This protocol is well-structured and clearly outlines the research objectives. However, to strengthen the design, here are some key areas that require further clarification:

Novelty and Regulatory Considerations:

The protocol mentions two potentially novel elements: a skin interstitial fluid sampling device and the vitamin D phosphate transdermal patch.
It's unclear if both are being investigated. Clarify if the sampling device is a secondary outcome measure or the primary focus of the study.
The mentioned phases in the Egyptian Clinical Trial law raise important regulatory considerations.
Since the patch is potentially novel, it likely falls under Article (4) requiring approval from the Egyptian Drug Authority, the Supreme Council, and potentially the General Intelligence Service due to the international collaboration.
According to Chapter Five. Article (10) in the Egyptian Clinical Trial law: The Medical interventions that originate outside the Arab Republic of Egypt, it is permitted to conduct the third and fourth stages after reviewing and approving the results of the first and second stages that were conducted in the country of origin by the Egyptian Drug Authority and the Supreme Council. Are we allowed to work under this article?
Sample Size and Group Allocation:

The protocol mentions 150 participants but lacks justification for the sample size.
Include a power calculation to ensure the study can detect a statistically significant effect size.
Details on group allocation are missing.
Specify how many participants will be in each treatment group (patch vs. placebo) and how randomization will be conducted.
Inclusion/exclusion criteria are Missing