|Fig. 1. Science cover feature. CLIP was revealed on March 16, 2015 by Prof. DeSimone at TED2015 in Vancouver and in a paper published in Science.|
Additive manufacturing (AM), commonly referred to as 3D printing, has soared in popularity in recent years in both academia and industry. In the past several decades, the technology has evolved to include many platforms that enable the regeneration of a 3D object from a computer aided design (CAD) file using a variety of materials. Typically, a CAD file is processed into a series of 2D cross sections of defined thickness that are sequentially directed to a printer, enabling the generation of a 3D object in layer-by-layer manner. The stereolithography apparatus (SLA) is one method of 3D printing that uses selective exposure of UV light to polymerize a photo-active resin one layer at a time. SLA, along with other AM platforms, enables the fabrication of geometries that are otherwise considered unmanufacturable. However, despite increasing popularity and potential, 3D printing has not developed beyond the realm of rapid prototyping given various limitations associated with layer-by-layer printing, the most notable being print speed, typically on the order of a few millimeters per hour. This slow production rate is not viable on a commercial scale. Further, layer-by-layer produced parts exhibit inherent structural weakness along the axis of printing due to a lack of significant chemical association between layers. CAD file conversion to a series of 2D slices imparts a “stair-casing” effect on angled structures, which has several drawbacks. “Stair-casing” results in a non-ideal surface finish that is merely an approximation of the original design and further represents a physical manifestation of the anisotropy of the final part. This effect can be mediated through the use of finer slicing, though at the expense of longer print times. Due to the deficiencies associated with layer-by-layer 3D printing, AM has been restricted to rapid prototyping, and its full potential in manufacturing has yet to be realized.
|Fig. 2. Comparison between traditional SLA and CLIP.
Continuous Liquid Interface Production (CLIP) is a recently developed AM platform that utilizes the selective exposure of UV light to initiate photopolymerization and solidify a part, similar to SLA. Free radical photopolymerization, used in the fabrication of thin films, is commonly conducted in an oxygen-free environment to avoid O2 inhibition. The free radical photopolymerization mechanism is inhibited in the presence of atmospheric oxygen and results in an incomplete cure as well as other consequences such as slow polymerization rates, long induction (onset of reaction) periods, low conversion, short polymer kinetic chain lengths, and tacky surface properties. Oxygen can quench either the excited-state photoinitiator or form a stable peroxy radical upon interaction with a free radical of a propagating chain. CLIP, however, turns O2 inhibition into an advantage by exposing the photopolymerizable resin via an oxygen-permeable build window, resulting in the formation of a dead zone at the surface of the window, or a region of uncured liquid resin beneath the growing part. The dead zone is present throughout the fabrication process and represents the liquid interface of the CLIP platform. The dead zone is the defining difference between CLIP and traditional SLA. As shown in Figure 2, the presence of the liquid interface allows for part production, resin renewal, and build elevator movement to occur in a single step, as opposed to the discrete steps of SLA.
|Fig. 3. CLIP Microneedles for Transdermal Drug Delivery. CLIP enables microneedles to be fabricated quickly with unprecedented control over size, shape and spacing. Novel geometries may afford improved penetration into the skin.|
The DeSimone lab is exploring the potential for CLIP to be used in the fabrication of very small defined structures for medical applications. Significant opportunity exists in personalized medicine to create tailored devices to meet an individual patient’s needs within an office setting or operating room. We are examining the incorporation of drugs, vaccines, and other agents, as well as the release properties of devices made using CLIP. We are also developing new materials to be used with CLIP. While standard 3D printing resins work with CLIP, the continuous nature of the new technology allows for the development of new chemistries with better mechanical and stability properties than traditional 3D printing resins.