# Nanotechnology, Nanomedicine, and the Kidney
## 1 Introduction  
Nanotechnology refers to the use, or application, of engineered macromolecules in the size range of 1-100 nanometers [1]. Materials involved range from polymers, metals, macromolecules, lipids, semiconductors, and chemicals [2, 3]. Due to their small size, nanomaterials, which contain more than 50% nanoparticles (NP), can exhibit novel physical (mechanical, optical, electrical, magnetic, and thermal) [4, 5, 6] properties that can add substantial value in a wide array of applications, including medicine [7], generating the portmanteau _nanomedicine_. Over the last decade, nanomedicine has developed tools used in medical imaging, disease diagnosis, and therapies for the prevention and treatment of diseases such as cancer [8], diabetes [9, 10], and cardiovascular diseases [10, 11, 12]. We further improved our understanding of how nanomaterials interact in a living organism, as well as their circulation, distribution, and excretion [13, 14]. Despite the proven benefits, concerns regarding the assumed toxicity of nanomaterials persist.  
The kidneys are vital organs that perform several essential functions. They generate urine, regulate blood pressure, balance electrolytes, and produce hormones. Kidney's can be damaged acutely by physical injury or nephotoxic insults, with can lead to acute kidney injury (AKI), a clinical condition that is often reversible. Chronic kidney disease (CKD), describes a gradual injury due to acquired or congenital diseases that involve immune regulatory disorders, diabetes, hypertension, metabolic disease, or other disorders. CKD, which can lead to end-stage renal disease (ESRD), the permanent loss of kidney function, requires dialysis or kidney transplantation. Between 2015 and 2018, CKD had an overall prevalence of 14.9% in the US [15].  
ESRD is a pressing issue, in 2018, in the US alone, the number of patients with ESRD was 131,636, a 2.3% increase compared to the previous year. ESRD prevalence in 2018 was 783,925, with 70.7% receiving dialysis and 29.3% of patients having a functioningtransplanted kidney [15]. The leading causes for CKD and ESRD are diabetes, hypertension, and cardiovascular disease (Figure 1). Controlling the underlying conditions, can slow the progression of CKD and help to prevent ESRD.  
The kidney is of key interest in nanotechnology and nanomedicine due to its essential functions in physiology, and further for its role and interaction in the filtration and excretion of particles used in nanomedicine therapies of distant organs. The main aspects of research in nanomedicine of the kidney are (i) the influence of the kidney on the pharmacokinetic of nanomaterials and nanodrugs, (ii) the nephrotoxic potential of nanomaterials, and (iii) the development of novel therapies to treat kidney diseases.  
In recent years, kidney-excretable nanoparticles with a vastly improved signal to background ratio have been produced. It enables more precise imaging, detection, and diagnosis of renal injury, while at the same time significantly reducing nephrotoxicity by lowering the amounts of diagnostic compounds necessary. Research on the interaction of nanomaterials and the kidney has further improved our understanding of the kidney's capacity for clearance of nanomaterial, targeting of kidney cells, and the accumulation of particles in the kidney. In this review, we briefly describe the kidney's role in the elimination of nanomaterials, and we summarize the recent advances in the imaging and treatment of kidney dysfunctions using nanotechnology.