Table 7.

Evolution of Drug Delivery Systems from 1950 to 2040.

Drug Delivery System Size Scale	Macroscale	Microscale and Nanoscale	Nanoscale (Targeted Delivery)
	Implants (e.g., Subcutaneous or intramuscular	Reservoir DDS (e.g., Oral tablets, drug-eluting stents, catheters)	Injected nanocarrier DDS (e.g., PEGylated drugs, PEGylated liposomes, PEGylated polymeric micelles, polymer-drug conjugates
	Inserts (e.g., Vaginal, ophthalmic)	Injected matrix or monolith depots (e.g., Degradable microparticles and phase separation)
	Ingested DDS (e.g., Osmotic pumps, hydrogels)	Early nanoparticles and PEGylation DDS (e.g., Polymeric micelles and liposomes)
	Topical DDS (e.g., Skin patches)
	1st Generation	2nd Generation	3rd Generation
Drug Delivery System Technologies	Basics of Controlled Release (1950–1980)	Smart Delivery System (1980–2010)	Modulated Delivery System (2010–2040)
	Oral delivery	Zero-order release	On-Off insulin release
	Transdermal delivery	Smart polymers and hydrogels	Targeted delivery
	Drug release mechanism	Peptide and protein delivery	Long term delivery system
		Nanoparticles	In-vitro and in-vivo correlation