What is an appropriate MNP-free control matrix for human tissue?

This is a very difficult question to answer given what we know about the ubiquity of human exposure to MNPs in modern life. In essence, contemporary human tissue that is completely MNP-free is unlikely to exist. The review mentions use of archived biological tissues dating from the pre-plastic era. While this is a viable option, the issue here is availability of such material. There is a commercial opportunity here for a reference standards supplier.

The use of artificial tissue analogues may be an option, especially for conducting recovery/validation experiments. Alternatively, tissues from (very) controlled environments could be a possibility. It is suggested that accompanying verification techniques (e.g., SEM) could be used to verify the absence of these materials. 

An appropriate MNP-free control matrix for human tissue could be considered a biological sample or synthetic that mimics one, that lacks MNPs but  is is similar to  the tissue that is studied. These matrices are essential for method validation, calibration, and control in experiments involving MNP detection or quantification, such as magnetic particle imaging, magnetic sensing, or biodistribution studies. MNP-free control matrices could be considered either blank tissue of the same type or  human or animal surrogate that are confirmed to be MNP-free through a pre-screening process, that were  collected before any exposure to MNP. Other MNP-free control that could be used are homogenized tissues that are prepared under the same conditions as test samples but without adding  MNPs. Also, agarose or gelatin gels doped with salts, lipids, or proteins that have the ability to mimic the magnetic and electrical properties of a tissue, but without MNPs could be considered  for being used as MNP-free control matrix. For in vitro studies, cell cultures or tissue lysates not exposed to MNPs also serve as effective controls. Additionally, artificial biological matrices such as  BSA in buffer or lipid emulsions can mimic tissue composition without the variability of real biological samples, making them particularly useful for reproducibility and calibration.

I am not aware of a specific MNP-free control matrix for human tissue. What is needed in a search for an MNP-free control matrix, is to anaytically verify the absence of MNP and secure that they remain absent in the control matrix over time.

It is doubtful such a matrix exists. Any historical (pre-1900s) biological samples that exist are likely contaminated by history of handling and storage.

Some tissues collected before 1950s, when plastics were not ubiquitous applied to daily life and stored in non-plastic containers or environment.

I have a difficult time imagine one. Maybe museum or cryo repository of large tissue samples that can be sliced to take an inner core that's free from surface contamination.

Artificial tissue simulants: Gelatin or agarose-based hydrogels doped with proteins or lipids to simulate tissue.

It is not easy or possible to find human tissues free of MNPs.  In the absence of human specimens that are free of MNPs, rodent or rabbit tissues that are devoid of MNPs can be used.  The rodents (e.g., rats or mice or rabbits) may be raised in plastic free environment to harvest organs and tissues, that can be used as MNP-free control matrix for human tissues.