# Biodegradable Contrast Agents for Photoacoustic Imaging
## 1 Introduction  
Photoacoustic imaging (PAI) is considered a promising volumetric imaging technique for biomedical applications based on hybrid contrast mechanism under a light illumination and emitted ultrasound capturing scheme which enables to provide spectroscopic optical contrast and ultrasonic high-resolution. Additionally, PAI can achieve deep tissue imaging (several centimeters) by overcoming the bottleneck of the typical optical penetration depth (theoretically \(\sim\)1 mm) [1; 2; 3; 4; 5; 6; 7; 8]. Furthermore, using the intrinsic optical absorption contrast between various components, including hemoglobin, melanoma, collagen and fat, PAI can visualize anatomical information such as vasculature distributions, melanin location, tendons and plaques, as well as physiological information such as the total amount of hemoglobin, oxygen saturation ratio, blood velocity and metabolic ratio [9; 10; 11]. In parallel, diverse PAI systems have been consistently developed with multiscale imaging ability [12]. Therefore, photoacoustic microscopy (PAM) achieves high spatial resolution with a focused excitation laser or a focused ultrasound transducer by scanning using a mechanical scanning stage or a high-speed optical scanner [13; 14; 15]. Also, photoacoustic computed tomography (PACT) delivers two- andthree-dimensional deep tissue PA images for various kinds of structural, functional and molecular imaging in macro- or micro-scale [16; 17; 18; 19; 20; 21]. These benefits can be widely utilized in basic scientific research and clinical translation approaches in areas including dermatology, oncology, ophthalmology and neurology [10; 11; 22; 23; 24; 25]. Although the intrinsic contrasts in biological tissue can provide multi-contrast PA images, the absorption peaks usually stay around the visible spectral range, which decreases the light penetration depth owing to strong light scattering and absorption. As a result, the intrinsic contrasts can only delineate the shallow-depth regions. Near-infrared light is suitable for deeper tissue imaging and exogenous contrast agents that absorb near-infrared light can significantly enhance the functionality of PAI [26]. In particular, high photoacoustic (PA) intensity signals are not obtained for transparent and colorless biological tissues such as the lymph nodes, bladder and intestines because they have relatively low absorption coefficients in the visible and near-infrared spectral regions. Thus, exogenous contrast agents with outstanding absorption capabilities in the near-infrared spectral region and the ability to target specific types of biological tissue can improve sensitivity, specificity and the contrast of PAI. Diverse exogenous contrast agents, such as metallic agents (e.g., gold-, silver- and copper agents) [27; 28; 29], carbon-based agents (e.g., carbon nanotubes) [30], organic agents (e.g., porphyrin, lipid and polypyrrole agents) [31; 32] and semi-conducting polymer agents [33] have been used as high-sensitivity PAI or theranostic contrast agents with superior absorption coefficients in the near-infrared spectral region. As one of the most widely used metallic agents, gold-based formulations, which produce a variety of gold structures, exhibit excellent PA signal enhancement [34; 35; 36]. However, their biocompatibility, biodegradability and renal clearance remain important issues for translation into real clinical applications. For instance, the clearance of gold nanoshells covered with polyethylene glycol (\(M_{w}\) = 5000) from the body is difficult [37]. The utilization of surfactants to synthesize gold nanorods may cause issues related to toxicity [38; 39]. Moreover, carbon-based agents obtained by pyrolysis have been studied as novel molecular imaging agents. Unfortunately, pyrolysis processes cause considerable chemical and physical changes and unavoidably result in the formation of cytotoxic compounds [39; 40; 41]. To compensate for these problems, several biocompatible and biodegradable PA contrast agents have been reported recently.