Tissue engineering (TE) is an interdisciplinary field, integrating engineering and medicine, that purpose to develop biological substitutes that will replace, repair or improve tissues and organs (Hasan et al., 2018). In 1970s was the first time that the concept of tissue engineering was introduced, by a pediatric orthopedic surgeon at Boston Children’s Hospital, W. T. Green, who performed several experiments aimed to generate cartilage from chondrocytes seeded in bone spicules (Melville et al., 2019). Twenty years later, TE was described by Robert Langer and Joseph Vacanti as “an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function” (Melville et al., 2019). In 2003, the Tissue Engineering Regenerative Medicine International Society was created, representing a huge milestone for the field, uniting scientists from all over the world to share and collaborate on their research, resulting in significant improvements (Melville et al., 2019).

TE is a field that is growing rapidly, providing new tools to manage complex diseases, and is a promising alternative to tissue harvesting, artificial tissues, and prostheses, since there are still high levels of graft rejection, low availability or unavailability of organ donation (Bodnar et al., 2018; Blum et al., 2021). In Figure 1 is shown the three factors that are essential for successful tissue regeneration, a combination of scaffolds or a framework, cell signaling and cells (Melville et al., 2019). To create a microenvironment of the human body, an extracellular matrix (EMC) is required, forming the basis of all organs and tissues ((Maheshwari et al., 2019).

In this scenario, 3D cell culture creates a microenvironment that can simulate the EMC found in the human body, taking into account morphological, biochemical, and mechanical factors (Fitzgerald et al., 2015; Asadi et al., 2020).

3D cell culture method enables in vivo conditions can be mimicked (Shao et al., 2021). For this, 3D cell culture using scaffolds has been improved in order to simulate the complexity of tumors in vivo (Shao et al., 2021). Therefore, the 3D models developed provide soluble gradients and allow the distribution of adhesions in all three spatial dimensions without polarity (Herreros-Pomares et al., 2021; Shao et al., 2021).

There are several types of platforms for 3D culture such as cell biology-based models (spheroids and organoids) and engineering-based models (scaffold and microfluidic platforms), cell biology-based have the advantage of having a greater similarity in the early details of cell development in vivo, while engineering-based models have better organization and composition of materials to develop ideal conditions important in tissue reconstruction (Zhuang et al., 2018). The methods approached through TE are limited mainly by the lack of appropriate techniques to develop physiological architectures that can mimic the EMC, in addition to the lack of control of cellular functions and their numerous properties (Hasan et al., 2018).

Photodynamic Therapy (PDT) is a minimally invasive therapeutic modality used for the treatment of several diseases, including cancer and non-malignant lesions (Mohammad-Hadi et al., 2018). Raab and von Tappeiner first introduced the term “photodynamic effect” into the literature through studies that showed that certain classes of dyes can sensitize microorganisms when exposed to sunlight, leading to cell death (Kessel, 2019). The advances were even greater when a group of physicians from the Mayo Clinic demonstrated that by employing a hematoporphyrin derivative, the fluorescence in the tumor tissues tended to increase, and the acronym “HPD” was used to refer to the material (Kessel, 2019). After numerous advances in the field, the terminology “Photodynamic Therapy” was introduced, based on the words used by von Tappeiner (Kessel, 2019).

The mechanism used by PDT consists in the interaction between a photosensitizer (PS) and a specific wavelength (Figure 1) of light in the presence of oxygen. The interaction leads to the formation of reactive oxygen species (ROS) and free radicals, such as singlet oxygen (¹O₂), that lead to the destruction of the target cells or tissue (Qidwai et al., 2020; Sun et al., 2020). Generally, studies using PDT is performed in monolayer, in other words, two-dimensional (2D) in vitro models, with advantages related to simplicity of application and reliability (Wu et al., 2020a; Demir Duman et al., 2020). However, the use of these models ultimately misses the cellular interactions with the EMC and does not reproduce the microenvironment correctly (Demir Duman et al., 2020). The use of animal studies also has limitations, such as the cost and the time of the experiment, which are usually long. In order to circumvent these factors in both techniques, more and more investment is being made in 3D culture models, where the microenvironment is optimally reproduced (Demir Duman et al., 2020). A direct advantage related to the use of PDT and his dependence on oxygen, is that the use of 3D models can incorporate the hypoxia that occurs in several tissues, like cancerous tissue (Demir Duman et al., 2020).

