In addition to what stated by indubioush, your device classification will depend on the risks associated with how your device will be used in its intended use environment. That is the key player in determining the classification, sometime it changes the classification of your device from the predicate you originally opted for. For example, you have a "synthetic gel which converts into a solid patch by in situ mixing at the surgical site to act as an implant" but as a predicate there are device(s) which are only "preformed solid bio-material patch made with tissues derived from animals or human origin" with the identical intended use with your device. If the predicate device is a class II device which have a long history of safety and effectiveness then you may end up getting a Class III designation based on FDA's evaluation since,
1. You have a new technology for the same intended use, which requires your device to have an in situ chemical reaction. For this you will have a higher risk than your predicate as there is a higher chance of device failure.
2. The degradation profile of your device is unknown. Because you are using synthetic materials which may degrade into harmful byproducts whereas your predicate is using materials which are biologically available inside human body.
The above two facts may lead to Clinical Investigations.
In summary, your predicate device has a "Moderate to High Risk" as it is an implant but with long history of safety and effectiveness and will need to provide enough "performance evidence" such as biocompatibility, degradation rate, animal testing etc. Whereas, besides the performance testing required by your predicate, you will need to provide additional evidences (in some instances "Clinical") as your product carries many unknown facts regarding its S&E during its intended use, as now you have a "High Risk".
Generally, manufacturer matches their device technologies for substantial equivalence with the existing devices in order to determine its risk profile prior to their submission to the FDA. I hope this will give an idea about how the product classification works when it comes to the US FDA regulation.