Biomaterials Break Physical Barriers to Boost Drug Delivery in Tumors

For decades, the story of cancer drug delivery has been dominated by chemistry: better ligands, smarter polymers, more potent payloads. Yet a growing body of evidence suggests that the reason so many nanomedicines fail in the clinic is not chemical at all — it is physical. A new review published in Biomedical Microdevices by Fathe Singh of the Council of Scientific & Industrial Research–Central Leather Research Institute in Chennai, India, argues that the field must confront the mechanical reality of solid tumours, and it lays out a comprehensive framework — “mechanotherapeutics” — for designing biomaterials that can physically overcome the barriers tumours build around themselves.

The core problem is one that has haunted nanomedicine since its inception. Laboratory results routinely show dramatic tumour accumulation and tumour shrinkage in mice, but clinical translation has remained stubbornly limited. The review attributes this gap primarily to the physical inaccessibility of tumour tissue. Solid tumours are not simply masses of malignant cells; they are mechanically abnormal environments. Their extracellular matrix (ECM) — the dense network of collagen, hyaluronan and other structural proteins that surrounds cells — becomes progressively densified and stiffened as the tumour grows. This matrix cross-linking exerts elevated solid stress on the tumour mass, compressing blood and lymphatic vessels and creating pockets of high interstitial fluid pressure (IFP) that push outward against anything trying to enter.

The consequence is a transport-limiting system. Blood vessels inside tumours are dysfunctional, leaky and irregularly shaped, so drug-bearing nanoparticles that escape into the bloodstream often exit through the wrong vessels or fail to exit at all. Those that do reach tumour tissue face a dense ECM that slows diffusion to a crawl, while elevated IFP suppresses the convective flow that normally carries macromolecules through tissue. The result is heterogeneous intratumoral distribution: drugs pool near the periphery of the tumour, close to blood vessels, while the tumour core — often the most hypoxic and aggressive region — remains untouched. Classic mathematical models of tumour transport, dating back to work by Baxter and Jain in the late 1980s, and decades of experimental work since, have established that this physical architecture, not just molecular targeting, determines whether a therapeutic payload reaches its target.

Singh’s review reframes nanomedicine failure as a transport-limited problem in which physical constraints are dominant — though not exclusive — determinants of therapeutic outcome. The framework categorises mechanotherapeutic strategies into three complementary approaches, each targeting a different aspect of the tumour’s mechanical armour.

The first category comprises stiffness-modulating systems that remodel the ECM itself. Enzymatic degradation of hyaluronan, collagen-targeting agents and matrix-loosening drugs have all demonstrated the ability to soften the tumour stroma and reduce diffusion distances. Landmark work on pancreatic ductal adenocarcinoma, one of the most fibrotic and drug-resistant cancers, showed that enzymatic targeting of the stroma could ablate physical barriers to treatment entirely. The review emphasises that these approaches must be carefully titrated: complete depletion of carcinoma-associated fibroblasts or wholesale matrix destruction can paradoxically accelerate tumour progression and induce immunosuppression, as has been demonstrated in pancreatic cancer models. The goal is controlled remodelling — enough softening to enable drug penetration without destabilising the tumour’s immunological containment.

The second approach involves deformable and penetration-optimised materials engineered to navigate structural constraints rather than brute-force through them. Particle size is a critical variable: sub-100-nanometre polymeric micelles accumulate in poorly permeable tumours in a size-dependent manner, and studies of size-shrinkable nanosystems show that designs capable of transitioning from larger accumulation-optimised particles to smaller penetration-optimised ones can achieve both high tumour retention and deep tissue infiltration. Particle shape and elasticity matter equally. Soft, deformable nanoparticles can squeeze through narrow interstitial gaps that rigid particles of equivalent diameter cannot traverse, and recent work quantifying size-dependent penetration depth of colloidal nanoparticles into cell spheroids confirms that mechanical pliability directly correlates with delivery depth. Singh argues that biomaterial design should explicitly incorporate mechanical properties — not just surface chemistry — as a design axis.

