Is injecting pure Hyaluronic acid and Hyaluronic acid serum bad to inject to cheeks?

Im a little confused by your question because of the terminology you use and the different forms of hyaluronic acid that is commercially available.  If you mean the stabilized fillers like belotero, juvederm, perlane, restylane, or voluma then the answer is yes they will give you visible predictable results that last from 6 mos to 2 years.  The duration is determined by location and product choice.If you mean serums of hyaluronic acid, or "pure" HA, that may be something different.  HA's are also in multiple skin care products or serums for example.  That form of HA has not been manufactured or stabilized in the way that the fillers mentioned above have been.  So stay away from any other form.  It would break down too quickly, and likely is an unapproved product.