created automatically during the preparation of CAD models and can also be made manually. In either situation, the supports must be removed manually after printing. Other forms of stereolithography build each layer by LCD masking, or using a DLP projector. Materials The liquid materials used for SLA printing are commonly referred to as "resins" and are thermoset polymers. A wide variety of resins are commercially available and it is also possible to use homemade resins to test different compositions for example. Material properties vary according to formulation configurations: "materials can be soft or hard, heavily filled with secondary materials like glass and ceramic, or imbued with mechanical properties like high heat deflection temperature or impact resistance". Recently, some studies have tested the possibility to green or reusable materials to produce "sustainable" resins. It is possible to classify the resins in the following categories: Standard resins, for general prototyping Engineering resins, for specific mechanical and thermal properties Dental and medical resins, for biocompatibility certifications Castable resins, for zero ash-content after burnout Biomaterial resins, formulated as aqueous solutions of synthetic polymers like polyethylene glycol, or biological polymers such as gelatin, dextran, or hyaluronic acid. Uses Medical modeling Stereolithographic models have been used in medicine since the 1990s, for creating accurate 3D models of various anatomical regions of a patient, based on data from computer scans. Medical modelling involves first acquiring a CT, MRI, or other scan. This data consists of a series of cross sectional images of the human anatomy. In these images different tissues show up as different levels of grey. Selecting a range of grey values enables specific tissues to be isolated. A region of interest is then selected and all the pixels connected to the target point within that grey value range are selected. This enables a specific organ to be selected. This process is referred to as segmentation. The segmented data may then be translated into a format suitable for stereolithography. While stereolithography is normally accurate, the accuracy of a medical model depends on many factors, especially the operator performing the segmentation correctly. There are potential errors possible when making medical models using stereolithography but these can be avoided with practice and well trained operators. Stereolithographic models are used as an aid to diagnosis, preoperative planning and implant design and manufacture. This might involve planning and rehearsing osteotomies, for example. Surgeons use models to help plan surgeries but