The present article seeks to provide a broad review of Tissue Engineering and Photodynamic Therapy, highlighting the main 3D models involved in TE and the most commonly used photosensitizers in PDT, as well as the applications, future prospects, and limitations that accompany both techniques.

Photodynamic therapy (PDT) is a therapeutic method that has been used in the treatment of several diseases, either as a single therapy or as a complement to conventional therapeutic protocols. This therapy is widely accepted by patients because it is less invasive than conventional ones, in addition to having few side effects and pain reduction. In addition, due to its therapeutic simplicity, it allows application in an outpatient setting, without the need for surgery (Li et al., 2017; Luo et al., 2017). The PDT mechanism is based on the correct combination of three components: photosensitizer (PS), monochromatic light at a specific wavelength (λ) and molecular oxygen (O₂) dissolved in a biological medium, PS is administered systemically, topical or oral, followed by exposure to visible light, resulting in a series of reactions that result in the death of target pathogenic cells (Figure 2), the three components do not show toxicity when separated (Calixto et al., 2016). After the internalization of the PS in the cells, irradiation is performed with a laser or LED at the wavelength where PS has greater energy absorption, the PS absorbs this energy and is excited to the singlet and triplet excited states, followed by an energy transfer to O₂, which leads to the production of reactive oxygen species defined as ROS that attack specific centers within cell systems, triggering the death of these tissues by processes of cell necrosis and/or apoptosis (Dai et al., 2012).

After the absorption of a photon of light, PS leaves the lower energy ground state (S⁰) and passes to the higher energy singlet state (S¹–Sⁿ), PS tends to return to the lower energy state, with this, part of the absorbed energy is used to return to the S0 state, through the physical relaxation process known as internal conversion or by radiative processes such as fluorescence emission (Figure 3) (Benov, 2015). However, part of the energy can be transferred by the Intersystem conversion mechanism, where the PS passes from the excited states S¹/Sⁿ and occupies the triplet excited state (T¹) of lower energy which can also be returned directly to S0 by the internal conversion process or through the radiative process of emission of phosphorescence, or triggering a series of photochemical interactions that give rise to two known photodynamic mechanisms (Type I and II) (Foot, 1991; Kwiatkowski et al., 2018).

In the Type I mechanism, PS is in the T1 state, there is a transfer of energy to the biomolecules or the abstraction of a hydrogen atom can also occur, in both cases these reactions result in the formation of free radicals or radical ions, in turn, these reduced species can transfer electrons to molecular oxygen diffused in the medium (which is in its triplet state), which leads to the generation of reactive oxygen species (ROS), hydrogen peroxides, anion superoxides. These reactions are outlined below (Benov, 2015).

Type I mechanism–Redox reactions with biomolecules P 0 → P 1 → P 3 P 3 + S → P + · + S - · ( energy   transfer ) S - · + O 2 3 → S + O 2 - · → HO · + HO · P = photosensitizer; S = organic substrate; + = cation and - = anion.

In the Type II mechanism PS in the T¹ state can transfer energy directly to molecular oxygen. This is possible due to the fact that molecular oxygen is also found in the T¹ conformation in its ground state, thus forming the reactive species in the singlet state, which has strong oxidizing properties, outlined below (Benov, 2015; Tedesco et al., 2017).

