Ultra-thin polymer films are defined as films with thicknesses below 100 nm. The thinnest freestanding films found in open literature are 20 nm thick and typically have diameters of less than 100 um. Such films are typically made by spin coating or dip coating a dilute solution of the polymer onto a substrate. Release the film from the substrate presents a problem. Typically a release or sacrificial layer such as sputtered salt or soap is dissolved to remove the film is removed from the substrate. The films are released by immersion in water. The film will float on the water surface and can be transferred to a holder, commonly a grid with small (typically sub-micron to 100 um) openings that define the free-standing area of film.
Most disadvantages of the current method are connected to the substrate. The substrate preparation can introduce roughness on the order of several nanometers, especially in the case of sputtered liftoff layers and the nonuniformity becomes more severe as the film thickness is reduced. Sacrificial liftoff layers can contaminate the film and decrease the strength of the film. At thicknesses lower than 30 nm, just the release of the film from the substrate becomes difficult to impossible. In addition, the shape of the holder, the liftoff technique itself, and the properties of the polymer that is used to produce the thin film limit the size of the film. Large films will often tear when lifted out of the water, and in some cases they tear while drying.
Biomedical membranes today are manufactured through such processes as microlithography, ion beam etching, electrochemical leaching (anodization) and sol-gel processes. PEEL offers a potentially simpler, faster way to produce biomedically useful membranes.
PEEL was originally developed to produce ultrathin films for the assembly of inertial confinement fusion targets. These targets have a fuel capsule that must be supported with minimal mass to avoid perturbations to the implosion.
LLNL Polyelectrolyte Enabled Liftoff (PEEL), is used to fabricate freestanding polymer films as thin as 10 nm that are capable of bearing loads ranging from milligrams to grams and deformations of up to forty percent (40%). PEEL employs robust, water-based, and self-optimizing surface chemistry to fabricate ultrathin films greater than 100 cm2 in area. The process is easily scalable in size and manufacturing quantity and applicable to a variety of polymeric materials.
The flexibility of PEEL is the key to its usefulness to industrial processes. It is scalable up to roll-to-roll level for high manufacturing volumes. It can use any kind of chemistry to generate membranes. Another significant benefit is that the film removal is water-based—it doesn’t need hazardous organic solvents. This makes PEEL a far more environmentally benign, safer process. PEEL allows its user to self-optimize the manufacturing process at any scale from surface micromachining to roll level.
The scalability, low cost, and environmentally benign chemistry of PEEL offer benefits to a wide range of membrane manufacturing processes in use today. It could help manufacturers overcome a cost and production barriers limiting the broader adoption of advanced membrane technologies that are thought too costly or too difficult to manufacture.
The PEEL method is:
- Low cost
- Scalable to large areas,
- Works reliably over numerous batches,
- Does not contaminate the water used to release the film,
- Environmentally benign
Membranes for biomedical uses are experiencing a surge of interest. Biocompatible membranes have applications ranging from hemodialysis to wound dressing, purifying biologically active materials, drug delivery through implantable devices with nanoporous membranes, and manufacturing artificial tissues such as blood vessels and cartilage for medical purposes. Biomembranes are also used in biosensors to detect medically hazardous compounds and diagnosis, and to immunoisolate or protect implanted cells or drug release devices from immune reactions.
Other potential uses involve the fabrication of separation membranes for carbon capture and for desalination.
The LLNL PEEL process is already in daily use at the National Ignition Facility (NIF), a national security and energy research facility, where it is used to fabricate compliant loadbearing polyvinyl formal (PVF) membranes that capture and center the hollow fuel spheres used as laser targets.
LLNL has filed for patent protection on the PEEL fabrication technology.
PEEL: Fabricating free-standing ultrathin polymer films over large areas by Salmaan Baxamusa, Staff Scientist