The formation of a hard sheet type material after lyophilization of nanosuspension could be due to several factors such as the presence of impurities, the use of inappropriate stabilizers, or the presence of high concentration of drug (Formulation aspects of intravenous...). In addition, centrifugation may not be sufficient to remove all impurities and aggregates from the nanosuspension. It is also possible that the lyophilization process itself may have caused aggregation of particles (Freeze-drying of drug nanosuspension...).
Could you provide more information about your nanosuspension such as its composition and preparation method? This could help me provide a more specific answer to your question.
Source:
(1) Formulation aspects of intravenous nanosuspensions - ScienceDirect. https://www.sciencedirect.com/science/article/pii/S0378517320305391.
(2) Freeze-drying of drug nanosuspension– study of formulation and .... https://www.sciencedirect.com/science/article/pii/S1773224722004397.
(3) Preparation and Characterization of Nanosuspension of ... - PubMed. https://pubmed.ncbi.nlm.nih.gov/27123422/.