Type II Mechanism–Mediated by the production of ¹O₂, as an example, lipid peroxidation. P 0 S → P 1 S → P 3 S P 3 S + O 2 3 → P 0 S + O 2 1 ( energy   transfer ) O 2 1 + S → S − OOH ( peroxides , etc . ) PS = photosensitizer and S = organic substrate.

In both mechanisms, a series of product responses are initiated, causing different effects and biological responses, such as oxidative stress to pathological tissue, and cell damage followed by death (Foot, 1991; Tedesco and Jesus, 2017).

PDT has mainly emerged as a new alternative treatment to conventional anti-cancer therapies that cause various side effects (Zhang et al., 2018). In the last 3 decades, several types of PS have been applied in pre-clinical and clinical studies (Table 1). In addition to some of these molecules reaching the market and showing great efficacy in the treatment of different types of cancer (Zhang et al., 2018).

Although the initial focus of the use of PDT was the treatment of several types of cancer, PDT can also be used in the treatment of many other diseases (Yoo et al., 2021). Table 2 presents a summary of some clinical and preclinical studies for non-cancer diseases that use PDT as a treatment.

There is a lot of effort to use PDT for the treatment of different types of diseases (Yoo et al., 2021). For that reason, it is extremely important to know the PS that has currently been employed in order to verify if there is potential for a new therapeutic application (Kou et al., 2017).

TE is an area in constant expansion and its use in association with PDT has shown promising results in some studies, especially in studies involving antitumor therapy (Table 3).

In the study by Cramer et al. (2019) a 3D model of malignant pleural mesothelioma (MPM) was developed to assess the effect of PDT using a 1st generation PS, Photofrin. First, they tested different combinations of scaffolds for cell growth: 1) agarose; 2) agarose-collagen type I; 3) agarose overlay preceded by hanging-drop; 4) GFR-matrigel. They observed that the combination containing collagen provided cell growth on the 7th day. However, some of the cell lines used did not grow under these conditions. The opposite was observed in GFR-Matrigel, cell growth was more efficient and consistent than the other combinations. Therefore, a 3D model containing Matrigel and type I collagen was used to evaluate the effect of PDT-Photofrin. For this, the protocol involved the use of erlotinib, an inhibitor of epidermal growth factor (EGFR) in order to confirm the hypothesis that this inhibition could improve the outcome of PDT. They concluded that the 3D model obtained can be used for future studies, allowing the analysis that the use of erlotinib was able to improve the cytotoxicity of PDT-Photofrin (Cramer et al., 2019).

Second-generation PS has been well described in the literature for clinical studies (Tables 1, 2). In this context, Aggarwal et al. (2015) reported in their work the development of a 3D model of inflammatory breast cancer (IBC MAME) for application in PDT. The IBC MAME model was obtained from reconstituted basement membrane (rBM) and different breast cancer cell lines. After 7 days the formation of structures occurred. Subsequently, experiments involving PDT were conducted using two protocols. In the first protocol, the photosensitizer derived from benzoporphyrin monoacid A (BPD) was used with a light dose ranging from 45 to 540 mJ/cm². For the second protocol, they combined BPD and with PS N-aspartyl chlorin e6 (NPe6) (NPe6/BPD), which were incubated for 60 min and activated sequentially. They observed that the NPe6/BPD-PDT protocol was more efficient in the photo death of tumor cells compared to the first protocol. In addition, the light dose required to obtain death above 90% for the NPe6/BPD-PDT protocol was 45 mJ/cm². Obtaining this same death rate for BPD-PDT required an 8-fold higher light dose (Aggarwal et al., 2015).

