What specific areas of research should be prioritized to address the identified knowledge gaps and uncertainties?

A top priority is the development and validation of methods that provide accurate, reproducible, quantitative, and qualitative data on the presence of MPs in biological samples. It is only when such methods are available that research into MP contamination of human and wildlife tissues can really make progress. A crucial area is the development of certified/standard reference materials that allow assessment of the accuracy of a method. In addition, the availability of reference particles that can be spiked into samples to act as matrix spikes is also a priority. Approaches to remove matrix effects while leaving MPs intact are also needed.

Standardization of methods, and, importantly, methods that allow for the  detection and quantification at the nanoscale. The environmental fate and transport of these materials also lacks some more detailed knowledge and more relevant ecotoxicological information is needed, particularly when considering environmentally relevant concentrations of MNPs and suitable level of weathering. 

I think that further on the line, the research on microplastic detection and recovery should focus on developing standardized, sensitive methods capable of monitoring both micro and nanoplastics. Understanding their sources, pathways, and ecological and human health impacts, and  also developing new and advanced effective removal technologies such as improved wastewater treatment, filtration, and biodegradation is essential for future addressing and identifying knowledge gaps and uncertainties. Studies should also promote some circular economy approaches meant to reduce plastic inputs and integrate recovery strategies into existing systems. Developing predictive modeling and risk assessment will also support evidence-based policy, regulatory frameworks, and targeted interventions in high-risk areas.

Here are some areas that researchers should be prioritized to address knowledge gaps and uncertainties. 
1. Improved analytical methods which are able to expand the sizes of the MNP to be detected and identify the types of MNPs with increased confidence. 
2. Effect of MNPs to the environment
3. Effect of MNPs to the human health

In general terms this would be any research that brings the level of QA/QC for plastics analysis up to the level of QA/QC applied for the analysis of soluble chemicals in any (certified) analytical lab. Basically, each of the steps that are part of the certification of any analytical lab, need to be assessed, implemented, and quality controlled for the analysis of plastics. The priority research area is thus the certification of the analyses of micro- and nanoplastcis in specific matrices.
Therupon, more (in an absolute numerical sense) and more diverse (in terms of properties relevant for the fate and effects of plastics) reference materials need to be developed and made available in sufficient quantities to allow use by any stakeholder independent of this being an academic, legislative, or commercial stakeholder.
Also, ringtesting needs to be promoted once sufficient materials for a sufficient diversity of matrices are available.
Finally, more effort is needed to develop alternative analytical methods to allow to cover the whole field of plastics polymers. This is amongst needed as the current methods are not able to do so.

This comment below is an overall comment to improve the presentation on the manuscript.
Overall, this article reads more from the perspective of MNP analysis in environmental samples, especially drinking water, using a spectroscopy or image based method.  There is little consideration or discussion that pertains to pyrolysis GCMS method which is widely used for biological samples.  The title of the paper is on biological samples and therefore, some more description relating to pyGCMS method would have made the manuscript much more relevant.  In such PyGCMS method, determination of background levels in various materials such as pipet tips, tubes is possible to control levels of contamination.  In other words, a bit more detailed focus on pyGCMS method would have helped the manuscript further, as most of the discussion pertain to FTIR, Raman and image based analysis.  

At the end of introduction section, on line 41:  I would add a sentence stating that analytical methods are still evolving and therefore best practice may change with more knowledge  becomes available.  

All figures have same captions.  It is really unusual to have same caption for all figures.  Each figure should have its own caption specifically depicting what it meant.

There are so many abbreviations used throughout without any expansion; e.g., PPE, HQI, MDA, MPDB, NCEI, GPML and many more.  Please expand these abbreviations at least when used for the first time.

Line 144:  'staff testing' should be changed as "staff training"

As mentioned above, sample collection section should at least mention avoidance of plastic sample collection devices.  This is common in human specimen collection.  Blood collection uses plastic syringes and tubes, but authors report data without paying attention to contamination from those plastic syringes and tubes  (may be inserted after line 289).

One statement that I do not agree with blank subtraction.  Authors recommend "do not subtract or add blank values".  This is not customary in mass spec based analysis.  In spectroscopy method it may not be followed, but in mass spec method, we subtract blank values.  Analysts measuring contaminants by mass spectrometry often subtract blank values to report concentrations in samples.  Otherwise it overestimates actual concentrations and spuriously report background levels of contamination.  I would at least suggest authors to reanalyze this section on "do not subtract or add blank values" to consider that in PyGCMS method, a common approach is to subtract blank value.  You may have negative value, but is better than reporting a false positive value.  

Proper protocols to separate microplastics' signals' origin between ambient sampling background and the biological sampled.

1. New combination of analytical methods for polymer confirmation and quantitation. 
2. Sample treatment strategy and methodology that enable new combination method applicable to biological matrices. 
3. Any model study demonstrates the matrices removal and succusses use of new combination method (acceptable recovery/accuracy, repeatability/precision)
4. Apply the whole development methodology into real world/real tissue sample studies.

-Production of relevant reference materials of a mixture of polymers, shapes and sizes that are pre-weighed followed by spike recovery experiments with tissues.  
-Simple exploration of background contamination in surgical rooms
-Method development research into solvent extraction for MNP quant on pyGCMS