The third category addresses pressure and perfusion. Pressure-alleviating strategies aim to lower IFP, restoring the transvascular and interstitial pressure gradients that drive convective drug transport. Vessel-normalising approaches, rooted in the pioneering work of Jain and colleagues, use anti-angiogenic agents in carefully timed regimens to prune the chaotic, poorly functional tumour vasculature into something resembling normal tissue — a window during which perfusion improves, hypoxia drops, and nanoparticles can actually reach the tumour interior. Combined strategies that simultaneously reduce solid stress and normalise vasculature have shown synergistic improvements in drug delivery and, importantly, in immunotherapy response, since better-perfused tumours are more accessible to immune cells.

What distinguishes this review from previous transport-focused discussions is its extension into mechanochemical coupling — the molecular machinery by which mechanical stress is translated into redox and metabolic adaptation within tumour cells. Singh identifies a representative signalling axis composed of reactive oxygen species (ROS), AMP-activated protein kinase (AMPK), and sirtuin 1 (SIRT1). Mechanical stress within the tumour microenvironment modulates ROS production, which in turn influences AMPK, the cell’s primary energy sensor, and SIRT1, a NAD+-dependent deacetylase that governs metabolic adaptation under stress. This axis links the physical state of the tumour to its metabolic and oxidative resilience, offering a molecular handle for responsive biomaterial design: materials could, in principle, be engineered to sense and modulate this signalling cascade in tandem with their delivery function, turning mechanical intervention into a coordinated biochemical one as well.

The framework also incorporates microdevice-enabled platforms as an experimental backbone. Microfluidic and tumour-on-chip systems allow researchers to recreate the tumour microenvironment — including ECM density, interstitial flow, solid stress and vascular geometry — under quantitatively controllable conditions. Vascularised cancer-on-chip models have demonstrated how perfusion directly affects tumour spheroid growth and drug delivery, while tumour-microenvironment-on-chip systems can simulate complex nanoparticle transport around tumours. Implantable microdevices capable of performing high-throughput in vivo drug sensitivity testing directly within tumours further bridge the gap between bench and bedside. Together, these platforms provide a quantitative and experimentally tractable way to evaluate transport behaviour and optimise delivery strategies before clinical translation.

The translational implications are significant. Imaging biomarkers such as elastography — an emerging branch of medical imaging that maps tissue stiffness non-invasively — could potentially be used to stratify patients by tumour stiffness, guiding which mechanotherapeutic interventions and biomaterial designs are most appropriate for a given tumour. The review notes that solid stress and elastic energy have been proposed as quantitative measures of tumour “mechanopathology,” opening the door to a precision mechanomedicine approach in which the mechanical phenotype of a patient’s tumour directly informs therapeutic strategy.

Singh is careful to frame the framework as physically informed and experimentally actionable rather than a complete solution. Physical constraints are described as dominant but not exclusive determinants of therapeutic outcome, and the review acknowledges that molecular resistance, tumour heterogeneity and immune evasion remain formidable challenges that no single strategy can address alone. The value of the mechanotherapeutic framework lies in its integration: by treating mechanics, transport, redox biology and metabolic signalling as a coupled system rather than isolated problems, it offers biomaterials designers a unified design language.

The review is published as Singh, F., “Mechanotherapeutic biomaterials: Overcoming physical barriers to enhance intratumoral drug delivery in solid tumours,” in Biomedical Microdevices, volume 28, article 51. As cancer nanomedicine enters its next phase of clinical translation, the message from this work is clear: the next generation of cancer therapeutics will need to be built not only with molecular precision but with mechanical intelligence — materials designed to soften the tumour’s scaffolding, squeeze through its corridors, relieve its internal pressures and, ultimately, deliver on the promise that nanomedicine has been chasing for three decades.