Broekgaarden et al. (2019) evaluated the efficiency of PDT using BPD alone and in combination with the chemotherapy drug oxyplatin in a 3D culture model of metastatic pancreatic cancer. 3D culture models were established from different pancreatic cancer cell lines on matrigel scaffolds, which were kept in culture for 18 days. After the eighth day, the PDT assay was conducted. They used in the PDT protocol 0.25 µM of BPD, incubation for 1.5 h, laser light of 690 nm, irradiance of 50 mW/cm² and light dose of 1–50 J/cm², the effects of the treatment were evaluated in the days 9, 11 and 18 post-treatment. For the chemotherapy protocol, oxyplatin was used for 72 h and the treatment effects were analyzed on day 11 and 18. The authors observed through the results that PDT combined with oxyplatin was more efficient than monotherapy. In addition, they noted that the effectiveness of the treatment was time-dependent. They concluded that PDT can prevent peritoneal carcinomatosis after surgery, which for the present study provides promising preclinical evidence (Broekgaarden et al., 2019).

Another study involving a 3D model of the pancreas was developed by Karimnia et al. (2021). The 3D model used presented in its composition co-culture of pancreatic cancer tumor cells and human fibroblasts and matrigel as a scaffold. After 7 days that the model was obtained, treatment with PDT was performed. The protocol involved the use of PS verteporfin (benzoporphyrin derivative monoacid ring A, BPD), incubation for 1 h, irradiation with a laser source of 690 nm, light dose variation from 5 to 20 J/cm² with irradiation of 100 mW/cm². They treated the 3D model with chemotherapeutic agents (gemcitabine and oxyplatin) in order to compare it with the response obtained by PDT after 24 h of treatment. They noted that the presence of fibroblasts in the 3D model promoted chemoresistance. In contrast, the response was increased to PDT when compared to monoculture. They concluded that PDT may be an efficient strategy to overcome tumor-promoting stromal interactions associated with poor therapeutic response in pancreatic cancer (Karimnia et al., 2021).

The generation of spheroids is frequently reported in the literature as an approach for evaluating the efficacy of drugs in vitro. This 3D model has advantages such as being relatively inexpensive with the possibility of co-culture (Gong et al., 2015; Khot et al., 2018; Yakavets et al., 2019; Brancato et al., 2020).

In this context, the surface of the culture plate used to obtain the spheroid plays an important role, mainly in the orientation of cellular behavior (Brancato et al., 2020). Therefore, some authors described in their work the use of agarose coated plate to obtain spheroids (Khot et al., 2018; Cramer et al., 2019; Yakavets et al., 2019).

Multicellular colorectal cancer spheroids to verify the effect of PDT-Hypericin compared to monolayer model was investigated by Khot et al. (2018). The authors concluded that the PDT-Hypericin effect was greater in the 2D culture than in the spheroids. However, using an ABCG2 protein inhibitor caused an increase in the effect of PDT-Hypericin (Khot et al., 2018). Yakavets et al. (2019) produced multicellular head and neck squamous cell carcinoma spheroids in co-culture. As in the work by Khot et al. (2018), the plate pre-coated with agarose to obtain the spheroid was used (Yakavets et al. (2019).

The PDT protocol employed by Yakavets et al. (2019) was based on the use of three second-generation PS, indocyanine green (ICG), temoporphyrin (mTHPC), and Chlorin e6 (Ce6) in co-culture spheroids compared to homospheroids. The authors concluded that tumor stromal components may limit the antitumor activity of anticancer therapies. In the case of the PS used, they observed that Ce6 had less accumulation in the co-culture spheroids, unlike mTHPC, whereas ICG accumulated equally in the two spheroid models compared (Yakavets et al., 2019).

In the data presented in Table 1, it was possible to observe that in the clinical trials for the treatment of cancer most of the PS used belong to the first and second generation. Therefore, 3D model studies involving 3rd generation PS for PDT are essential to enable the expansion of its clinical use (Chen et al., 2001; Pogue et al., 2001; Cramer et al., 2019; Agostinis et al., 2011, 2012; Lamberti et al., 2014; Bacellar et al., 2015; Spring et al., 2016; Banerjee et al., 2017; Kwiatkowski et al., 2018; Dos Santos et al. al., 2019).

