This method demonstrates a fabrication strategy for creating sophisticated, biocompatible hydrogel-based micro devices, which are incorporated with moving components which can be magnetically actuated. The main advantage of this technique is that it is a rapid and simple method for micro fabricating complex devices out of PET-based hydrogels, which are FDA approved for use in humans. This results in an entirely biocompatible and implantable device.
To begin, review the accompanying text protocol and use it to fabricate the PDMS chamber and design the photomasks as described in the accompanying text protocol. Next, create the top lid for the PDMS chamber using an untreated piece of a number two glass cover slip. Starting from the zero level of the device, lower the bottom substrate using the micrometer head to the desired height.
Deposit a small volume of the PEGDA prepolymer sufficient to cover the bottom substrate and then place the top substrate onto the PDMS chamber. It is important to ensure that there are no air bubbles trapped between the top and bottom substrates. Air bubbles will result in large pores or defects forming in the polymerized hydrogel.
Next, place a photomask with the desired design on top of the top substrate. Ensure that the mask is in full contact with the top substrate and align to the substrate's bottom. Place on protective UV goggles.
Then expose the hydrogel prepolymer to UV light through the photomask. The power and duration of the UV exposure depends on the type of system and PEGDA formulation. After the hydrogel layer has been polymerized, lift the top substrate off from the PDMS chamber.
The polymerized layers should be adhered onto the top substrate. Wet this polymerized layer with excess, uncrosslinked prepolymer and store it away from light and to prevent the layer from drying out and cracking. Save it for use later to seal the assembled device.
To create the bottom support structures, begin by taking a PDMS-coated glass cover slip and use it as the top substrate of the PDMS chamber. Deposit more hydrogel prepolymer onto the bottom substrate and cover the PDMS well with the PDMS-coated glass cover slip. This is to ensure that the polymerized layers remain on the bottom substrate, allowing the user to build layers upwards.
Place a photomask on top of the sample, taking care to align it properly. Then, again expose the sample to UV light. Following exposure, remove the top substrate, add more PEGDA prepolymer, and lower the bottom substrate using the micrometer head to the desired level.
This level should correspond to the thickness of the second layer of hydrogel to be polymerized. At this point, cover the well again with the PDMS-coated glass top substrate and properly align the photomask. Then, expose the sample to UV light.
Repeat this process as many times as necessary to build up the desired number of hydrogel layers. To assemble and seal the device, first remove the top PDMS-coated cover glass and using a pair of tweezers, place the preformed hydrogel components, including gears and iron doped components, onto the support structures. To seal the device, first bring the bottom substrate to the final desired height of the assembled device using the micrometer screw gauge.
This should be the final height of the device, taking into account the thickness of the layers, interior components and any clearances given for moving components. Next, place the preformed hydrogel layer, adhered onto the untreated glass cover slip, onto the partially assembled device. Carefully place the preformed layer such that it is correctly aligned to the structures below it.
Then, place a photomask that allows for the ceiling of the device but protects the interior moving components from UV exposure. Doing the ceiling of the device, it is important to ensure proper alignment of the photomask so that the device is sealed, while avoiding exposure of the moving components to UV lights. This prevents the moving components from being polymerized onto the supporting structures.
Once properly aligned, expose the entire structure to UV light. Lift the glass cover slip from the fabrication stage. The sealed device should adhere to the top substrate.
If the device remains adhered to the bottom substrate, carefully lift the device with a pair of flat-tipped tweezers. Carefully remove any excess unpolymerized PEGDA using vacuum suction. Then, using a pair of flat tweezers, carefully lift the device off from the glass cover slip.
Place the device into saline solution or DI water for at least 30 minutes to allow for stabilization and expansion of the device and the interior components. To actuate the device, place a neodymium magnet below or above the device within one to two centimeters from the device. While moving the magnet, the movement of the iron oxide doped components will shadow the movement of the magnet.
This functional, single-geared device was fabricated in less than 15 minutes. The device can be actuated with a magnet and figure 7B shows the different positions of the iron oxide segment when it is actuated with a magnet. This strategy can be used for creating various different types of hydrogel-based micro devices.
For instance, here's a simple gate valve which controls the release of drugs from a single reservoir. The linear movement of the iron oxide doped hydrogel component gates the diffusion of a hypothetical drug out through an outlet. And here, a gated linear manifold controls the release of drugs from multiple reservoirs.
Each reservoir contains hypothetical drugs and the movement of the iron oxide doped component gates the movement of drugs out of these reservoirs through a window of hydrogel that allows for the diffusion of these drugs out of the exterior. In addition, a sophisticated design based on the Geneva drive can produce intermittent movement. A driving gear with the pin is able to engage a larger driven gear and produce a 60 degree rotation by the gear for each full rotation of the driving gear.
This fabrication strategy is a versatile method for creating various types of hydrogel-based devices. Although we have demonstrated its use in creating PEG-based devices, this strategy can be used for any type of photo polymerizable hydrogel. This technique creates what we believe is the next generation of implantable devices:sophisticated, entirely biocompatible devices which can be magnetically controlled and actuated without the need of an onboard power source.