In the studies by Demir Duman et al. (2020), Etcheverry et al. (2020) and Wu et al. (2020) the effects of PDT with different third-generation PS on different 3D tumor models were studied. These being 1) nanocomposites of gold nanorods (Au NRs) with the cationic porphyrin TMPyP (5,10,15,20-tetrakis (1-methyl 4-pyridinium)porphyrin tetra (p-toluenesulfonate); 2) m-THPC and 3) pegylated liposomes containing mTHPC, respectively. Results and protocols were varied. There was a consensus that PDT was efficient and further studies on other photosensitizers are needed to prove the generality of the 3D models developed in the studies described (Demir Duman et al., 2020; Etcheverry et al., 2020; Wu et al. (2020).

Therefore, the association of Tissue Engineering and Photodynamic Therapy protocols resulted in great advances for the understanding of therapeutic processes based on the interest in the interaction of monochromatic light with biological tissues. Tissue Engineering is a field of science in full expansion and would also contribute to a better understanding of photodynamic mechanisms. This scientific review article can directly contribute to the organization of what is considered a state of the art in this field of knowledge. Updating and presenting important information for the direction of works that wish to use these advanced protocols. There is no doubt about the great potential for using these combined concepts, which are at the Frontier of knowledge and can help in the development of new biological assays available for application in various clinical treatments and chronic pathologies such as antitumor, anticancer and chronic psoriasis.

The development of tissue engineering has been described in the literature for many decades. In recent years it has gained evidence mainly through the appeal to use alternatives to animal testing. Until now, the studies have involved non-systemic evaluation of drug behavior in human cells/tissues, replacement of damaged parts of the body, cosmetic testing, among others (Langer and Vacanti, 1993; Morales, 2008; Doke and Dhawale, 2015).

However, the biggest challenge related to this technology is to obtain models that faithfully emulate all the characteristics of the human biological structure. In addition, the pathophysiology of certain diseases often has different expressions between species, which becomes another limitation for reproducing the methodologies (Guiro and Arinzeh, 2015; Hapach et al., 2019). Other possible obstacles to the development and application of 3D models mainly involve cell types, as it is necessary to use at least 2 cell types to be able to recreate the original structure and production costs (Langer and Vacanti, 2016).

Despite the challenges, the production of 3D Cell Culture Models is an excellent tool to assess the possibility of transposing data directly to humans (Ravi et al., 2015; Ceylan and Bölgen, 2016). In this context, there are several types of photosensitizing agents available for use in PDT that require biological models to be tested (Desmet et al., 2018; Yakavets et al., 2019; Etcheverry et al., 2020). However, the challenges associated with the structural characteristics of PS need to be overcome for application in PDT and, consequently, make its clinical use unfeasible. They are usually molecules of high molecular weight and lipophilicity, such as porphyrins, which lead to low permeability and make it difficult to incorporate it into conventional pharmaceutical forms (Webber, 2016).

Another factor to be considered for the application of PDT includes the low selectivity of PS action. Therefore, this contributes to the use of nanotechnology (Sharma et al., 2017). Therefore, a strategy to overcome these limitations associated with PS would be the use of delivery systems such as polymeric nanoparticles and liposomes (Sharma et al., 2017).

Therefore, the evolution of TE is related to the possibility of obtaining new tools such as the implantation of biofabricated tissues, elaboration of synthetic scaffolds capable of simulating the tissue’s microenvironment, production of mini-organs, valves, cartilages, among others from 3D bioprinting (Nguyen and Pentoney, 2017; W et al., 2017; Tarassoli et al., 2018). And more recently, the concepts of organ-in-a-chip and human body-on-chip were introduced, which are small three-dimensional biomimetic systems that aim to mimic characteristics of the organs they represent (Low et al.; Chen et al., 2021). In addition to being interconnected to form larger systems with different types of cells on which physical forces act, they have several applications such as analysis of pharmacokinetic, pharmacodynamic and toxicological properties of drugs, organ-organ interaction (Low et al., 2021; Chen et al., 2021).